More work on child process diagnostics buffering

26 files changed, 1312 insertions, 591 deletions

diff --git a/libbuild2/bin/def-rule.cxx b/libbuild2/bin/def-rule.cxx
index 421074c..257fad8 100644
--- a/libbuild2/bin/def-rule.cxx
+++ b/libbuild2/bin/def-rule.cxx
@@ -33,8 +33,10 @@ namespace build2
     };
 
     static void
-    read_dumpbin (istream& is, symbols& syms)
+    read_dumpbin (diag_buffer& dbuf, ifdstream& is, symbols& syms)
     {
+      // Note: io_error is handled by the caller.
+
       // Lines that describe symbols look like:
       //
       // 0   1        2      3          4            5 6
@@ -72,29 +74,27 @@ namespace build2
       //
       // Note that an UNDEF data symbol with non-zero OFFSET is a "common
       // symbol", equivalent to the nm `C` type.
-
-      // Map of read-only (.rdata, .xdata) and uninitialized (.bss) sections
-      // to their types (R and B, respectively). If a section is not found in
-      // this map, then it's assumed to be normal data (.data).
       //
-      map<string, char> sections;
-
-      string l;
-      while (!eof (getline (is, l)))
+      // We keep a map of read-only (.rdata, .xdata) and uninitialized (.bss)
+      // sections to their types (R and B, respectively). If a section is not
+      // found in this map, then it's assumed to be normal data (.data).
+      //
+      auto parse_line = [&syms,
+                         secs = map<string, char> ()] (const string& l) mutable
       {
         size_t b (0), e (0), n;
 
         // IDX (note that it can be more than 3 characters).
         //
         if (next_word (l, b, e) == 0)
-          continue;
+          return;
 
         // OFFSET (always 8 characters).
         //
         n = next_word (l, b, e);
 
         if (n != 8)
-          continue;
+          return;
 
         string off (l, b, n);
 
@@ -103,7 +103,7 @@ namespace build2
         n = next_word (l, b, e);
 
         if (n == 0)
-          continue;
+          return;
 
         string sec (l, b, n);
 
@@ -112,7 +112,7 @@ namespace build2
         n = next_word (l, b, e);
 
         if (l.compare (b, n, "notype") != 0)
-          continue;
+          return;
 
         bool dat;
         if (l[e] == ' ' && l[e + 1] == '(' && l[e + 2] == ')')
@@ -128,7 +128,7 @@ namespace build2
         n = next_word (l, b, e);
 
         if (n == 0)
-          continue;
+          return;
 
         string vis (l, b, n);
 
@@ -137,14 +137,14 @@ namespace build2
         n = next_word (l, b, e);
 
         if (n != 1 || l[b] != '|')
-          continue;
+          return;
 
         // SYMNAME
         //
         n = next_word (l, b, e);
 
         if (n == 0)
-          continue;
+          return;
 
         string s (l, b, n);
 
@@ -162,23 +162,23 @@ namespace build2
           };
 
           if      (cmp (".rdata", 6) ||
-                   cmp (".xdata", 6))    sections.emplace (move (sec), 'R');
-          else if (cmp (".bss",   4))    sections.emplace (move (sec), 'B');
+                   cmp (".xdata", 6))    secs.emplace (move (sec), 'R');
+          else if (cmp (".bss",   4))    secs.emplace (move (sec), 'B');
 
-          continue;
+          return;
         }
 
         // We can only export extern symbols.
         //
         if (vis != "External")
-          continue;
+          return;
 
         if (dat)
         {
           if (sec != "UNDEF")
           {
-            auto i (sections.find (sec));
-            switch (i == sections.end () ? 'D' : i->second)
+            auto i (secs.find (sec));
+            switch (i == secs.end () ? 'D' : i->second)
             {
             case 'D': syms.d.insert (move (s)); break;
             case 'R': syms.r.insert (move (s)); break;
@@ -196,20 +196,54 @@ namespace build2
           if (sec != "UNDEF")
             syms.t.insert (move (s));
         }
+      };
+
+      // Read until we reach EOF on all streams.
+      //
+      // Note that if dbuf is not opened, then we automatically get inactive
+      // nullfd entry.
+      //
+      fdselect_set fds {is.fd (), dbuf.is.fd ()};
+      fdselect_state& ist (fds[0]);
+      fdselect_state& dst (fds[1]);
+
+      for (string l; ist.fd != nullfd || dst.fd != nullfd; )
+      {
+        if (ist.fd != nullfd && getline_non_blocking (is, l))
+        {
+          if (eof (is))
+            ist.fd = nullfd;
+          else
+          {
+            parse_line (l);
+            l.clear ();
+          }
+
+          continue;
+        }
+
+        ifdselect (fds);
+
+        if (dst.ready)
+        {
+          if (!dbuf.read ())
+            dst.fd = nullfd;
+        }
       }
     }
 
     static void
-    read_posix_nm (istream& is, symbols& syms)
+    read_posix_nm (diag_buffer& dbuf, ifdstream& is, symbols& syms)
     {
+      // Note: io_error is handled by the caller.
+
       // Lines that describe symbols look like:
       //
       // <NAME> <TYPE> <VALUE> <SIZE>
       //
       // The types that we are interested in are T, D, R, and B.
       //
-      string l;
-      while (!eof (getline (is, l)))
+      auto parse_line = [&syms] (const string& l)
       {
         size_t b (0), e (0), n;
 
@@ -218,7 +252,7 @@ namespace build2
         n = next_word (l, b, e);
 
         if (n == 0)
-          continue;
+          return;
 
         string s (l, b, n);
 
