aboutsummaryrefslogtreecommitdiff
path: root/libbuild2/algorithm.cxx
blob: 76f7c4ccb17a252567e5baee73b8e8ace92a15f4 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
// file      : libbuild2/algorithm.cxx -*- C++ -*-
// license   : MIT; see accompanying LICENSE file

#include <libbuild2/algorithm.hxx>

#include <libbuild2/scope.hxx>
#include <libbuild2/target.hxx>
#include <libbuild2/rule.hxx>
#include <libbuild2/file.hxx> // import()
#include <libbuild2/search.hxx>
#include <libbuild2/context.hxx>
#include <libbuild2/filesystem.hxx>
#include <libbuild2/diagnostics.hxx>
#include <libbuild2/prerequisite.hxx>

using namespace std;
using namespace butl;

namespace build2
{
  const target&
  search (const target& t, const prerequisite& p)
  {
    assert (t.ctx.phase == run_phase::match);

    const target* r (p.target.load (memory_order_consume));

    if (r == nullptr)
      r = &search_custom (p, search (t, p.key ()));

    return *r;
  }

  const target*
  search_existing (const prerequisite& p)
  {
    context& ctx (p.scope.ctx);

    assert (ctx.phase == run_phase::match || ctx.phase == run_phase::execute);

    const target* r (p.target.load (memory_order_consume));

    if (r == nullptr)
    {
      r = search_existing (ctx, p.key ());

      if (r != nullptr)
        search_custom (p, *r);
    }

    return r;
  }

  const target&
  search (const target& t, const prerequisite_key& pk)
  {
    assert (t.ctx.phase == run_phase::match);

    // If this is a project-qualified prerequisite, then this is import's
    // business.
    //
    if (pk.proj)
      return import (t.ctx, pk);

    if (const target* pt = pk.tk.type->search (t, pk))
      return *pt;

    return create_new_target (t.ctx, pk);
  }

  pair<target&, ulock>
  search_locked (const target& t, const prerequisite_key& pk)
  {
    assert (t.ctx.phase == run_phase::match && !pk.proj);

    if (const target* pt = pk.tk.type->search (t, pk))
      return {const_cast<target&> (*pt), ulock ()};

    return create_new_target_locked (t.ctx, pk);
  }

  const target*
  search_existing (context& ctx, const prerequisite_key& pk)
  {
    return pk.proj
      ? import_existing (ctx, pk)
      : search_existing_target (ctx, pk);
  }

  const target&
  search_new (context& ctx, const prerequisite_key& pk)
  {
    assert (ctx.phase == run_phase::load || ctx.phase == run_phase::match);

    if (const target* pt = search_existing_target (ctx, pk))
      return *pt;

    return create_new_target (ctx, pk);
  }

  pair<target&, ulock>
  search_new_locked (context& ctx, const prerequisite_key& pk)
  {
    assert (ctx.phase == run_phase::load || ctx.phase == run_phase::match);

    if (const target* pt = search_existing_target (ctx, pk))
      return {const_cast<target&> (*pt), ulock ()};

    return create_new_target_locked (ctx, pk);
  }

  const target&
  search (const target& t, name n, const scope& s, const target_type* tt)
  {
    assert (t.ctx.phase == run_phase::match);

    auto rp (s.find_target_type (n, location (), tt));
    tt = rp.first;
    optional<string>& ext (rp.second);

    if (tt == nullptr)
      fail << "unknown target type " << n.type << " in name " << n;

    if (!n.dir.empty ())
      n.dir.normalize (false, true); // Current dir collapses to an empty one.

    // @@ OUT: for now we assume the prerequisite's out is undetermined.
    //         Would need to pass a pair of names.
    //
    return search (t,
                   *tt,
                   n.dir,
                   dir_path (),
                   n.value,
                   ext ? &*ext : nullptr,
                   &s,
                   n.proj);
  }

  const target*
  search_existing (const name& cn, const scope& s, const dir_path& out)
  {
    // See also scope::find_prerequisite_key().
    //
    name n (cn);
    auto rp (s.find_target_type (n, location ()));
    const target_type* tt (rp.first);
    optional<string>& ext (rp.second);

    // For now we treat an unknown target type as an unknown target. Seems
    // logical.
    //
    if (tt == nullptr)
      return nullptr;

    if (!n.dir.empty ())
    {
      // More often than not a non-empty directory will already be normalized.
      //
      // Note that we collapse current dir to an empty one.
      //
      if (!n.dir.normalized () || n.dir.string () == ".")
        n.dir.normalize (false, true);
    }

    bool q (cn.qualified ());

    // @@ OUT: for now we assume the prerequisite's out is undetermined.
    //         Would need to pass a pair of names.
    //
    prerequisite_key pk {
      n.proj, {tt, &n.dir, q ? &empty_dir_path : &out, &n.value, ext}, &s};

    return q
      ? import_existing (s.ctx, pk)
      : search_existing_target (s.ctx, pk);
  }

  const target*
  search_existing (const names& ns, const scope& s)
  {
    if (size_t n = ns.size ())
    {
      if (n == (ns[0].pair ? 2 : 1))
      {
        return search_existing (ns[0], s, n == 1 ? dir_path () : ns[1].dir);
      }
    }

    fail << "invalid target name: " << ns << endf;
  }

  // target_lock
  //
  // Note that the stack may contain locks for targets from multiple nested
  // contexts. This should be harmless (deadlock detection-wise) since
  // contexts are assumed non-overlapping.
  //
  static
#ifdef __cpp_thread_local
  thread_local
#else
  __thread
#endif
  const target_lock* target_lock_stack = nullptr;

  const target_lock* target_lock::
  stack () noexcept
  {
    return target_lock_stack;
  }

  const target_lock* target_lock::
  stack (const target_lock* s) noexcept
  {
    const target_lock* r (target_lock_stack);
    target_lock_stack = s;
    return r;
  }

  // If the work_queue is absent, then we don't wait.
  //
  target_lock
  lock_impl (action a, const target& ct, optional<scheduler::work_queue> wq)
  {
    context& ctx (ct.ctx);

    assert (ctx.phase == run_phase::match);

    // Most likely the target's state is (count_touched - 1), that is, 0 or
    // previously executed, so let's start with that.
    //
    size_t b (ctx.count_base ());
    size_t e (b + target::offset_touched - 1);

    size_t appl (b + target::offset_applied);
    size_t busy (b + target::offset_busy);

    atomic_count& task_count (ct[a].task_count);

    while (!task_count.compare_exchange_strong (
             e,
             busy,
             memory_order_acq_rel,  // Synchronize on success.
             memory_order_acquire)) // Synchronize on failure.
    {
      // Wait for the count to drop below busy if someone is already working
      // on this target.
      //
      if (e >= busy)
      {
        // Check for dependency cycles. The cycle members should be evident
        // from the "while ..." info lines that will follow.
        //
        if (dependency_cycle (a, ct))
          fail << "dependency cycle detected involving target " << ct;

        if (!wq)
          return target_lock {a, nullptr, e - b};

        // We also unlock the phase for the duration of the wait. Why?
        // Consider this scenario: we are trying to match a dir{} target whose
        // buildfile still needs to be loaded. Let's say someone else started
        // the match before us. So we wait for their completion and they wait
        // to switch the phase to load. Which would result in a deadlock
        // unless we release the phase.
        //
        phase_unlock u (ct.ctx, true /* unlock */, true /* delay */);
        e = ctx.sched.wait (busy - 1, task_count, u, *wq);
      }

      // We don't lock already applied or executed targets.
      //
      if (e >= appl)
        return target_lock {a, nullptr, e - b};
    }

    // We now have the lock. Analyze the old value and decide what to do.
    //
    target& t (const_cast<target&> (ct));
    target::opstate& s (t[a]);

    size_t offset;
    if (e <= b)
    {
      // First lock for this operation.
      //
      s.rule = nullptr;
      s.dependents.store (0, memory_order_release);

      offset = target::offset_touched;
    }
    else
    {
      offset = e - b;
      assert (offset == target::offset_touched ||
              offset == target::offset_tried   ||
              offset == target::offset_matched);
    }

    return target_lock {a, &t, offset};
  }

  void
  unlock_impl (action a, target& t, size_t offset)
  {
    context& ctx (t.ctx);

    assert (ctx.phase == run_phase::match);

    atomic_count& task_count (t[a].task_count);

    // Set the task count and wake up any threads that might be waiting for
    // this target.
    //
    task_count.store (offset + ctx.count_base (), memory_order_release);
    ctx.sched.resume (task_count);
  }

  target&
  add_adhoc_member (target& t,
                    const target_type& tt,
                    dir_path dir,
                    dir_path out,
                    string n)
  {
    tracer trace ("add_adhoc_member");

    const_ptr<target>* mp (&t.adhoc_member);
    for (; *mp != nullptr && !(*mp)->is_a (tt); mp = &(*mp)->adhoc_member) ;

    if (*mp != nullptr) // Might already be there.
      return **mp;

    target* m (nullptr);
    {
      pair<target&, ulock> r (
        t.ctx.targets.insert_locked (tt,
                                     move (dir),
                                     move (out),
                                     move (n),
                                     nullopt /* ext     */,
                                     target_decl::implied,
                                     trace,
                                     true /* skip_find */));

      if (r.second) // Inserted.
      {
        m = &r.first;
        m->group = &t;
      }
    }

    assert (m != nullptr);
    *mp = m;

    return *m;
  };

  static bool
  trace_target (const target& t, const vector<name>& ns)
  {
    for (const name& n: ns)
    {
      if (n.untyped () || n.qualified () || n.pattern)
        fail << "unsupported trace target name '" << n << "'" <<
          info << "unqualified, typed, non-pattern name expected";

      if (!n.dir.empty ())
      {
        if (n.dir.relative () || !n.dir.normalized ())
          fail << "absolute and normalized trace target directory expected";

        if (t.dir != n.dir)
          continue;
      }

      if (n.type == t.type ().name && n.value == t.name)
        return true;
    }

    return false;
  }

  void
  set_rule_trace (target_lock& l, const rule_match* rm)
  {
    action a (l.action);
    target& t (*l.target);

