From 0798dc8be4509c057e2ba62e611f53e6f873ea67 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Thu, 20 Oct 2022 07:06:18 +0200
Subject: Implement parallel execution of post hoc prerequisites

---
 libbuild2/algorithm.ixx |  4 ++--
 libbuild2/operation.cxx | 53 ++++++++++++++++++++++++++++++++++++++++++-------
 2 files changed, 48 insertions(+), 9 deletions(-)

diff --git a/libbuild2/algorithm.ixx b/libbuild2/algorithm.ixx
index 3398595..3c6c2fa 100644
--- a/libbuild2/algorithm.ixx
+++ b/libbuild2/algorithm.ixx
@@ -431,7 +431,7 @@ namespace build2
 
     target_state r (match_impl (a, t, 0, nullptr).second);
 
-    if (fail && r == target_state::failed)
+    if (r == target_state::failed && fail)
       throw failed ();
 
     return r;
@@ -509,7 +509,7 @@ namespace build2
     assert (ctx.phase == run_phase::match);
     target_state r (match_impl (a, t, sc, &tc).second);
 
-    if (fail && !ctx.keep_going && r == target_state::failed)
+    if (r == target_state::failed && fail && !ctx.keep_going)
       throw failed ();
 
     return r;
diff --git a/libbuild2/operation.cxx b/libbuild2/operation.cxx
index 908ebd6..3c7aee4 100644
--- a/libbuild2/operation.cxx
+++ b/libbuild2/operation.cxx
@@ -371,7 +371,7 @@ namespace build2
           // the face of cycles does not work well (we will deadlock for the
           // reverse execution mode).
           //
-          // @@ TODO: match in parallel.
+          // @@ PERF: match in parallel (need match_direct_async(), etc).
           //
           for (const target* pt: p.prerequisite_targets)
           {
@@ -496,8 +496,7 @@ namespace build2
                  << " post hoc prerequisites of " << t;
           });
 
-        // @@ TODO: execute in parallel.
-        //
+#if 0
         for (const target* pt: p.prerequisite_targets)
         {
           target_state s (execute_direct_sync (a, *pt, false /* fail */));
@@ -510,7 +509,45 @@ namespace build2
               break;
           }
         }
+#else
+        // Note: similar logic/reasoning to below except we use direct
+        // execution.
+        //
+        atomic_count tc (0);
+        wait_guard wg (ctx, tc);
+
+        for (const target* pt: p.prerequisite_targets)
+        {
+          target_state s (execute_direct_async (a, *pt, 0, tc, false /*fail*/));
+
+          if (s == target_state::failed)
+          {
+            posthoc_fail = true;
+
+            if (!ctx.keep_going)
+              break;
+          }
+        }
+
+        wg.wait ();
 
+        // Process the result.
+        //
+        for (const target* pt: p.prerequisite_targets)
+        {
+          // Similar to below, no need to wait.
+          //
+          target_state s (pt->executed_state (a, false /* fail */));
+
+          if (s == target_state::failed)
+          {
+            // Note: no need to keep going.
+            //
+            posthoc_fail = true;
+            break;
+          }
+        }
+#endif
         if (posthoc_fail && !ctx.keep_going)
           break;
       }
@@ -668,13 +705,15 @@ namespace build2
       // targets so it seems the best we can do is just fail them all.
       //
       if (!posthoc_fail)
-        at.state = t.executed_state (a, false);
+      {
+        // Note that here we call executed_state() directly instead of
+        // execute_complete() since we know there is no need to wait.
+        //
+        at.state = t.executed_state (a, false /* fail */);
+      }
       else
         at.state = /*t.state[a].state =*/ target_state::failed;
 
-      // Note that here we call executed_state() directly instead of
-      // execute_complete() since we know there is no need to wait.
-      //
       switch (at.state)
       {
       case target_state::unknown:
-- 
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