@@ -227,7 +261,7 @@ namespace build2
         n = next_word (l, b, e);
 
         if (n != 1)
-          continue;
+          return;
 
         switch (l[b])
         {
@@ -238,6 +272,39 @@ namespace build2
         case 'C': syms.c.insert (move (s)); break;
         case 'T': syms.t.insert (move (s)); break;
         }
+      };
+
+      // Read until we reach EOF on all streams.
+      //
+      // Note that if dbuf is not opened, then we automatically get inactive
+      // nullfd entry.
+      //
+      fdselect_set fds {is.fd (), dbuf.is.fd ()};
+      fdselect_state& ist (fds[0]);
+      fdselect_state& dst (fds[1]);
+
+      for (string l; ist.fd != nullfd || dst.fd != nullfd; )
+      {
+        if (ist.fd != nullfd && getline_non_blocking (is, l))
+        {
+          if (eof (is))
+            ist.fd = nullfd;
+          else
+          {
+            parse_line (l);
+            l.clear ();
+          }
+
+          continue;
+        }
+
+        ifdselect (fds);
+
+        if (dst.ready)
+        {
+          if (!dbuf.read ())
+            dst.fd = nullfd;
+        }
       }
     }
 
@@ -656,6 +723,8 @@ namespace build2
       if (verb == 1)
         text << "def " << t;
 
+      diag_buffer dbuf (ctx);
+
       // Extract symbols from each object file.
       //
       symbols syms;
@@ -674,22 +743,37 @@ namespace build2
 
         // Both dumpbin.exe and nm send their output to stdout. While nm sends
         // diagnostics to stderr, dumpbin sends it to stdout together with the
-        // output.
+        // output. To keep things uniform we will buffer stderr in both cases.
         //
-        process pr (run_start (nm,
-                               args,
-                               0     /* stdin */,
-                               -1    /* stdout */));
+        process pr (
+          run_start (nm,
+                     args,
+                     0                                /* stdin */,
+                     -1                               /* stdout */,
+                     dbuf.open (args[0],
+                                false /* force */,
+                                fdstream_mode::non_blocking |
+                                fdstream_mode::skip)  /* stderr */));
+
         bool io (false);
         try
         {
-          ifdstream is (
-            move (pr.in_ofd), fdstream_mode::skip, ifdstream::badbit);
+          // Note that while we read both streams until eof in the normal
+          // circumstances, we cannot use fdstream_mode::skip for the
+          // exception case on both of them: we may end up being blocked
+          // trying to read one stream while the process may be blocked
+          // writing to the other. So in case of an exception we only skip the
+          // diagnostics and close stdout hard. The latter should happen first
+          // so the order of the dbuf/is variables is important.
+          //
+          ifdstream is (move (pr.in_ofd),
+                        fdstream_mode::non_blocking,
+                        ifdstream::badbit);
 
           if (lid == "msvc" || nid == "msvc")
-            read_dumpbin (is, syms);
+            read_dumpbin (dbuf, is, syms);
           else
-            read_posix_nm (is, syms);
+            read_posix_nm (dbuf, is, syms);
 
           is.close ();
         }
@@ -701,7 +785,7 @@ namespace build2
           io = true;
         }
 
-        if (!run_finish_code (args.data (), pr) || io)
+        if (!run_finish_code (dbuf, args, pr) || io)
           fail << "unable to extract symbols from " << arg;
       }
 
diff --git a/libbuild2/bin/guess.cxx b/libbuild2/bin/guess.cxx
index e935913..e9759b8 100644
--- a/libbuild2/bin/guess.cxx
+++ b/libbuild2/bin/guess.cxx
@@ -92,7 +92,7 @@ namespace build2
     static global_cache<ar_info> ar_cache;
 
     const ar_info&
-    guess_ar (const path& ar, const path* rl, const char* paths)
+    guess_ar (context& ctx, const path& ar, const path* rl, const char* paths)
     {
       tracer trace ("bin::guess_ar");
 
@@ -234,7 +234,11 @@ namespace build2
         // (yes, it goes to stdout) but that seems harmless.
         //
         sha256 cs;
-        arr = run<guess_result> (3, are, "--version", f, false, false, &cs);
+        arr = run<guess_result> (ctx,
+                                 3,
+                                 are, "--version",
+                                 f,
+                                 false , false, &cs);
 
         if (!arr.empty ())
           arr.checksum = cs.string ();
@@ -254,10 +258,10 @@ namespace build2
             : guess_result ();
         };
 
-        // Redirect STDERR to STDOUT and ignore exit status.
+        // Redirect stderr to stdout and ignore exit status.
         //
         sha256 cs;
-        arr = run<guess_result> (3, are, f, false, true, &cs);
+        arr = run<guess_result> (ctx, 3, are, f, false, true, &cs);
 
         if (!arr.empty ())
         {
@@ -300,7 +304,11 @@ namespace build2
           };
 
           sha256 cs;
-          rlr = run<guess_result> (3, rle, "--version", f, false, false, &cs);
+          rlr = run<guess_result> (ctx,
+                                   3,
+                                   rle, "--version",
+                                   f,
+                                   false, false, &cs);
 
           if (!rlr.empty ())
             rlr.checksum = cs.string ();
@@ -317,10 +325,10 @@ namespace build2
               : guess_result ();
           };
 