    // Note: see similar code in execute_impl() for execute.
    //
    if (trace_target (t, *t.ctx.trace_match))
    {
      diag_record dr (info);

      dr << "matching to " << diag_do (a, t);

      if (rm != nullptr)
      {
        const rule& r (rm->second);

        if (const adhoc_rule* ar = dynamic_cast<const adhoc_rule*> (&r))
        {
          dr << info (ar->loc);

          if (ar->pattern != nullptr)
            dr << "using ad hoc pattern rule ";
          else
            dr << "using ad hoc recipe ";
        }
        else
          dr << info << "using rule ";

        dr << rm->first;
      }
      else
        dr << info << "using directly-assigned recipe";
    }

    t[a].rule = rm;
  }

  // Return the matching rule or NULL if no match and try_match is true.
  //
  const rule_match*
  match_rule (action a, target& t, const rule* skip, bool try_match)
  {
    const scope& bs (t.base_scope ());

    // Match rules in project environment.
    //
    auto_project_env penv;
    if (const scope* rs = bs.root_scope ())
      penv = auto_project_env (*rs);

    match_extra& me (t[a].match_extra);

    // First check for an ad hoc recipe.
    //
    // Note that a fallback recipe is preferred over a non-fallback rule.
    //
    if (!t.adhoc_recipes.empty ())
    {
      auto df = make_diag_frame (
        [a, &t](const diag_record& dr)
        {
          if (verb != 0)
            dr << info << "while matching ad hoc recipe to " << diag_do (a, t);
        });

      auto match = [a, &t, &me] (const adhoc_rule& r, bool fallback) -> bool
      {
        me.init (fallback);

        if (auto* f = (a.outer ()
                       ? t.ctx.current_outer_oif
                       : t.ctx.current_inner_oif)->adhoc_match)
          return f (r, a, t, string () /* hint */, me);
        else
          return r.match (a, t, string () /* hint */, me);
      };

      // The action could be Y-for-X while the ad hoc recipes are always for
      // X. So strip the Y-for part for comparison (but not for the match()
      // calls; see below for the hairy inner/outer semantics details).
      //
      action ca (a.inner ()
                 ? a
                 : action (a.meta_operation (), a.outer_operation ()));

      auto b (t.adhoc_recipes.begin ()), e (t.adhoc_recipes.end ());
      auto i (find_if (
                b, e,
                [&match, ca] (const shared_ptr<adhoc_rule>& r)
                {
                  auto& as (r->actions);
                  return (find (as.begin (), as.end (), ca) != as.end () &&
                          match (*r, false));
                }));

      if (i == e)
      {
        // See if we have a fallback implementation.
        //
        // See the adhoc_rule::reverse_fallback() documentation for details on
        // what's going on here.
        //
        i = find_if (
          b, e,
          [&match, ca, &t] (const shared_ptr<adhoc_rule>& r)
          {
            auto& as (r->actions);

            // Note that the rule could be there but not match (see above),
            // thus this extra check.
            //
            return (find (as.begin (), as.end (), ca) == as.end () &&
                    r->reverse_fallback (ca, t.type ())            &&
                    match (*r, true));
          });
      }

      if (i != e)
        return &(*i)->rule_match;
    }

    // If this is an outer operation (Y-for-X), then we look for rules
    // registered for the outer id (X; yes, it's really outer). Note that we
    // still pass the original action to the rule's match() function so that
    // it can distinguish between a pre/post operation (Y-for-X) and the
    // actual operation (X).
    //
    // If you are then wondering how would a rule for Y ever match in case of
    // Y-for-X, the answer is via a rule that matches for X and then, in case
    // of Y-for-X, matches an inner rule for just Y (see match_inner()).
    //
    meta_operation_id mo (a.meta_operation ());
    operation_id o (a.inner () ? a.operation () : a.outer_operation ());

    // Our hint semantics applies regardless of the meta-operation. This works
    // reasonably well except for the default/fallback rules provided by some
    // meta-operations (e.g., dist, config), which naturally do not match the
    // hint.
    //
    // The way we solve this problem is by trying a hint-less match as a
    // fallback for non-perform meta-operations. @@ Ideally we would want to
    // only consider such default/fallback rules, which we may do in the
    // future (we could just decorate their names with some special marker,
    // e.g., `dist.file.*` but that would be visible in diagnostics).
    //
    // It seems the only potential problem with this approach is the inability
    // by the user to specify the hint for this specific meta-operation (e.g.,
    // to resolve an ambiguity between two rules or override a matched rule),
    // which seems quite remote at the moment. Maybe/later we can invent a
    // syntax for that.
    //
    const string* hint;
    for (bool retry (false);; retry = true)
    {
      hint = retry
        ? &empty_string
        : &t.find_hint (o); // MT-safe (target locked).

      for (auto tt (&t.type ()); tt != nullptr; tt = tt->base)
      {
        // Search scopes outwards, stopping at the project root. For retry
        // only look in the root and global scopes.
        //
        for (const scope* s (retry ? bs.root_scope () : &bs);
             s != nullptr;
             s = s->root () ? &s->global_scope () : s->parent_scope ())
        {
          const operation_rule_map* om (s->rules[mo]);

          if (om == nullptr)
            continue; // No entry for this meta-operation id.

          // First try the map for the actual operation. If that doesn't yeld
          // anything, try the wildcard map.
          //
          for (operation_id oi (o), oip (o); oip != 0; oip = oi, oi = 0)
          {
            const target_type_rule_map* ttm ((*om)[oi]);

            if (ttm == nullptr)
              continue; // No entry for this operation id.

            if (ttm->empty ())
              continue; // Empty map for this operation id.

            auto i (ttm->find (tt));

            if (i == ttm->end () || i->second.empty ())
              continue; // No rules registered for this target type.

            const auto& rules (i->second); // Name map.

            // Filter against the hint, if any.
            //
            auto rs (hint->empty ()
                     ? make_pair (rules.begin (), rules.end ())
                     : rules.find_sub (*hint));

            for (auto i (rs.first); i != rs.second; ++i)
            {
              const rule_match* r (&*i);

              // In a somewhat hackish way we reuse operation wildcards to
              // plumb the ad hoc rule's reverse operation fallback logic.
              //
              // The difficulty is two-fold:
              //
              // 1. It's difficult to add the fallback flag to the rule map
              //    because of rule_match which is used throughout.
              //
              // 2. Even if we could do that, we pass the reverse action to
              //    reverse_fallback() rather than it returning (a list) of
              //    reverse actions, which would be necessary to register them.
              //
              using fallback_rule = adhoc_rule_pattern::fallback_rule;

              auto find_fallback = [mo, o, tt] (const fallback_rule& fr)
                -> const rule_match*
              {
                for (const shared_ptr<adhoc_rule>& ar: fr.rules)
                  if (ar->reverse_fallback (action (mo, o), *tt))
                    return &ar->rule_match;

                return nullptr;
              };

              if (oi == 0)
              {
                if (auto* fr =
                      dynamic_cast<const fallback_rule*> (&r->second.get ()))
                {
                  if ((r = find_fallback (*fr)) == nullptr)
                    continue;
                }
              }

              const string& n (r->first);
              const rule& ru (r->second);

              if (&ru == skip)
                continue;

              me.init (oi == 0 /* fallback */);
              {
                auto df = make_diag_frame (
                  [a, &t, &n](const diag_record& dr)
                  {
                    if (verb != 0)
                      dr << info << "while matching rule " << n << " to "
                         << diag_do (a, t);
                  });

                if (!ru.match (a, t, *hint, me))
                  continue;
              }

              // Do the ambiguity test.
              //
              bool ambig (false);

              diag_record dr;
              for (++i; i != rs.second; ++i)
              {
                const rule_match* r1 (&*i);

                if (oi == 0)
                {
                  if (auto* fr =
                        dynamic_cast<const fallback_rule*> (&r1->second.get ()))
                  {
                    if ((r1 = find_fallback (*fr)) == nullptr)
                      continue;
                  }
                }

                const string& n1 (r1->first);
                const rule& ru1 (r1->second);

                {
                  auto df = make_diag_frame (
                    [a, &t, &n1](const diag_record& dr)
                    {
                      if (verb != 0)
                        dr << info << "while matching rule " << n1 << " to "
                           << diag_do (a, t);
                    });

                  // @@ TODO: this makes target state in match() undetermined
                  //    so need to fortify rules that modify anything in match
                  //    to clear things.
                  //
                  // @@ Can't we temporarily swap things out in target?
                  //
                  match_extra me1;
                  me1.init (oi == 0);
                  if (!ru1.match (a, t, *hint, me1))
                    continue;
                }

                if (!ambig)
                {
                  dr << fail << "multiple rules matching " << diag_doing (a, t)
                     << info << "rule " << n << " matches";
                  ambig = true;
                }

                dr << info << "rule " << n1 << " also matches";
              }

              if (!ambig)
                return r;
              else
                dr << info << "use rule hint to disambiguate this match";
            }
          }
        }
      }

      if (mo == perform_id || hint->empty () || retry)
        break;
    }

    me.free ();

    if (!try_match)
    {
      diag_record dr (fail);

      if (hint->empty ())
        dr << "no rule to ";
      else
        dr << "no rule with hint " << *hint << " to ";

      dr << diag_do (a, t);