-          // Redirect STDERR to STDOUT and ignore exit status.
+          // Redirect stderr to stdout and ignore exit status.
           //
           sha256 cs;
-          rlr = run<guess_result> (3, rle, f, false, true, &cs);
+          rlr = run<guess_result> (ctx, 3, rle, f, false, true, &cs);
 
           if (!rlr.empty ())
           {
@@ -385,7 +393,7 @@ namespace build2
     static global_cache<ld_info> ld_cache;
 
     const ld_info&
-    guess_ld (const path& ld, const char* paths)
+    guess_ld (context& ctx, const path& ld, const char* paths)
     {
       tracer trace ("bin::guess_ld");
 
@@ -497,12 +505,12 @@ namespace build2
                   : guess_result (move (id), move (l), move (ver)));
         };
 
-        // Redirect STDERR to STDOUT and ignore exit status. Note that in case
+        // Redirect stderr to stdout and ignore exit status. Note that in case
         // of link.exe we will hash the diagnostics (yes, it goes to stdout)
         // but that seems harmless.
         //
         sha256 cs;
-        r = run<guess_result> (3, env, "--version", f, false, true, &cs);
+        r = run<guess_result> (ctx, 3, env, "--version", f, false, true, &cs);
 
         if (!r.empty ())
           r.checksum = cs.string ();
@@ -533,7 +541,7 @@ namespace build2
         };
 
         sha256 cs;
-        r = run<guess_result> (3, env, "-v", f, false, false, &cs);
+        r = run<guess_result> (ctx, 3, env, "-v", f, false, false, &cs);
 
         if (!r.empty ())
           r.checksum = cs.string ();
@@ -560,7 +568,7 @@ namespace build2
         // option.
         //
         sha256 cs;
-        r = run<guess_result> (3, env, "-version", f, false, false, &cs);
+        r = run<guess_result> (ctx, 3, env, "-version", f, false, false, &cs);
 
         if (!r.empty ())
           r.checksum = cs.string ();
@@ -598,7 +606,7 @@ namespace build2
     static global_cache<rc_info> rc_cache;
 
     const rc_info&
-    guess_rc (const path& rc, const char* paths)
+    guess_rc (context& ctx, const path& rc, const char* paths)
     {
       tracer trace ("bin::guess_rc");
 
@@ -654,7 +662,7 @@ namespace build2
         // option.
         //
         sha256 cs;
-        r = run<guess_result> (3, env, "--version", f, false, false, &cs);
+        r = run<guess_result> (ctx, 3, env, "--version", f, false, false, &cs);
 
         if (!r.empty ())
           r.checksum = cs.string ();
@@ -687,7 +695,7 @@ namespace build2
         };
 
         sha256 cs;
-        r = run<guess_result> (3, env, "/?", f, false, false, &cs);
+        r = run<guess_result> (ctx, 3, env, "/?", f, false, false, &cs);
 
         if (!r.empty ())
           r.checksum = cs.string ();
@@ -715,7 +723,7 @@ namespace build2
     static global_cache<nm_info> nm_cache;
 
     const nm_info&
-    guess_nm (const path& nm, const char* paths)
+    guess_nm (context& ctx, const path& nm, const char* paths)
     {
       tracer trace ("bin::guess_nm");
 
@@ -799,7 +807,7 @@ namespace build2
         // option.
         //
         sha256 cs;
-        r = run<guess_result> (3, env, "--version", f, false, false, &cs);
+        r = run<guess_result> (ctx, 3, env, "--version", f, false, false, &cs);
 
         if (!r.empty ())
           r.checksum = cs.string ();
diff --git a/libbuild2/bin/guess.hxx b/libbuild2/bin/guess.hxx
index 52c0e1b..7dc7b33 100644
--- a/libbuild2/bin/guess.hxx
+++ b/libbuild2/bin/guess.hxx
@@ -54,7 +54,7 @@ namespace build2
     // attemplated and the returned ranlib_* members will be left empty.
     //
     const ar_info&
-    guess_ar (const path& ar, const path* ranlib, const char* paths);
+    guess_ar (context&, const path& ar, const path* ranlib, const char* paths);
 
     // ld information.
     //
@@ -100,7 +100,7 @@ namespace build2
     };
 
     const ld_info&
-    guess_ld (const path& ld, const char* paths);
+    guess_ld (context&, const path& ld, const char* paths);
 
     // rc information.
     //
@@ -132,7 +132,7 @@ namespace build2
     };
 
     const rc_info&
-    guess_rc (const path& rc, const char* paths);
+    guess_rc (context&, const path& rc, const char* paths);
 
     // nm information.
     //
@@ -166,7 +166,7 @@ namespace build2
     };
 
     const nm_info&
-    guess_nm (const path& nm, const char* paths);
+    guess_nm (context&, const path& nm, const char* paths);
   }
 }
 
diff --git a/libbuild2/bin/init.cxx b/libbuild2/bin/init.cxx
index 4b6090a..d7c5b5a 100644
--- a/libbuild2/bin/init.cxx
+++ b/libbuild2/bin/init.cxx
@@ -236,9 +236,9 @@ namespace build2
         //
         if (!hint && config_sub)
         {
-          s = run<string> (3,
-                           *config_sub,
-                           s.c_str (),
+          s = run<string> (ctx,
+                           3,
+                           *config_sub, s.c_str (),
                            [] (string& l, bool) {return move (l);});
           l5 ([&]{trace << "config.sub target: '" << s << "'";});
         }
@@ -692,7 +692,7 @@ namespace build2
                            nullptr,
                            config::save_default_commented)));
 
-        const ar_info& ari (guess_ar (ar, ranlib, pat.paths));
+        const ar_info& ari (guess_ar (rs.ctx, ar, ranlib, pat.paths));
 
         // If this is a configuration with new values, then print the report
         // at verbosity level 2 and up (-v).
@@ -843,7 +843,7 @@ namespace build2
                            path (apply_pattern (ld_d, pat.pattern)),
                            config::save_default_commented)));
 
-        const ld_info& ldi (guess_ld (ld, pat.paths));
+        const ld_info& ldi (guess_ld (rs.ctx, ld, pat.paths));
 
         // If this is a configuration with new values, then print the report
         // at verbosity level 2 and up (-v).
@@ -1004,7 +1004,7 @@ namespace build2
                            path (apply_pattern (rc_d, pat.pattern)),
                            config::save_default_commented)));
 
-        const rc_info& rci (guess_rc (rc, pat.paths));
+        const rc_info& rci (guess_rc (rs.ctx, rc, pat.paths));
 
         // If this is a configuration with new values, then print the report
         // at verbosity level 2 and up (-v).
@@ -1116,7 +1116,7 @@ namespace build2
                            path (apply_pattern (nm_d, pat.pattern)),
                            config::save_default_commented)));
 
-        const nm_info& nmi (guess_nm (nm, pat.paths));
+        const nm_info& nmi (guess_nm (rs.ctx, nm, pat.paths));
 
         // If this is a configuration with new values, then print the report
         // at verbosity level 2 and up (-v).
diff --git a/libbuild2/cc/compile-rule.cxx b/libbuild2/cc/compile-rule.cxx
index 3a750a1..7221175 100644
--- a/libbuild2/cc/compile-rule.cxx
+++ b/libbuild2/cc/compile-rule.cxx
@@ -4608,7 +4608,7 @@ namespace build2
               continue;
             }
             else
-              run_finish (args, pr); // Throws.
+              run_finish (args, pr); // Throws. @@ DBUF
           }
           catch (const process_error& e)
           {
@@ -5008,7 +5008,7 @@ namespace build2
             info << "then run failing command to display compiler diagnostics";
         }
         else
-          run_finish (args, pr); // Throws.
+          run_finish (args, pr); // Throws. @@ DBUF
       }
       catch (const process_error& e)
       {
@@ -7218,7 +7218,7 @@ namespace build2
             args.push_back (nullptr);
           }
 
-          dbuf.finish (args, pr);
+          run_finish (dbuf, args, pr);
         }
         catch (const process_error& e)
         {
@@ -7275,8 +7275,7 @@ namespace build2
                         env.empty () ? nullptr : env.data ());
 
             dbuf.read ();
-
-            dbuf.finish (args, pr);
+            run_finish (dbuf, args, pr);
           }
           catch (const process_error& e)
           {
diff --git a/libbuild2/cc/gcc.cxx b/libbuild2/cc/gcc.cxx
index 7999c82..6b24516 100644
--- a/libbuild2/cc/gcc.cxx
+++ b/libbuild2/cc/gcc.cxx
@@ -132,113 +132,94 @@ namespace build2
       if (verb >= 3)
         print_process (env, args);
 
+      // Open pipe to stderr, redirect stdin and stdout to /dev/null.
+      //
+      process pr (run_start (
+                    env,
+                    args,
+                    -2,      /* stdin */
+                    -2,      /* stdout */
+                    {-1, -1} /* stderr */));
       try
       {
-        //@@ TODO: why don't we use run_start() here? Because it's unable to
-        //   open pipe for stderr and we need to change it first, for example,
-        //   making the err parameter a file descriptor rather than a flag.
-        //
+        ifdstream is (
+          move (pr.in_efd), fdstream_mode::skip, ifdstream::badbit);
 
-        // Open pipe to stderr, redirect stdin and stdout to /dev/null.
+        // Normally the system header paths appear between the following
+        // lines:
         //
-        process pr (xc,
-                    args.data (),
-                    -2,     /* stdin */
-                    -2,     /* stdout */
-                    -1,     /* stderr */
-                    nullptr /* cwd */,
-                    env.vars);
-
-        try
+        // #include <...> search starts here:
+        // End of search list.
+        //
+        // The exact text depends on the current locale. What we can rely on
+        // is the presence of the "#include <...>" substring in the "opening"
+        // line and the fact that the paths are indented with a single space
+        // character, unlike the "closing" line.
+        //
+        // Note that on Mac OS we will also see some framework paths among
+        // system header paths, followed with a comment. For example:
+        //
+        //  /Library/Frameworks (framework directory)
+        //
+        // For now we ignore framework paths and to filter them out we will
+        // only consider valid paths to existing directories, skipping those
+        // which we fail to normalize or stat. @@ Maybe this is a bit too
+        // loose, especially compared to gcc_library_search_dirs()?
+        //
+        string s;
+        for (bool found (false); getline (is, s); )
         {
-          ifdstream is (
-            move (pr.in_efd), fdstream_mode::skip, ifdstream::badbit);
-
-          // Normally the system header paths appear between the following
-          // lines:
-          //
-          // #include <...> search starts here:
-          // End of search list.
-          //
-          // The exact text depends on the current locale. What we can rely on
-          // is the presence of the "#include <...>" substring in the
-          // "opening" line and the fact that the paths are indented with a
-          // single space character, unlike the "closing" line.
-          //
-          // Note that on Mac OS we will also see some framework paths among
-          // system header paths, followed with a comment. For example:
-          //
-          //  /Library/Frameworks (framework directory)
-          //
-          // For now we ignore framework paths and to filter them out we will
-          // only consider valid paths to existing directories, skipping those
-          // which we fail to normalize or stat. @@ Maybe this is a bit too
-          // loose, especially compared to gcc_library_search_dirs()?
-          //
-          string s;
-          for (bool found (false); getline (is, s); )
+          if (!found)
+            found = s.find ("#include <...>") != string::npos;
+          else
           {
-            if (!found)
-              found = s.find ("#include <...>") != string::npos;
-            else
-            {
-              if (s[0] != ' ')
-                break;
+            if (s[0] != ' ')
+              break;
 