      // Try to give some hints of the common causes.
      //
      switch (t.decl)
      {
      case target_decl::prereq_new:
        {
          dr << info << "target " << t << " is not declared in any buildfile";

          if (t.is_a<file> ())
            dr << info << "perhaps it is a missing source file?";

          break;
        }
      case target_decl::prereq_file:
        {
          // It's an existing file so it's an unlikely case.
          //
          break;
        }
      case target_decl::implied:
        {
          // While the "in a buildfile" is not exactly accurate, we assume
          // it's unlikely we will end up here in other cases.
          //
          dr << info << "target " << t << " is implicitly declared in a "
             << "buildfile";

          if (const scope* rs = bs.root_scope ())
          {
            if (t.out.empty () && rs->src_path () != rs->out_path ())
            {
              name n (t.as_name ()[0]);
              n.dir.clear ();
              dr << info << "perhaps it should be declared as being in the "
                 << "source tree: " << n << "@./ ?";
            }
          }

          break;
        }
      case target_decl::real:
        {
          // If we had a location, maybe it would make sense to mention this
          // case.
          //
          break;
        }
      }

      if (verb < 4)
        dr << info << "re-run with --verbose=4 for more information";
    }

    return nullptr;
  }

  recipe
  apply_impl (action a,
              target& t,
              const pair<const string, reference_wrapper<const rule>>& m)
  {
    const scope& bs (t.base_scope ());

    // Apply rules in project environment.
    //
    auto_project_env penv;
    if (const scope* rs = bs.root_scope ())
      penv = auto_project_env (*rs);

    const rule& ru (m.second);
    match_extra& me (t[a].match_extra);

    auto df = make_diag_frame (
      [a, &t, &m](const diag_record& dr)
      {
        if (verb != 0)
          dr << info << "while applying rule " << m.first << " to "
             << diag_do (a, t);
      });

    auto* f ((a.outer ()
              ? t.ctx.current_outer_oif
              : t.ctx.current_inner_oif)->adhoc_apply);

    auto* ar (f == nullptr ? nullptr : dynamic_cast<const adhoc_rule*> (&ru));

    recipe re (ar != nullptr ? f (*ar, a, t, me) : ru.apply (a, t, me));

    me.free ();
    return re;
  }

  // If step is true then perform only one step of the match/apply sequence.
  //
  // If try_match is true, then indicate whether there is a rule match with
  // the first half of the result.
  //
  static pair<bool, target_state>
  match_impl (target_lock& l,
              bool step = false,
              bool try_match = false)
  {
    assert (l.target != nullptr);

    action a (l.action);
    target& t (*l.target);
    target::opstate& s (t[a]);

    try
    {
      // Intercept and handle matching an ad hoc group member.
      //
      if (t.adhoc_group_member ())
      {
        assert (!step);

        const target& g (*t.group);

        // It feels natural to "convert" this call to the one for the group,
        // including the try_match part. Semantically, we want to achieve the
        // following:
        //
        // [try_]match (a, g);
        // match_recipe (l, group_recipe);
        //
        auto df = make_diag_frame (
          [a, &t](const diag_record& dr)
          {
            if (verb != 0)
              dr << info << "while matching group rule to " << diag_do (a, t);
          });

        pair<bool, target_state> r (match_impl (a, g, 0, nullptr, try_match));

        if (r.first)
        {
          if (r.second != target_state::failed)
          {
            match_inc_dependents (a, g);
            match_recipe (l, group_recipe);
          }
        }
        else
          l.offset = target::offset_tried;

        return r; // Group state (must be consistent with matched_state()).
      }

      // Continue from where the target has been left off.
      //
      switch (l.offset)
      {
      case target::offset_tried:
        {
          if (try_match)
            return make_pair (false, target_state::unknown);

          // To issue diagnostics ...
        }
        // Fall through.
      case target::offset_touched:
        {
          // Match.
          //

          // Clear the rule-specific variables, resolved targets list, and the
          // auxiliary data storage before calling match(). The rule is free
          // to modify these in its match() (provided that it matches) in
          // order to, for example, convey some information to apply().
          //
          clear_target (a, t);

          const rule_match* r (match_rule (a, t, nullptr, try_match));

          assert (l.offset != target::offset_tried); // Should have failed.

          if (r == nullptr) // Not found (try_match == true).
          {
            l.offset = target::offset_tried;
            return make_pair (false, target_state::unknown);
          }

          set_rule (l, r);
          l.offset = target::offset_matched;

          if (step)
            // Note: s.state is still undetermined.
            return make_pair (true, target_state::unknown);

          // Otherwise ...
        }
        // Fall through.
      case target::offset_matched:
        {
          // Apply.
          //
          set_recipe (l, apply_impl (a, t, *s.rule));
          l.offset = target::offset_applied;
          break;
        }
      default:
        assert (false);
      }
    }
    catch (const failed&)
    {
      // As a sanity measure clear the target data since it can be incomplete
      // or invalid (mark()/unmark() should give you some ideas).
      //
      clear_target (a, t);

      s.state = target_state::failed;
      l.offset = target::offset_applied;
    }

    return make_pair (true, s.state);
  }

  // If try_match is true, then indicate whether there is a rule match with
  // the first half of the result.
  //
  pair<bool, target_state>
  match_impl (action a,
              const target& ct,
              size_t start_count,
              atomic_count* task_count,
              bool try_match)
  {
    // If we are blocking then work our own queue one task at a time. The
    // logic here is that we may have already queued other tasks before this
    // one and there is nothing bad (except a potentially deep stack trace)
    // about working through them while we wait. On the other hand, we want
    // to continue as soon as the lock is available in order not to nest
    // things unnecessarily.
    //
    // That's what we used to do but that proved to be too deadlock-prone. For
    // example, we may end up popping the last task which needs a lock that we
    // are already holding. A fuzzy feeling is that we need to look for tasks
    // (compare their task_counts?) that we can safely work on (though we will
    // need to watch out for indirections). So perhaps it's just better to keep
    // it simple and create a few extra threads.
    //
    target_lock l (
      lock_impl (a,
                 ct,
                 task_count == nullptr
                 ? optional<scheduler::work_queue> (scheduler::work_none)
                 : nullopt));

    if (l.target != nullptr)
    {
      assert (l.offset < target::offset_applied); // Shouldn't lock otherwise.

      if (try_match && l.offset == target::offset_tried)
        return make_pair (false, target_state::unknown);

      if (task_count == nullptr)
        return match_impl (l, false /* step */, try_match);

      // Pass "disassembled" lock since the scheduler queue doesn't support
      // task destruction.
      //
      target_lock::data ld (l.release ());

      // Also pass our diagnostics and lock stacks (this is safe since we
      // expect the caller to wait for completion before unwinding its stack).
      //
      if (ct.ctx.sched.async (
            start_count,
            *task_count,
            [a, try_match] (const diag_frame* ds,
                            const target_lock* ls,
                            target& t, size_t offset)
            {
              // Switch to caller's diag and lock stacks.
              //
              diag_frame::stack_guard dsg (ds);
              target_lock::stack_guard lsg (ls);

              try
              {
                phase_lock pl (t.ctx, run_phase::match); // Throws.
                {
                  target_lock l {a, &t, offset}; // Reassemble.
                  match_impl (l, false /* step */, try_match);
                  // Unlock within the match phase.
                }
              }
              catch (const failed&) {} // Phase lock failure.
            },
            diag_frame::stack (),
            target_lock::stack (),
            ref (*ld.target),
            ld.offset))
        return make_pair (true, target_state::postponed); // Queued.

      // Matched synchronously, fall through.
    }
    else
    {
      // Already applied, executed, or busy.
      //
      if (l.offset >= target::offset_busy)
        return make_pair (true, target_state::busy);

      // Fall through.
    }

    return ct.try_matched_state (a, false);
  }

  static group_view
  resolve_members_impl (action a, const target& g, target_lock&& l)
  {
    // Note that we will be unlocked if the target is already applied.
    //
    group_view r;

    // Continue from where the target has been left off.
    //
    switch (l.offset)
    {
    case target::offset_touched:
    case target::offset_tried:
      {
        // Match (locked).
        //
        if (match_impl (l, true).second == target_state::failed)
          throw failed ();

        if ((r = g.group_members (a)).members != nullptr)
          break;

        // To apply ...
      }
      // Fall through.
    case target::offset_matched:
      {
        // @@ Doing match without execute messes up our target_count. Does
        //    not seem like it will be easy to fix (we don't know whether
        //    someone else will execute this target).
        //
        //    What if we always do match & execute together? After all,
        //    if a group can be resolved in apply(), then it can be
        //    resolved in match()! Feels a bit drastic.
        //
        //    But, this won't be a problem if the target returns noop_recipe.
        //    And perhaps it's correct to fail if it's not noop_recipe but
        //    nobody executed it? Maybe not.
        //
        //    Another option would be to have a count for such "matched but
        //    may not be executed" targets and then make sure target_count
        //    is less than that at the end. Though this definitelt makes it
        //    less exact (since we can end up executed this target but not
        //    some other). Maybe we can increment and decrement such targets
        //    in a separate count (i.e., mark their recipe as special or some
        //    such).
        //

        // Apply (locked).
        //
        if (match_impl (l, true).second == target_state::failed)
          throw failed ();

        if ((r = g.group_members (a)).members != nullptr)
          break;

        // Unlock and to execute ...
        //
        l.unlock ();
      }
      // Fall through.
    case target::offset_applied:
      {
        // Execute (unlocked).
        //
        // Note that we use execute_direct_sync() rather than execute_sync()
        // here to sidestep the dependents count logic. In this context, this
        // is by definition the first attempt to execute this rule (otherwise
        // we would have already known the members list) and we really do need
        // to execute it now.
        //
        {
          phase_switch ps (g.ctx, run_phase::execute);
          execute_direct_sync (a, g);
        }

        r = g.group_members (a);
        break;
      }
    }

    return r;
  }

  group_view
  resolve_members (action a, const target& g)
  {
    group_view r;

    if (a.outer ())
      a = a.inner_action ();

    // We can be called during execute though everything should have been
    // already resolved.
    //
    switch (g.ctx.phase)
    {
    case run_phase::match:
      {
        // Grab a target lock to make sure the group state is synchronized.
        //
        target_lock l (lock_impl (a, g, scheduler::work_none));
        r = g.group_members (a);