-              dir_path d;
-              try
-              {
-                string ds (s, 1, s.size () - 1);
+            dir_path d;
+            try
+            {
+              string ds (s, 1, s.size () - 1);
 
 #ifdef _WIN32
-                if (path_traits::is_separator (ds[0]))
-                  add_current_drive (ds);
+              if (path_traits::is_separator (ds[0]))
+                add_current_drive (ds);
 #endif
-                d = dir_path (move (ds));
+              d = dir_path (move (ds));
 
-                if (d.relative () || !exists (d, true))
-                  continue;
-
-                d.normalize ();
-              }
-              catch (const invalid_path&)
-              {
+              if (d.relative () || !exists (d, true))
                 continue;
-              }
 
-              if (find (r.begin (), r.end (), d) == r.end ())
-                r.emplace_back (move (d));
+              d.normalize ();
             }
+            catch (const invalid_path&)
+            {
+              continue;
+            }
+
+            if (find (r.begin (), r.end (), d) == r.end ())
+              r.emplace_back (move (d));
           }
+        }
 
-          is.close (); // Don't block.
+        is.close (); // Don't block.
 
-          if (!pr.wait ())
-          {
-            // We have read stderr so better print some diagnostics.
-            //
-            diag_record dr (fail);
+        if (!run_wait (args, pr))
+        {
+          // We have read stderr so better print some diagnostics.
+          //
+          diag_record dr (fail);
 
-            dr << "failed to extract " << x_lang << " header search paths" <<
-              info << "command line: ";
+          dr << "failed to extract " << x_lang << " header search paths" <<
+            info << "command line: ";
 
-            print_process (dr, args);
-          }
-        }
-        catch (const io_error&)
-        {
-          pr.wait ();
-          fail << "error reading " << x_lang << " compiler -v -E output";
+          print_process (dr, args);
         }
       }
-      catch (const process_error& e)
+      catch (const io_error&)
       {
-        error << "unable to execute " << args[0] << ": " << e;
-
-        if (e.child)
-          exit (1);
-
-        throw failed ();
+        run_wait (args, pr);
+        fail << "error reading " << x_lang << " compiler -v -E output";
       }
 
       // It's highly unlikely not to have any system directories. More likely
@@ -302,6 +283,9 @@ namespace build2
 
       // Open pipe to stdout.
       //
+      // Note: this function is called in the serial load phase and so no
+      // diagnostics buffering is needed.
+      //
       process pr (run_start (env,
                              args,
                              0, /* stdin */
diff --git a/libbuild2/cc/guess.cxx b/libbuild2/cc/guess.cxx
index e851ea8..5e8dbbc 100644
--- a/libbuild2/cc/guess.cxx
+++ b/libbuild2/cc/guess.cxx
@@ -181,12 +181,12 @@ namespace build2
       // could also be because there is something wrong with the compiler or
       // options but that we simply leave to blow up later).
       //
-      process pr (run_start (3     /* verbosity */,
+      process pr (run_start (3       /* verbosity */,
                              xp,
                              args,
-                             -1    /* stdin */,
-                             -1    /* stdout */,
-                             false /* error  */));
+                             -1      /* stdin */,
+                             -1      /* stdout */,
+                             {-1, 1} /* stderr (to stdout) */));
       string l, r;
       try
       {
@@ -817,7 +817,8 @@ namespace build2
     // Note: allowed to change pre if succeeds.
     //
     static guess_result
-    guess (const char* xm,
+    guess (context& ctx,
+           const char* xm,
            lang xl,
            const path& xc,
            const strings& x_mo,
@@ -1009,7 +1010,7 @@ namespace build2
 #endif
 
       string cache;
-      auto run = [&cs, &env, &args, &cache] (
+      auto run = [&ctx, &cs, &env, &args, &cache] (
         const char* o,
         auto&& f,
         bool checksum = false) -> guess_result
@@ -1017,9 +1018,10 @@ namespace build2
         args[args.size () - 2] = o;
         cache.clear ();
         return build2::run<guess_result> (
+          ctx,
           3                          /* verbosity */,
           env,
-          args.data (),
+          args,
           forward<decltype (f)> (f),
           false                      /* error */,
           false                      /* ignore_exit */,
@@ -1318,7 +1320,11 @@ namespace build2
         //
         const char* evars[] = {"CL=", "_CL_=", nullptr};
 
-        r = build2::run<guess_result> (3, process_env (xp, evars), f, false);
+        r = build2::run<guess_result> (ctx,
+                                       3,
+                                       process_env (xp, evars),
+                                       f,
+                                       false);
 
         if (r.empty ())
         {
@@ -1634,7 +1640,8 @@ namespace build2
                                      "LIB", "LINK", "_LINK_", nullptr};
 
     static compiler_info
-    guess_msvc (const char* xm,
+    guess_msvc (context&,
+                const char* xm,
                 lang xl,
                 const path& xc,
                 const string* xv,
@@ -1905,7 +1912,8 @@ namespace build2
       "SDKROOT", "MACOSX_DEPLOYMENT_TARGET", nullptr};
 
     static compiler_info
-    guess_gcc (const char* xm,
+    guess_gcc (context& ctx,
+               const char* xm,
                lang xl,
                const path& xc,
                const string* xv,
@@ -2018,7 +2026,7 @@ namespace build2
         //
         auto f = [] (string& l, bool) {return move (l);};
 
-        t = run<string> (3, xp, args.data (), f, false);
+        t = run<string> (ctx, 3, xp, args, f, false);
 
         if (t.empty ())
         {
@@ -2026,7 +2034,7 @@ namespace build2
                         << "falling back to -dumpmachine";});
 
           args[args.size () - 2] = "-dumpmachine";
-          t = run<string> (3, xp, args.data (), f, false);
+          t = run<string> (ctx, 3, xp, args, f, false);
         }
 
         if (t.empty ())
@@ -2169,9 +2177,9 @@ namespace build2
       process pr (run_start (3     /* verbosity */,
                              xp,
                              args,
-                             -2    /* stdin  (/dev/null) */,
-                             -1    /* stdout             */,
-                             false /* error  (2>&1)      */));
+                             -2      /* stdin  (to /dev/null) */,
+                             -1      /* stdout                */,
+                             {-1, 1} /* stderr (to stdout)    */));
 
       clang_msvc_info r;
 
@@ -2357,7 +2365,8 @@ namespace build2
       nullptr};
 
     static compiler_info
-    guess_clang (const char* xm,
+    guess_clang (context& ctx,
+                 const char* xm,
                  lang xl,
                  const path& xc,
                  const string* xv,
@@ -2604,7 +2613,7 @@ namespace build2
         // for LC_ALL.
         //
         auto f = [] (string& l, bool) {return move (l);};
-        t = run<string> (3, xp, args.data (), f, false);
+        t = run<string> (ctx, 3, xp, args, f, false);
 
         if (t.empty ())
           fail << "unable to extract target architecture from " << xc
@@ -2832,7 +2841,8 @@ namespace build2
     }
 
     static compiler_info
-    guess_icc (const char* xm,
+    guess_icc (context& ctx,
+               const char* xm,
                lang xl,
                const path& xc,
                const string* xv,
@@ -2896,7 +2906,7 @@ namespace build2
         //
         // @@ TODO: running without the mode options.
         //
-        s = run<string> (3, env, "-V", f, false);
+        s = run<string> (ctx, 3, env, "-V", f, false);
 
         if (s.empty ())
           fail << "unable to extract signature from " << xc << " -V output";
@@ -3022,7 +3032,7 @@ namespace build2
 
         // The -V output is sent to STDERR.
         //
-        t = run<string> (3, env, args.data (), f, false);
+        t = run<string> (ctx, 3, env, args, f, false);
 
         if (t.empty ())
           fail << "unable to extract target architecture from " << xc
@@ -3073,7 +3083,7 @@ namespace build2
         //
         {
           auto f = [] (string& l, bool) {return move (l);};
-          t = run<string> (3, xp, "-dumpmachine", f);
+          t = run<string> (ctx, 3, xp, "-dumpmachine", f);
         }
 
         if (t.empty ())
@@ -3154,7 +3164,8 @@ namespace build2
     static global_cache<compiler_info> cache;
 
     const compiler_info&
-    guess (const char* xm,
+    guess (context& ctx,
+           const char* xm,
            lang xl,
            const string& ec,
            const path& xc,
@@ -3228,7 +3239,7 @@ namespace build2
 
       if (pre.type != invalid_compiler_type)
       {
-        gr = guess (xm, xl, xc, x_mo, xi, pre, cs);
+        gr = guess (ctx, xm, xl, xc, x_mo, xi, pre, cs);
 
         if (gr.empty ())
         {
@@ -3244,13 +3255,14 @@ namespace build2
       }
 
       if (gr.empty ())
-        gr = guess (xm, xl, xc, x_mo, xi, pre, cs);
+        gr = guess (ctx, xm, xl, xc, x_mo, xi, pre, cs);
 
       if (gr.empty ())
         fail << "unable to guess " << xl << " compiler type of " << xc <<
           info << "use config." << xm << ".id to specify explicitly";
 
       compiler_info (*gf) (
+        context&,
         const char*, lang, const path&, const string*, const string*,
         const strings&,
         const strings*, const strings*,
@@ -3270,7 +3282,8 @@ namespace build2
       case compiler_type::icc: gf = &guess_icc;     break;
       }
 
-      compiler_info r (gf (xm, xl, xc, xv, xt,
+      compiler_info r (gf (ctx,
+                           xm, xl, xc, xv, xt,
                            x_mo, c_po, x_po, c_co, x_co, c_lo, x_lo,
                            move (gr), cs));
 
diff --git a/libbuild2/cc/guess.hxx b/libbuild2/cc/guess.hxx
index 53acc15..7cbbd87 100644
--- a/libbuild2/cc/guess.hxx
+++ b/libbuild2/cc/guess.hxx
@@ -253,7 +253,8 @@ namespace build2
     // that most of it will be the same, at least for C and C++.
     //
     const compiler_info&
-    guess (const char* xm,        // Module (for var names in diagnostics).
+    guess (context&,
+           const char* xm,        // Module (for var names in diagnostics).
            lang xl,               // Language.
            const string& ec,      // Environment checksum.
            const path& xc,        // Compiler path.
diff --git a/libbuild2/cc/link-rule.cxx b/libbuild2/cc/link-rule.cxx
index 5d212e6..ff03fd9 100644
--- a/libbuild2/cc/link-rule.cxx
+++ b/libbuild2/cc/link-rule.cxx
@@ -2970,7 +2970,7 @@ namespace build2
                 }
 