        // If the group members are alrealy known or there is nothing else
        // we can do, then unlock and return.
        //
        if (r.members == nullptr && l.offset != target::offset_executed)
          r = resolve_members_impl (a, g, move (l));

        break;
      }
    case run_phase::execute: r = g.group_members (a); break;
    case run_phase::load:    assert (false);
    }

    return r;
  }

  void
  resolve_group_impl (action, const target&, target_lock l)
  {
    match_impl (l, true /* step */, true /* try_match */);
  }

  template <typename R, typename S>
  static void
  match_prerequisite_range (action a, target& t,
                            R&& r,
                            const S& ms,
                            const scope* s)
  {
    auto& pts (t.prerequisite_targets[a]);

    // Start asynchronous matching of prerequisites. Wait with unlocked phase
    // to allow phase switching.
    //
    wait_guard wg (t.ctx, t.ctx.count_busy (), t[a].task_count, true);

    size_t i (pts.size ()); // Index of the first to be added.
    for (auto&& p: forward<R> (r))
    {
      // Ignore excluded.
      //
      include_type pi (include (a, t, p));

      if (!pi)
        continue;

      prerequisite_target pt (ms
                              ? ms (a, t, p, pi)
                              : prerequisite_target (&search (t, p), pi));

      if (pt.target == nullptr || (s != nullptr && !pt.target->in (*s)))
        continue;

      match_async (a, *pt.target, t.ctx.count_busy (), t[a].task_count);
      pts.push_back (move (pt));
    }

    wg.wait ();

    // Finish matching all the targets that we have started.
    //
    for (size_t n (pts.size ()); i != n; ++i)
    {
      const target& pt (*pts[i]);
      match_complete (a, pt);
    }
  }

  void
  match_prerequisites (action a, target& t,
                       const match_search& ms,
                       const scope* s)
  {
    match_prerequisite_range (a, t, group_prerequisites (t), ms, s);
  }

  void
  match_prerequisite_members (action a, target& t,
                              const match_search_member& msm,
                              const scope* s)
  {
    match_prerequisite_range (a, t, group_prerequisite_members (a, t), msm, s);
  }

  void
  match_members (action a, target& t, const target* const* ts, size_t n)
  {
    // Pretty much identical to match_prerequisite_range() except we don't
    // search.
    //
    wait_guard wg (t.ctx, t.ctx.count_busy (), t[a].task_count, true);

    for (size_t i (0); i != n; ++i)
    {
      const target* m (ts[i]);

      if (m == nullptr || marked (m))
        continue;

      match_async (a, *m, t.ctx.count_busy (), t[a].task_count);
    }

    wg.wait ();

    // Finish matching all the targets that we have started.
    //
    for (size_t i (0); i != n; ++i)
    {
      const target* m (ts[i]);

      if (m == nullptr || marked (m))
        continue;

      match_complete (a, *m);
    }
  }

  void
  match_members (action a,
                 target& t,
                 prerequisite_targets& ts,
                 size_t s,
                 pair<uintptr_t, uintptr_t> imv)
  {
    size_t n (ts.size ());

    wait_guard wg (t.ctx, t.ctx.count_busy (), t[a].task_count, true);

    for (size_t i (s); i != n; ++i)
    {
      const prerequisite_target& pt (ts[i]);
      const target* m (pt.target);

      if (m == nullptr ||
          marked (m)   ||
          (imv.first != 0 && (pt.include & imv.first) != imv.second))
        continue;

      match_async (a, *m, t.ctx.count_busy (), t[a].task_count);
    }

    wg.wait ();

    for (size_t i (s); i != n; ++i)
    {
      const prerequisite_target& pt (ts[i]);
      const target* m (pt.target);

      if (m == nullptr ||
          marked (m)   ||
          (imv.first != 0 && (pt.include & imv.first) != imv.second))
        continue;

      match_complete (a, *m);
    }
  }

  const fsdir*
  inject_fsdir (action a, target& t, bool prereq, bool parent)
  {
    tracer trace ("inject_fsdir");

    // If t is a directory (name is empty), say foo/bar/, then t is bar and
    // its parent directory is foo/.
    //
    const dir_path& d (parent && t.name.empty () ? t.dir.directory () : t.dir);

    const scope& bs (t.ctx.scopes.find_out (d));
    const scope* rs (bs.root_scope ());

    // If root scope is NULL, then this can mean that we are out of any
    // project or if the directory is in src_root. In both cases we don't
    // inject anything unless explicitly requested.
    //
    // Note that we also used to bail out if this is the root of the
    // project. But that proved not to be such a great idea in case of
    // subprojects (e.g., tests/).
    //
    const fsdir* r (nullptr);
    if (rs != nullptr && !d.sub (rs->src_path ()))
    {
      l6 ([&]{trace << d << " for " << t;});

      // Target is in the out tree, so out directory is empty.
      //
      r = &search<fsdir> (t, d, dir_path (), string (), nullptr, nullptr);
    }
    else if (prereq)
    {
      // See if one was mentioned explicitly.
      //
      for (const prerequisite& p: group_prerequisites (t))
      {
        if (p.is_a<fsdir> ())
        {
          const target& pt (search (t, p));

          if (pt.dir == d)
          {
            r = &pt.as<fsdir> ();
            break;
          }
        }
      }
    }

    if (r != nullptr)
    {
      // Make it ad hoc so that it doesn't end up in prerequisite_targets
      // after execution.
      //
      match_sync (a, *r);
      t.prerequisite_targets[a].emplace_back (r, include_type::adhoc);
    }

    return r;
  }

  // Execute the specified recipe (if any) and the scope operation callbacks
  // (if any/applicable) then merge and return the resulting target state.
  //
  static target_state
  execute_recipe (action a, target& t, const recipe& r)
  {
    target_state ts (target_state::unchanged);

    try
    {
      auto df = make_diag_frame (
        [a, &t](const diag_record& dr)
        {
          if (verb != 0)
            dr << info << "while " << diag_doing (a, t);
        });

      // If this is a dir{} target, see if we have any operation callbacks
      // in the corresponding scope.
      //
      const dir* op_t (t.is_a<dir> ());
      const scope* op_s (nullptr);

      using op_iterator = scope::operation_callback_map::const_iterator;
      pair<op_iterator, op_iterator> op_p;

      if (op_t != nullptr)
      {
        op_s = &t.ctx.scopes.find_out (t.dir); // Always out.

        if (op_s->out_path () == t.dir && !op_s->operation_callbacks.empty ())
        {
          op_p = op_s->operation_callbacks.equal_range (a);

          if (op_p.first == op_p.second)
            op_s = nullptr; // Ignore.
        }
        else
          op_s = nullptr; // Ignore.
      }

      if (r != nullptr || op_s != nullptr)
      {
        const scope& bs (t.base_scope ());

        // Execute recipe/callbacks in project environment.
        //
        auto_project_env penv;
        if (const scope* rs = bs.root_scope ())
          penv = auto_project_env (*rs);

        // Pre operations.
        //
        // Note that here we assume the dir{} target cannot be part of a group
        // and as a result we (a) don't try to avoid calling post callbacks in
        // case of a group failure and (b) merge the pre and post states with
        // the group state.
        //
        if (op_s != nullptr)
        {
          for (auto i (op_p.first); i != op_p.second; ++i)
            if (const auto& f = i->second.pre)
              ts |= f (a, *op_s, *op_t);
        }

        // Recipe.
        //
        if (r != nullptr)
          ts |= r (a, t);

        // Post operations.
        //
        if (op_s != nullptr)
        {
          for (auto i (op_p.first); i != op_p.second; ++i)
            if (const auto& f = i->second.post)
              ts |= f (a, *op_s, *op_t);
        }
      }

      // See the recipe documentation for details on what's going on here.
      // Note that if the result is group, then the group's state can be
      // failed.
      //
      switch (t[a].state = ts)
      {
      case target_state::changed:
      case target_state::unchanged:
        break;
      case target_state::postponed:
        ts = t[a].state = target_state::unchanged;
        break;
      case target_state::group:
        ts = (*t.group)[a].state;
        break;
      default:
        assert (false);
      }
    }
    catch (const failed&)
    {
      ts = t[a].state = target_state::failed;
    }

    return ts;
  }

  void
  update_backlink (const file& f, const path& l, bool changed, backlink_mode m)
  {
    using mode = backlink_mode;

    const path& p (f.path ());
    dir_path d (l.directory ());

    // At low verbosity levels we print the command if the target changed or
    // the link does not exist (we also treat errors as "not exist" and let
    // the link update code below handle it).
    //
    // Note that in the changed case we print it even if the link is not
    // actually updated to signal to the user that the updated out target is
    // now available in src.
    //
    if (verb <= 2)
    {
      if (changed || !butl::entry_exists (l,
                                          false /* follow_symlinks */,
                                          true  /* ignore_errors */))
      {
        const char* c (nullptr);
        switch (m)
        {
        case mode::link:
        case mode::symbolic:  c = verb >= 2 ? "ln -s" : "ln";         break;
        case mode::hard:      c = "ln";                               break;
        case mode::copy:
        case mode::overwrite: c = l.to_directory () ? "cp -r" : "cp"; break;
        }

        // Note: 'ln foo/ bar/' means a different thing.
        //
        if (verb >= 2)
          text << c << ' ' << p.string () << ' ' << l.string ();
        else
          text << c << ' ' << f << " -> " << d;
      }
    }

    // What if there is no such subdirectory in src (some like to stash their
    // executables in bin/ or some such). The easiest is probably just to
    // create it even though we won't be cleaning it up.
    //
    if (!exists (d))
      mkdir_p (d, 2 /* verbosity */);

    update_backlink (f.ctx, p, l, m);
  }

  void
  update_backlink (context& ctx,
                   const path& p, const path& l, bool changed, backlink_mode m)
  {
    // As above but with a slightly different diagnostics.

    using mode = backlink_mode;

    dir_path d (l.directory ());

    if (verb <= 2)
    {
      if (changed || !butl::entry_exists (l,
                                          false /* follow_symlinks */,
                                          true  /* ignore_errors */))
      {
        const char* c (nullptr);
        switch (m)
        {
        case mode::link:
        case mode::symbolic:  c = verb >= 2 ? "ln -s" : "ln";         break;
        case mode::hard:      c = "ln";                               break;
        case mode::copy:
        case mode::overwrite: c = l.to_directory () ? "cp -r" : "cp"; break;
        }

        if (verb >= 2)
          text << c << ' ' << p.string () << ' ' << l.string ();
        else
          text << c << ' ' << p.string () << " -> " << d;
      }
    }

    if (!exists (d))
      mkdir_p (d, 2 /* verbosity */);

    update_backlink (ctx, p, l, m);
  }

  static inline void
  try_rmbacklink (const path& l,
                  backlink_mode m,
                  bool ie /* ignore_errors */= false)
  {
    // Note that this function should not be called in the dry-run mode.
    //
    // See also clean_backlink() below.

    using mode = backlink_mode;

    if (l.to_directory ())
    {
      switch (m)
      {
      case mode::link:
      case mode::symbolic:
      case mode::hard:      try_rmsymlink (l, true /* directory */, ie); break;
      case mode::copy:      try_rmdir_r   (path_cast<dir_path> (l), ie); break;
      case mode::overwrite:                                              break;
      }
    }
    else
    {
      // try_rmfile() should work for symbolic and hard file links.
      //
      switch (m)
      {
      case mode::link:
      case mode::symbolic:
      case mode::hard:
      case mode::copy:      try_rmfile (l, ie); break;
      case mode::overwrite:                     break;
      }
    }
  }

  void
  update_backlink (context& ctx,
                   const path& p, const path& l, backlink_mode om,
                   uint16_t verbosity)
  {
    using mode = backlink_mode;

    bool d (l.to_directory ());
    mode m (om); // Keep original mode.

    auto print = [&p, &l, &m, verbosity, d] ()
    {
      if (verb >= verbosity)
      {
        const char* c (nullptr);
        switch (m)
        {
        case mode::link:
        case mode::symbolic:  c = "ln -sf";           break;
        case mode::hard:      c = "ln -f";            break;
        case mode::copy:
        case mode::overwrite: c = d ? "cp -r" : "cp"; break;
        }

        text << c << ' ' << p.string () << ' ' << l.string ();
      }
    };

    // Note that none of mk*() or cp*() functions that we use here handle
    // the dry-run mode.
    //
    if (!ctx.dry_run)
    try
    {
      try
      {
        // Normally will be there.
        //
        try_rmbacklink (l, m);

        // Skip (ad hoc) targets that don't exist.
        //
        if (!(d ? dir_exists (p) : file_exists (p)))
          return;

        switch (m)
        {
        case mode::link:
          if (!d)
          {
            mkanylink (p, l, false /* copy */);
            break;
          }
          // Directory hardlinks are not widely supported so for them we will
          // only try the symlink.
          //
          // Fall through.

        case mode::symbolic:  mksymlink  (p, l, d);  break;
        case mode::hard:
          {
            // The target can be a symlink (or a symlink chain) with a
            // relative target that, unless the (final) symlink and the
            // hardlink are in the same directory, will result in a dangling
            // link.
            //
            mkhardlink (followsymlink (p), l, d);
            break;
          }
        case mode::copy:
        case mode::overwrite:
          {
            if (d)
            {
              // Currently, for a directory, we do a "copy-link": we make the
              // target directory and then link each entry. (For now this is
              // only used to "link" a Windows DLL assembly with only files
              // inside. We also have to use hard links; see the relevant
              // comment in cc/link-rule for details. Maybe we can invent a
              // special Windows-only "assembly link" for this).
              //
              dir_path fr (path_cast<dir_path> (p));
              dir_path to (path_cast<dir_path> (l));

              try_mkdir (to);

              for (const auto& de:
                     dir_iterator (fr, false /* ignore_dangling */))
              {
                path f (fr / de.path ());
                path t (to / de.path ());

                update_backlink (ctx, f, t, mode::hard, verb_never);
              }
            }
            else
              cpfile (p, l, (cpflags::overwrite_content |
                             cpflags::copy_timestamps));

            break;
          }
        }
      }
      catch (system_error& e)
      {
        // Translate to mkanylink()-like failure.
        //
        entry_type t (entry_type::unknown);
        switch (m)
        {
        case mode::link:
        case mode::symbolic:  t = entry_type::symlink;  break;
        case mode::hard:      t = entry_type::other;    break;
        case mode::copy:
        case mode::overwrite: t = entry_type::regular;  break;
        }

        throw pair<entry_type, system_error> (t, move (e));
      }
    }
    catch (const pair<entry_type, system_error>& e)
    {
      const char* w (e.first == entry_type::regular ? "copy"     :
                     e.first == entry_type::symlink ? "symlink"  :
                     e.first == entry_type::other   ? "hardlink" : nullptr);
      print ();
      fail << "unable to make " << w << ' ' << l << ": " << e.second;
    }

    print ();
  }

  void
  clean_backlink (context& ctx,
                  const path& l, uint16_t v /*verbosity*/, backlink_mode m)
  {
    // Like try_rmbacklink() but with diagnostics and error handling.
    //
    // Note that here the dry-run mode is handled by the filesystem functions.

    using mode = backlink_mode;

    if (l.to_directory ())
    {
      switch (m)
      {
      case mode::link:
      case mode::symbolic:
      case mode::hard:  rmsymlink (ctx, l, true /* directory */, v);     break;
      case mode::copy:  rmdir_r (ctx, path_cast<dir_path> (l), true, v); break;
      case mode::overwrite:                                              break;
      }
    }
    else
    {
      // remfile() should work for symbolic and hard file links.
      //
      switch (m)
      {
      case mode::link:
      case mode::symbolic:
      case mode::hard:
      case mode::copy:      rmfile (ctx, l, v); break;
      case mode::overwrite:                     break;
      }
    }
  }

  // If target/link path are syntactically to a directory, then the backlink
  // is assumed to be to a directory, otherwise -- to a file.
  //
  struct backlink: auto_rm<path>
  {
    using path_type = build2::path;

    reference_wrapper<const path_type> target;
    backlink_mode mode;

    backlink (const path_type& t, path_type&& l, backlink_mode m, bool active)
        : auto_rm<path_type> (move (l), active), target (t), mode (m)
    {
      assert (t.to_directory () == path.to_directory ());
    }

    ~backlink ()
    {
      if (active)
      {
        try_rmbacklink (path, mode, true /* ignore_errors */);
        active = false;
      }
    }

    backlink (backlink&&) = default;
    backlink& operator= (backlink&&) = default;
  };

  // Normally (i.e., on sane platforms that don't have things like PDBs, etc)
  // there will be just one backlink so optimize for that.
  //
  using backlinks = small_vector<backlink, 1>;

  static optional<backlink_mode>
  backlink_test (const target& t, const lookup& l)
  {
    using mode = backlink_mode;

    optional<mode> r;
    const string& v (cast<string> (l));

    if      (v == "true")      r = mode::link;
    else if (v == "symbolic")  r = mode::symbolic;
    else if (v == "hard")      r = mode::hard;
    else if (v == "copy")      r = mode::copy;
    else if (v == "overwrite") r = mode::overwrite;
    else if (v != "false")
      fail << "invalid backlink variable value '" << v << "' "
           << "specified for target " << t;

    return r;
  }

  static optional<backlink_mode>
  backlink_test (action a, target& t)
  {
    context& ctx (t.ctx);

    // Note: the order of these checks is from the least to most expensive.

    // Only for plain update/clean.
    //
    if (a.outer () || (a != perform_update_id && a != perform_clean_id))
      return nullopt;

    // Only file-based targets in the out tree can be backlinked.
    //
    if (!t.out.empty () || !t.is_a<file> ())
      return nullopt;

    // Neither an out-of-project nor in-src configuration can be forwarded.
    //
    const scope& bs (t.base_scope ());
    const scope* rs (bs.root_scope ());
    if (rs == nullptr || bs.src_path () == bs.out_path ())
      return nullopt;

    // Only for forwarded configurations.
    //
    if (!cast_false<bool> (rs->vars[ctx.var_forwarded]))
      return nullopt;

    lookup l (t.state[a][ctx.var_backlink]);

    // If not found, check for some defaults in the global scope (this does
    // not happen automatically since target type/pattern-specific lookup
    // stops at the project boundary).
    //
    if (!l.defined ())
      l = ctx.global_scope.lookup (*ctx.var_backlink, t.key ());

    return l ? backlink_test (t, l) : nullopt;
  }

  static backlinks
  backlink_collect (action a, target& t, backlink_mode m)
  {
    using mode = backlink_mode;

    context& ctx (t.ctx);
    const scope& s (t.base_scope ());

    backlinks bls;
    auto add = [&bls, &s] (const path& p, mode m)
    {
      bls.emplace_back (p,
                        s.src_path () / p.leaf (s.out_path ()),
                        m,
                        !s.ctx.dry_run /* active */);
    };

    // First the target itself.
    //
    add (t.as<file> ().path (), m);

    // Then ad hoc group file/fsdir members, if any.
    //
    for (const target* mt (t.adhoc_member);
         mt != nullptr;
         mt = mt->adhoc_member)
    {
      const path* p (nullptr);

      if (const file* f = mt->is_a<file> ())
      {
        p = &f->path ();

        if (p->empty ()) // The "trust me, it's somewhere" case.
          p = nullptr;
      }
      else if (const fsdir* d = mt->is_a<fsdir> ())
        p = &d->dir;

      if (p != nullptr)
      {
        // Check for a custom backlink mode for this member. If none, then
        // inherit the one from the group (so if the user asked to copy .exe,
        // we will also copy .pdb).
        //
        // Note that we want to avoid group or tt/patter-spec lookup. And
        // since this is an ad hoc member (which means it was either declared
        // in the buildfile or added by the rule), we assume that the value,
        // if any, will be set as a target or rule-specific variable.
        //
        lookup l (mt->state[a].vars[ctx.var_backlink]);

        if (!l)
          l = mt->vars[ctx.var_backlink];

        optional<mode> bm (l ? backlink_test (*mt, l) : m);

        if (bm)
          add (*p, *bm);
      }
    }

    return bls;
  }

  static inline backlinks
  backlink_update_pre (action a, target& t, backlink_mode m)
  {
    return backlink_collect (a, t, m);
  }

  static void
  backlink_update_post (target& t, target_state ts, backlinks& bls)
  {
    if (ts == target_state::failed)
      return; // Let auto rm clean things up.