                 dbuf.read ();
-                dbuf.finish (args, pr, 2 /* verbosity */);
+                run_finish (dbuf, args, pr, 2 /* verbosity */);
               }
               catch (const process_error& e)
               {
@@ -4116,23 +4116,10 @@ namespace build2
 
         if (!ctx.dry_run)
         {
-#if 0
-          run (rl,
+          run (dbuf,
+               rl,
                args,
-               dir_path () /* cwd */,
                env_ptrs.empty () ? nullptr : env_ptrs.data ());
-#else
-          process pr (rl,
-                      args,
-                      0                   /* stdin  */,
-                      1                   /* stdout */,
-                      dbuf.open (args[0]) /* stderr */,
-                      nullptr             /* cwd    */,
-                      env_ptrs.empty () ? nullptr : env_ptrs.data ());
-
-          dbuf.read ();
-          dbuf.finish (args, pr);
-#endif
         }
       }
 
diff --git a/libbuild2/cc/module.cxx b/libbuild2/cc/module.cxx
index 6583001..9063ebc 100644
--- a/libbuild2/cc/module.cxx
+++ b/libbuild2/cc/module.cxx
@@ -30,6 +30,8 @@ namespace build2
     {
       tracer trace (x, "guess_init");
 
+      context& ctx (rs.ctx);
+
       bool cc_loaded (cast_false<bool> (rs["cc.core.guess.loaded"]));
 
       // Adjust module priority (compiler). Also order cc module before us
@@ -157,6 +159,7 @@ namespace build2
         // we are now folding *.std options into mode options.
         //
         x_info = &build2::cc::guess (
+          ctx,
           x, x_lang,
           rs.root_extra->environment_checksum,
           move (xc),
@@ -183,7 +186,8 @@ namespace build2
 
         if (config_sub)
         {
-          ct = run<string> (3,
+          ct = run<string> (ctx,
+                            3,
                             *config_sub,
                             xi.target.c_str (),
                             [] (string& l, bool) {return move (l);});
diff --git a/libbuild2/cc/msvc.cxx b/libbuild2/cc/msvc.cxx
index 765b1c6..3165602 100644
--- a/libbuild2/cc/msvc.cxx
+++ b/libbuild2/cc/msvc.cxx
@@ -433,9 +433,9 @@ namespace build2
       //
       process pr (run_start (ld,
                              args,
-                             0     /* stdin */,
-                             -1    /* stdout */,
-                             false /* error */));
+                             0       /* stdin */,
+                             -1      /* stdout */,
+                             {-1, 1} /* stderr (to stdout) */));
 
       bool obj (false), dll (false);
       string s;
diff --git a/libbuild2/context.cxx b/libbuild2/context.cxx
index 0108d27..4858c4c 100644
--- a/libbuild2/context.cxx
+++ b/libbuild2/context.cxx
@@ -255,7 +255,8 @@ namespace build2
       // Did the user ask us to use config.guess?
       //
       string orig (config_guess
-                   ? run<string> (3,
+                   ? run<string> (*this,
+                                  3,
                                   *config_guess,
                                   [](string& l, bool) {return move (l);})
                    : BUILD2_HOST_TRIPLET);
diff --git a/libbuild2/diagnostics.cxx b/libbuild2/diagnostics.cxx
index 629febf..3b0415c 100644
--- a/libbuild2/diagnostics.cxx
+++ b/libbuild2/diagnostics.cxx
@@ -50,12 +50,10 @@ namespace build2
   // diag_buffer
   //
   process::pipe diag_buffer::
-  open (const char* args0, bool force, bool blocking)
+  open (const char* args0, bool force, fdstream_mode m)
   {
     assert (state_ == state::closed && args0 != nullptr);
 
-    assert (blocking); // @@ TODO (also in read() and close () below).
-
     serial = ctx_.sched.serial ();
     nobuf = !serial && ctx_.no_diag_buffer;
 
@@ -71,7 +69,9 @@ namespace build2
         //
         r = process::pipe (p.in.get (), move (p.out));
 
-        is.open (move (p.in), fdstream_mode::text);
+        m |= fdstream_mode::text;
+
+        is.open (move (p.in), m);
       }
       catch (const io_error& e)
       {
@@ -96,6 +96,26 @@ namespace build2
     {
       try
       {
+        // Copy buffers directly.
+        //
+        auto copy = [this] (fdstreambuf& sb)
+        {
+          const char* p (sb.gptr ());
+          size_t n (sb.egptr () - p);
+
+          // Allocate at least fdstreambuf::buffer_size to reduce
+          // reallocations and memory fragmentation.
+          //
+          size_t i (buf.size ());
+          if (i == 0 && n < fdstreambuf::buffer_size)
+            buf.reserve (fdstreambuf::buffer_size);
+
+          buf.resize (i + n);
+          memcpy (buf.data () + i, p, n);
+
+          sb.gbump (static_cast<int> (n));
+        };
+
         if (is.blocking ())
         {
           if (serial || nobuf)
@@ -139,30 +159,30 @@ namespace build2
             fdstreambuf& sb (*static_cast<fdstreambuf*> (is.rdbuf ()));
 