    // Make backlinks.
    //
    for (auto b (bls.begin ()), i (b); i != bls.end (); ++i)
    {
      const backlink& bl (*i);

      if (i == b)
        update_backlink (t.as<file> (),
                         bl.path,
                         ts == target_state::changed,
                         bl.mode);
      else
        update_backlink (t.ctx, bl.target, bl.path, bl.mode);
    }

    // Cancel removal.
    //
    if (!t.ctx.dry_run)
    {
      for (backlink& bl: bls)
        bl.cancel ();
    }
  }

  static void
  backlink_clean_pre (action a, target& t, backlink_mode m)
  {
    backlinks bls (backlink_collect (a, t, m));

    for (auto b (bls.begin ()), i (b); i != bls.end (); ++i)
    {
      // Printing anything at level 1 will probably just add more noise.
      //
      backlink& bl (*i);
      bl.cancel ();
      clean_backlink (t.ctx, bl.path, i == b ? 2 : 3 /* verbosity */, bl.mode);
    }
  }

  static target_state
  execute_impl (action a, target& t)
  {
    context& ctx (t.ctx);

    target::opstate& s (t[a]);

    assert (s.task_count.load (memory_order_consume) == t.ctx.count_busy ()
            && s.state == target_state::unknown);

    target_state ts;
    try
    {
      // Handle target backlinking to forwarded configurations.
      //
      // Note that this function will never be called if the recipe is noop
      // which is ok since such targets are probably not interesting for
      // backlinking.
      //
      backlinks bls;
      optional<backlink_mode> blm (backlink_test (a, t));

      if (blm)
      {
        if (a == perform_update_id)
          bls = backlink_update_pre (a, t, *blm);
        else
          backlink_clean_pre (a, t, *blm);
      }

      // Note: see similar code in set_rule_trace() for match.
      //
      if (ctx.trace_execute != nullptr && trace_target (t, *ctx.trace_execute))
      {
        diag_record dr (info);

        dr << diag_doing (a, t);

        if (s.rule != nullptr)
        {
          const rule& r (s.rule->second);

          if (const adhoc_rule* ar = dynamic_cast<const adhoc_rule*> (&r))
          {
            dr << info (ar->loc);

            if (ar->pattern != nullptr)
              dr << "using ad hoc pattern rule ";
            else
              dr << "using ad hoc recipe ";
          }
          else
            dr << info << "using rule ";

          dr << s.rule->first;
        }
        else
          dr << info << "using directly-assigned recipe";
      }

      ts = execute_recipe (a, t, s.recipe);

      if (blm)
      {
        if (a == perform_update_id)
          backlink_update_post (t, ts, bls);
      }
    }
    catch (const failed&)
    {
      // If we could not backlink the target, then the best way to signal the
      // failure seems to be to mark the target as failed.
      //
      ts = s.state = target_state::failed;
    }

    // Clear the recipe to release any associated memory. Note that
    // s.recipe_group_action may be used further (see, for example,
    // group_state()) and should retain its value.
    //
    //
    if (!s.recipe_keep)
      s.recipe = nullptr;

    // Decrement the target count (see set_recipe() for details).
    //
    // Note that here we cannot rely on s.state being group because of the
    // postponment logic (see excute_recipe() for details).
    //
    if (a.inner () && !s.recipe_group_action)
      ctx.target_count.fetch_sub (1, memory_order_relaxed);

    // Decrement the task count (to count_executed) and wake up any threads
    // that might be waiting for this target.
    //
    size_t tc (s.task_count.fetch_sub (
                 target::offset_busy - target::offset_executed,
                 memory_order_release));
    assert (tc == ctx.count_busy ());
    ctx.sched.resume (s.task_count);

    return ts;
  }

  target_state
  execute_impl (action a,
                const target& ct,
                size_t start_count,
                atomic_count* task_count)
  {
    target& t (const_cast<target&> (ct)); // MT-aware.
    target::opstate& s (t[a]);

    context& ctx (t.ctx);

    // Update dependency counts and make sure they are not skew.
    //
    size_t gd (ctx.dependency_count.fetch_sub (1, memory_order_relaxed));
    size_t td (s.dependents.fetch_sub (1, memory_order_release));
    assert (td != 0 && gd != 0);

    // Handle the "last" execution mode.
    //
    // This gets interesting when we consider interaction with groups. It seem
    // to make sense to treat group members as dependents of the group, so,
    // for example, if we try to clean the group via three of its members,
    // only the last attempt will actually execute the clean. This means that
    // when we match a group member, inside we should also match the group in
    // order to increment the dependents count. This seems to be a natural
    // requirement: if we are delegating to the group, we need to find a
    // recipe for it, just like we would for a prerequisite.
    //
    // Note that we are also going to treat the group state as postponed.
    // This is not a mistake: until we execute the recipe, we want to keep
    // returning postponed. And once the recipe is executed, it will reset the
    // state to group (see group_action()). To put it another way, the
    // execution of this member is postponed, not of the group.
    //
    // Note also that the target execution is postponed with regards to this
    // thread. For other threads the state will still be unknown (until they
    // try to execute it).
    //
    if (ctx.current_mode == execution_mode::last && --td != 0)
      return target_state::postponed;

    // Try to atomically change applied to busy.
    //
    size_t tc (ctx.count_applied ());

    size_t exec (ctx.count_executed ());
    size_t busy (ctx.count_busy ());

    if (s.task_count.compare_exchange_strong (
          tc,
          busy,
          memory_order_acq_rel,  // Synchronize on success.
          memory_order_acquire)) // Synchronize on failure.
    {
      // Handle the noop recipe.
      //
      if (s.state == target_state::unchanged)
      {
        // There could still be scope operations.
        //
        if (t.is_a<dir> ())
          execute_recipe (a, t, nullptr /* recipe */);

        s.task_count.store (exec, memory_order_release);
        ctx.sched.resume (s.task_count);
      }
      else
      {
        if (task_count == nullptr)
          return execute_impl (a, t);

        // Pass our diagnostics stack (this is safe since we expect the
        // caller to wait for completion before unwinding its diag stack).
        //
        if (ctx.sched.async (start_count,
                             *task_count,
                             [a] (const diag_frame* ds, target& t)
                             {
                               diag_frame::stack_guard dsg (ds);
                               execute_impl (a, t);
                             },
                             diag_frame::stack (),
                             ref (t)))
          return target_state::unknown; // Queued.

        // Executed synchronously, fall through.
      }
    }
    else
    {
      // Either busy or already executed.
      //
      if (tc >= busy) return target_state::busy;
      else            assert (tc == exec);
    }

    return t.executed_state (a, false);
  }

  target_state
  execute_direct_impl (action a,
                       const target& ct,
                       size_t start_count,
                       atomic_count* task_count)
  {
    context& ctx (ct.ctx);

    target& t (const_cast<target&> (ct)); // MT-aware.
    target::opstate& s (t[a]);

    // Similar logic to execute_impl() above.
    //
    size_t tc (ctx.count_applied ());

    size_t exec (ctx.count_executed ());
    size_t busy (ctx.count_busy ());

    if (s.task_count.compare_exchange_strong (
          tc,
          busy,
          memory_order_acq_rel,  // Synchronize on success.
          memory_order_acquire)) // Synchronize on failure.
    {
      if (s.state == target_state::unknown)
      {
        if (task_count == nullptr)
          return execute_impl (a, t);

        if (ctx.sched.async (start_count,
                             *task_count,
                             [a] (const diag_frame* ds, target& t)
                             {
                               diag_frame::stack_guard dsg (ds);
                               execute_impl (a, t);
                             },
                             diag_frame::stack (),
                             ref (t)))
          return target_state::unknown; // Queued.

        // Executed synchronously, fall through.
      }
      else
      {
        assert (s.state == target_state::unchanged ||
                s.state == target_state::failed);

        if (s.state == target_state::unchanged)
        {
          if (t.is_a<dir> ())
            execute_recipe (a, t, nullptr /* recipe */);
        }

        s.task_count.store (exec, memory_order_release);
        ctx.sched.resume (s.task_count);
      }
    }
    else
    {
      // Either busy or already executed.
      //
      if (tc >= busy) return target_state::busy;
      else            assert (tc == exec);
    }

    return t.executed_state (a, false);
  }

  bool
  update_during_match (tracer& trace, action a, const target& t, timestamp ts)
  {
    assert (a == perform_update_id);

    // Note: this function is used to make sure header dependencies are up to
    // date (and which is where it originated).
    //
    // There would normally be a lot of headers for every source file (think
    // all the system headers) and just calling execute_direct_sync() on all
    // of them can get expensive. At the same time, most of these headers are
    // existing files that we will never be updating (again, system headers,
    // for example) and the rule that will match them is the fallback
    // file_rule. That rule has an optimization: it returns noop_recipe (which
    // causes the target state to be automatically set to unchanged) if the
    // file is known to be up to date. So we do the update "smartly".
    //
    // Also, now that we do header pre-generation by default, there is a good
    // chance the header has already been updated. So we also detect that and
    // avoid switching the phase.
    //
    const path_target* pt (t.is_a<path_target> ());

    if (pt == nullptr)
      ts = timestamp_unknown;

    target_state os (t.matched_state (a));

    if (os == target_state::unchanged)
    {
      if (ts == timestamp_unknown)
        return false;
      else
      {
        // We expect the timestamp to be known (i.e., existing file).
        //
        timestamp mt (pt->mtime ());
        assert (mt != timestamp_unknown);
        return mt > ts;
      }
    }
    else
    {
      // We only want to return true if our call to execute() actually caused
      // an update. In particular, the target could already have been in
      // target_state::changed because of the dynamic dependency extraction
      // run for some other target.
      //
      target_state ns;
      if (os != target_state::changed)
      {
        phase_switch ps (t.ctx, run_phase::execute);
        ns = execute_direct_sync (a, t);
      }
      else
        ns = os;

      if (ns != os && ns != target_state::unchanged)
      {
        l6 ([&]{trace << "updated " << t
                      << "; old state " << os
                      << "; new state " << ns;});
        return true;
      }
      else
        return ts != timestamp_unknown ? pt->newer (ts, ns) : false;
    }
  }

  bool
  update_during_match_prerequisites (tracer& trace,
                                     action a, target& t,
                                     uintptr_t mask)
  {
    prerequisite_targets& pts (t.prerequisite_targets[a]);

    // On the first pass detect and handle unchanged tragets. Note that we
    // have to do it in a separate pass since we cannot call matched_state()
    // once we've switched the phase.
    //
    size_t n (0);

    for (prerequisite_target& p: pts)
    {
      if ((p.include & mask) != 0)
      {
        if (p.target != nullptr)
        {
          const target& pt (*p.target);

          target_state os (pt.matched_state (a));

          if (os != target_state::unchanged)
          {
            ++n;
            p.data = static_cast<uintptr_t> (os);
            continue;
          }
        }

        p.data = 0;
      }
    }

    // If all unchanged, we are done.
    //
    if (n == 0)
      return false;

    // Provide additional information on what's going on.
    //
    auto df = make_diag_frame (
      [&t](const diag_record& dr)
      {
        if (verb != 0)
          dr << info << "while updating during match prerequisites of "
             << "target " << t;
      });

    context& ctx (t.ctx);

    phase_switch ps (ctx, run_phase::execute);

    bool r (false);

    // @@ Maybe we should optimize for n == 1? Maybe we should just call
    //    smarter update_during_match() in this case?
    //
#if 0
    for (prerequisite_target& p: pts)
    {
      if ((p.include & mask) != 0 && p.data != 0)
      {
        const target& pt (*p.target);

        target_state os (static_cast<target_state> (p.data));
        target_state ns (execute_direct_sync (a, pt));

        if (ns != os && ns != target_state::unchanged)
        {
          l6 ([&]{trace << "updated " << pt
                        << "; old state " << os
                        << "; new state " << ns;});
          r = true;
        }

        p.data = 0;
      }
    }
#else

    // Start asynchronous execution of prerequisites. Similar logic to
    // straight_execute_members().
    //
    // Note that the target's task count is expected to be busy (since this
    // function is called during match). And there don't seem to be any
    // problems in using it for execute.
    //
    atomic_count& tc (t[a].task_count);

    size_t busy (ctx.count_busy ());

    wait_guard wg (ctx, busy, tc);

    for (prerequisite_target& p: pts)
    {
      if ((p.include & mask) != 0 && p.data != 0)
      {
        execute_direct_async (a, *p.target, busy, tc);
      }
    }

    wg.wait ();

    // Finish execution and process the result.
    //
    for (prerequisite_target& p: pts)
    {
      if ((p.include & mask) != 0 && p.data != 0)
      {
        const target& pt (*p.target);
        target_state ns (execute_complete (a, pt));
        target_state os (static_cast<target_state> (p.data));

        if (ns != os && ns != target_state::unchanged)
        {
          l6 ([&]{trace << "updated " << pt
                        << "; old state " << os
                        << "; new state " << ns;});
          r = true;
        }

        p.data = 0;
      }
    }
#endif

    return r;
  }

  static inline void
  blank_adhoc_member (const target*&)
  {
  }

  static inline void
  blank_adhoc_member (prerequisite_target& pt)
  {
    if (pt.adhoc ())
      pt.target = nullptr;
  }

  template <typename T>
  target_state
  straight_execute_members (context& ctx, action a, atomic_count& tc,
                            T ts[], size_t n, size_t p)
  {
    target_state r (target_state::unchanged);

    size_t busy (ctx.count_busy ());

    // Start asynchronous execution of prerequisites.
    //
    wait_guard wg (ctx, busy, tc);

    n += p;
    for (size_t i (p); i != n; ++i)
    {
      const target*& mt (ts[i]);

      if (mt == nullptr) // Skipped.
        continue;

      target_state s (execute_async (a, *mt, busy, tc));

      if (s == target_state::postponed)
      {
        r |= s;
        mt = nullptr;
      }
    }

    wg.wait ();

    // Now all the targets in prerequisite_targets must be either still busy
    // or executed and synchronized (and we have blanked out all the postponed
    // ones).
    //
    for (size_t i (p); i != n; ++i)
    {
      if (ts[i] == nullptr)
        continue;

      const target& mt (*ts[i]);
      r |= execute_complete (a, mt);

      blank_adhoc_member (ts[i]);
    }

    return r;
  }

  template <typename T>
  target_state
  reverse_execute_members (context& ctx, action a, atomic_count& tc,
                           T ts[], size_t n, size_t p)
  {
    // Pretty much as straight_execute_members() but in reverse order.
    //
    target_state r (target_state::unchanged);

    size_t busy (ctx.count_busy ());

    wait_guard wg (ctx, busy, tc);

    n = p - n;
    for (size_t i (p); i != n; )
    {
      const target*& mt (ts[--i]);

      if (mt == nullptr)
        continue;

      target_state s (execute_async (a, *mt, busy, tc));

      if (s == target_state::postponed)
      {
        r |= s;
        mt = nullptr;
      }
    }

    wg.wait ();

    for (size_t i (p); i != n; )
    {
      if (ts[--i] == nullptr)
        continue;

      const target& mt (*ts[i]);
      r |= execute_complete (a, mt);

      blank_adhoc_member (ts[i]);
    }

    return r;
  }

  // Instantiate only for what we need.
  //
  template LIBBUILD2_SYMEXPORT target_state
  straight_execute_members<const target*> (
    context&, action, atomic_count&, const target*[], size_t, size_t);

  template LIBBUILD2_SYMEXPORT target_state
  reverse_execute_members<const target*> (
    context&, action, atomic_count&, const target*[], size_t, size_t);

  template LIBBUILD2_SYMEXPORT target_state
  straight_execute_members<prerequisite_target> (
    context&, action, atomic_count&, prerequisite_target[], size_t, size_t);

  template LIBBUILD2_SYMEXPORT target_state
  reverse_execute_members<prerequisite_target> (
    context&, action, atomic_count&, prerequisite_target[], size_t, size_t);

  pair<optional<target_state>, const target*>
  execute_prerequisites (const target_type* tt,
                         action a, const target& t,
                         const timestamp& mt, const execute_filter& ef,
                         size_t n)
  {
    assert (a == perform_update_id);

    context& ctx (t.ctx);

    size_t busy (ctx.count_busy ());

    auto& pts (t.prerequisite_targets[a]);

    if (n == 0)
      n = pts.size ();

    // Pretty much as straight_execute_members() but hairier.
    //
    target_state rs (target_state::unchanged);

    wait_guard wg (ctx, busy, t[a].task_count);

    for (size_t i (0); i != n; ++i)
    {
      const target*& pt (pts[i]);

      if (pt == nullptr) // Skipped.
        continue;

      target_state s (execute_async (a, *pt, busy, t[a].task_count));

      if (s == target_state::postponed)
      {
        rs |= s;
        pt = nullptr;
      }
    }

    wg.wait ();

    bool e (mt == timestamp_nonexistent);
    const target* rt (nullptr);

    for (size_t i (0); i != n; ++i)
    {
      prerequisite_target& p (pts[i]);

      if (p == nullptr)
        continue;

      const target& pt (*p.target);
      target_state s (execute_complete (a, pt));
      rs |= s;

      // Should we compare the timestamp to this target's?
      //
      if (!e && (p.adhoc () || !ef || ef (pt, i)))
      {
        // If this is an mtime-based target, then compare timestamps.
        //
        if (const mtime_target* mpt = pt.is_a<mtime_target> ())
        {
          if (mpt->newer (mt, s))
            e = true;
        }
        else
        {
          // Otherwise we assume the prerequisite is newer if it was changed.
          //
          if (s == target_state::changed)
            e = true;
        }
      }

      if (p.adhoc ())
        p.target = nullptr; // Blank out.
      else if (tt != nullptr)
      {
        if (rt == nullptr && pt.is_a (*tt))
          rt = &pt;
      }
    }

    assert (tt == nullptr || rt != nullptr);

    return pair<optional<target_state>, const target*> (
      e ? optional<target_state> () : rs, rt);
  }

  pair<optional<target_state>, const target*>
  reverse_execute_prerequisites (const target_type* tt,
                                 action a, const target& t,
                                 const timestamp& mt, const execute_filter& ef,
                                 size_t n)
  {
    assert (a == perform_update_id);

    context& ctx (t.ctx);

    size_t busy (ctx.count_busy ());

    auto& pts (t.prerequisite_targets[a]);

    if (n == 0)
      n = pts.size ();

    // Pretty much as reverse_execute_members() but hairier.
    //
    target_state rs (target_state::unchanged);

    wait_guard wg (ctx, busy, t[a].task_count);

    for (size_t i (n); i != 0; )
    {
      const target*& pt (pts[--i]);

      if (pt == nullptr) // Skipped.
        continue;

      target_state s (execute_async (a, *pt, busy, t[a].task_count));

      if (s == target_state::postponed)
      {
        rs |= s;
        pt = nullptr;
      }
    }

    wg.wait ();

    bool e (mt == timestamp_nonexistent);
    const target* rt (nullptr);

    for (size_t i (n); i != 0; )
    {
      prerequisite_target& p (pts[--i]);

      if (p == nullptr)
        continue;

      const target& pt (*p.target);
      target_state s (execute_complete (a, pt));
      rs |= s;

      // Should we compare the timestamp to this target's?
      //
      if (!e && (p.adhoc () || !ef || ef (pt, i)))
      {
        // If this is an mtime-based target, then compare timestamps.
        //
        if (const mtime_target* mpt = pt.is_a<mtime_target> ())
        {
          if (mpt->newer (mt, s))
            e = true;
        }
        else
        {
          // Otherwise we assume the prerequisite is newer if it was changed.
          //
          if (s == target_state::changed)
            e = true;
        }
      }

      if (p.adhoc ())
        p.target = nullptr; // Blank out.
      else if (tt != nullptr)
      {
        // Note that here we need last.
        //
        if (pt.is_a (*tt))
          rt = &pt;
      }
    }

    assert (tt == nullptr || rt != nullptr);

    return pair<optional<target_state>, const target*> (
      e ? optional<target_state> () : rs, rt);
  }

  target_state
  noop_action (action a, const target& t)
  {
    text << "noop action triggered for " << diag_doing (a, t);
    assert (false); // We shouldn't be called (see set_recipe()).
    return target_state::unchanged;
  }

  target_state
  group_action (action a, const target& t)
  {
    context& ctx (t.ctx);

    // If the group is busy, we wait, similar to prerequisites.
    //
    const target& g (*t.group);

    // This is execute_sync(a, t, false) but that saves a call to
    // executed_state() (which we don't need).
    //
    target_state gs (execute_impl (a, g, 0, nullptr));

    if (gs == target_state::busy)
      ctx.sched.wait (ctx.count_executed (),
                      g[a].task_count,
                      scheduler::work_none);

    // Return target_state::group to signal to execute() that this target's
    // state comes from the group (which, BTW, can be failed).
    //
    // There is just one small problem: if the returned group state is
    // postponed, then this means the group hasn't been executed yet. And if
    // we return target_state::group, then this means any state queries (see
    // executed_state()) will be directed to the target which might still not
    // be executed or, worse, is being executed as we query.
    //
    // So in this case we return target_state::postponed (which will result in
    // the member being treated as unchanged). This is how it is done for
    // prerequisites and seeing that we've been acting as if the group is our
    // prerequisite, there is no reason to deviate (see the recipe return
    // value documentation for details).
    //
    return gs != target_state::postponed ? target_state::group : gs;
  }

  target_state
  default_action (action a, const target& t)
  {
    return execute_prerequisites (a, t);
  }

  static target_state
  clean_extra (context& ctx,
               const path& fp,
               const clean_extras& es,
               path& ep, bool& ed)
  {
    assert (!fp.empty ()); // Must be assigned.

    target_state er (target_state::unchanged);

    for (const char* e: es)
    {
      size_t n;
      if (e == nullptr || (n = strlen (e)) == 0)
        continue;

      path p;
      bool d;

      if (path::traits_type::absolute (e))
      {
        p = path (e);
        d = p.to_directory ();
      }
      else
      {
        if ((d = (e[n - 1] == '/')))
          --n;

        p = fp;
        for (; *e == '-'; ++e)
          p = p.base ();

        p.append (e, n);
      }

      target_state r (target_state::unchanged);

      if (d)
      {
        dir_path dp (path_cast<dir_path> (p));

        switch (rmdir_r (ctx, dp, true, 3))
        {
        case rmdir_status::success:
          {
            r = target_state::changed;
            break;
          }
        case rmdir_status::not_empty:
          {
            if (verb >= 3)
              text << dp << " is current working directory, not removing";
            break;
          }
        case rmdir_status::not_exist:
          break;
        }
      }
      else
      {
        if (rmfile (ctx, p, 3))
          r = target_state::changed;
      }

      if (r == target_state::changed && ep.empty ())
      {
        ed = d;
        ep = move (p);
      }

      er |= r;
    }

    return er;
  }

  target_state
  perform_clean_extra (action a, const file& ft,
                       const clean_extras& extras,
                       const clean_adhoc_extras& adhoc_extras)
  {
    context& ctx (ft.ctx);

    // Clean the extras first and don't print the commands at verbosity level
    // below 3. Note the first extra file/directory that actually got removed
    // for diagnostics below.
    //
    // Note that dry-run is taken care of by the filesystem functions.
    //
    target_state er (target_state::unchanged);
    bool ed (false);
    path ep;

    const path& fp (ft.path ());

    if (!fp.empty () && !extras.empty ())
      er |= clean_extra (ctx, fp, extras, ep, ed);

    target_state tr (target_state::unchanged);

    // Check if we were asked not to actually remove the files. The extras are
    // tricky: some of them, like depdb should definitely be removed. But
    // there could also be those that shouldn't. Currently we only use this
    // for auto-generated source code where the only extra file, if any, is
    // depdb so for now we treat them as "to remove" but in the future we may
    // need to have two lists.
    //
    bool clean (cast_true<bool> (ft[ctx.var_clean]));

    // Now clean the ad hoc group file members, if any.
    //
    for (const target* m (ft.adhoc_member);
         m != nullptr;
         m = m->adhoc_member)
    {
      const file* mf (m->is_a<file> ());
      const path* mp (mf != nullptr ? &mf->path () : nullptr);

      if (mf == nullptr || mp->empty ())
        continue;

      if (!adhoc_extras.empty ())
      {
        auto i (find_if (adhoc_extras.begin (),
                         adhoc_extras.end (),
                         [mf] (const clean_adhoc_extra& e)
                         {
                           return mf->is_a (e.type);
                         }));

        if (i != adhoc_extras.end ())
          er |= clean_extra (ctx, *mp, i->extras, ep, ed);
      }

      if (!clean)
        continue;

      // Make this "primary target" for diagnostics/result purposes if the
      // primary target is unreal.
      //
      if (fp.empty ())
      {
        if (rmfile (*mp, *mf))
          tr = target_state::changed;
      }
      else
      {
        target_state r (rmfile (ctx, *mp, 3)
                        ? target_state::changed
                        : target_state::unchanged);

        if (r == target_state::changed && ep.empty ())
          ep = *mp;

        er |= r;
      }
    }

    // Now clean the primary target and its prerequisited in the reverse order
    // of update: first remove the file, then clean the prerequisites.
    //
    if (clean && !fp.empty () && rmfile (fp, ft))
      tr = target_state::changed;

    // Update timestamp in case there are operations after us that could use
    // the information.
    //
    ft.mtime (timestamp_nonexistent);

    // We factor the result of removing the extra files into the target state.
    // While strictly speaking removing them doesn't change the target state,
    // if we don't do this, then we may end up removing the file but still
    // saying that everything is clean (e.g., if someone removes the target
    // file but leaves the extra laying around). That would be confusing.
    //
    // What would also be confusing is if we didn't print any commands in
    // this case.
    //
    if (tr != target_state::changed && er == target_state::changed)
    {
      if (verb > (ctx.current_diag_noise ? 0 : 1) && verb < 3)
      {
        if (ed)
          text << "rm -r " << path_cast<dir_path> (ep);
        else
          text << "rm " << ep;
      }
    }

    // Clean prerequisites.
    //
    tr |= reverse_execute_prerequisites (a, ft);

    tr |= er;
    return tr;
  }

  target_state
  perform_clean_group_extra (action a, const mtime_target& g,
                             const clean_extras& extras)
  {
    context& ctx (g.ctx);

    target_state er (target_state::unchanged);
    bool ed (false);
    path ep;

    if (!extras.empty ())
      er |= clean_extra (ctx, g.dir / path (g.name), extras, ep, ed);

    target_state tr (target_state::unchanged);

    if (cast_true<bool> (g[g.ctx.var_clean]))
    {
      for (group_view gv (g.group_members (a)); gv.count != 0; --gv.count)
      {
        if (const target* m = gv.members[gv.count - 1])
        {
          if (rmfile (m->as<file> ().path (), *m))
            tr |= target_state::changed;
        }
      }
    }

    g.mtime (timestamp_nonexistent);

    if (tr != target_state::changed && er == target_state::changed)
    {
      if (verb > (ctx.current_diag_noise ? 0 : 1) && verb < 3)
      {
        if (ed)
          text << "rm -r " << path_cast<dir_path> (ep);
        else
          text << "rm " << ep;
      }
    }

    tr |= reverse_execute_prerequisites (a, g);

    tr |= er;
    return tr;
  }

  target_state
  perform_clean (action a, const target& t)
  {
    const file& f (t.as<file> ());
    assert (!f.path ().empty ());
    return perform_clean_extra (a, f, {});
  }

  target_state
  perform_clean_depdb (action a, const target& t)
  {
    const file& f (t.as<file> ());
    assert (!f.path ().empty ());
    return perform_clean_extra (a, f, {".d"});
  }

  target_state
  perform_clean_group (action a, const target& t)
  {
    return perform_clean_group_extra (a, t.as<mtime_target> (), {});
  }

  target_state
  perform_clean_group_depdb (action a, const target& t)
  {
    path d;
    clean_extras extras;
    {
      group_view gv (t.group_members (a));
      if (gv.count != 0)
      {
        for (size_t i (0); i != gv.count; ++i)
        {
          if (const target* m = gv.members[i])
          {
            d = m->as<file> ().path () + ".d";
            break;
          }
        }

        assert (!d.empty ());
        extras.push_back (d.string ().c_str ());
      }
    }

    return perform_clean_group_extra (a, t.as<mtime_target> (), extras);
  }
}