             while (is.peek () != istream::traits_type::eof ())
-            {
-              const char* p (sb.gptr ());
-              size_t n (sb.egptr () - p);
-
-              // Allocate at least fdstreambuf::buffer_size to reduce
-              // reallocations and memory fragmentation.
-              //
-              size_t i (buf.size ());
-              if (i == 0 && n < fdstreambuf::buffer_size)
-                buf.reserve (fdstreambuf::buffer_size);
-
-              buf.resize (i + n);
-              memcpy (buf.data () + i, p, n);
-
-              sb.gbump (static_cast<int> (n));
-            }
+              copy (sb);
           }
 
           r = false;
         }
         else
         {
-          // @@ TODO (maybe we can unify the two?)
-          r = true;
+          // We do not support finishing off after the custom processing in
+          // the non-blocking mode (but could probably do if necessary).
+          //
+          assert (!(serial || nobuf));
+
+          fdstreambuf& sb (*static_cast<fdstreambuf*> (is.rdbuf ()));
+
+          // Try not to allocate the buffer if there is no diagnostics (the
+          // common case).
+          //
+          // Note that we must read until blocked (0) or EOF (-1).
+          //
+          streamsize n;
+          while ((n = sb.in_avail ()) > 0)
+            copy (sb);
+
+          r = (n != -1);
         }
 
         if (!r)
@@ -216,10 +236,53 @@ namespace build2
   }
 
   void diag_buffer::
-  close (const char* const* args, size_t args_size,
+  close (const char* const* args,
          const process_exit& pe,
          uint16_t v,
-         const location& loc)
+         const location& loc,
+         bool omit_normall)
+  {
+    tracer trace ("diag_buffer::close");
+
+    assert (state_ != state::closed);
+
+    // We need to make sure the command line we print on the unsuccessful exit
+    // is inseparable from any buffered diagnostics. So we prepare the record
+    // first and then write both while holding the diagnostics stream lock.
+    //
+    diag_record dr;
+    if (!pe)
+    {
+      // Note: see similar code in run_finish_impl().
+      //
+      if (omit_normall && pe.normal ())
+      {
+        l4 ([&]{trace << "process " << args[0] << " " << pe;});
+      }
+      else
+      {
+        // It's unclear whether we should print this only if printing the
+        // command line (we could also do things differently for
+        // normal/abnormal exit). Let's print this always and see how it
+        // wears.
+        //
+        // Note: make sure keep the above trace is not printing.
+        //
+        dr << error (loc) << "process " << args[0] << " " << pe;
+
+        if (verb >= 1 && verb <= v)
+        {
+          dr << info << "command line: ";
+          print_process (dr, args);
+        }
+      }
+    }
+
+    close (move (dr));
+  }
+
+  void diag_buffer::
+  close (diag_record&& dr)
   {
     assert (state_ != state::closed);
 
@@ -231,9 +294,18 @@ namespace build2
       {
         try
         {
-          // @@ TODO: is it ok to call peek() in non-blocking?
-          //
-          assert (is.peek () == ifdstream::traits_type::eof ());
+          if (is.good ())
+          {
+            if (is.blocking ())
+            {
+              assert (is.peek () == ifdstream::traits_type::eof ());
+            }
+            else
+            {
+              assert (is.rdbuf ()->in_avail () == -1);
+            }
+          }
+
           is.close ();
         }
         catch (const io_error& e)
@@ -245,28 +317,6 @@ namespace build2
       state_ = state::eof;
     }
 
-    // We need to make sure the command line we print on the unsuccessful exit
-    // is inseparable from any buffered diagnostics. So we prepare the record
-    // first and then write both while holding the diagnostics stream lock.
-    //
-    diag_record dr;
-    if (!pe)
-    {
-      // Note: see similar code in run_finish_impl().
-
-      // It's unclear whether we should print this only if printing the
-      // command line (we could also do things differently for normal/abnormal
-      // exit). Let's print this always and see how it wears.
-      //
-      dr << error (loc) << "process " << args[0] << " " << pe;
-
-      if (verb >= 1 && verb <= v)
-      {
-        dr << info << "command line: ";
-        print_process (dr, args, args_size);
-      }
-    }
-
     if (!buf.empty () || !dr.empty ())
     {
       diag_stream_lock l;
@@ -290,31 +340,6 @@ namespace build2
     state_ = state::closed;
   }
 
-  void diag_buffer::
-  finish (const char* const* args, size_t args_size,
-          process& pr,
-          uint16_t v,
-          const location& loc)
-  {
-    // Note: see similar code in run_finish_impl().
-    //
-    try
-    {
-      pr.wait ();
-    }
-    catch (const process_error& e)
-    {
-      fail (loc) << "unable to execute " << args[0] << ": " << e << endf;
-    }
-
-    const process_exit& pe (*pr.exit);
-
-    close (args, args_size, pe, v, loc);
-
-    if (!pe)
-      throw failed ();
-  }
-
   // print_process()
   //
   void
diff --git a/libbuild2/diagnostics.hxx b/libbuild2/diagnostics.hxx
index 48dfe36..1659a09 100644
--- a/libbuild2/diagnostics.hxx
+++ b/libbuild2/diagnostics.hxx
@@ -44,8 +44,9 @@ namespace build2
 
   public:
     // If buffering is necessary or force is true, open a pipe and return the
-    // child process end of it. Otherwise, return stderr. If blocking is
-    // false, then make reading from the parent end of the pipe non-blocking.
+    // child process end of it. Otherwise, return stderr. If mode is
+    // non_blocking, then make reading from the parent end of the pipe
+    // non-blocking.
     //
     // The args0 argument is the child process program name for diagnostics.
     // It is expected to remain valid until the call to close() and should
@@ -61,8 +62,14 @@ namespace build2
     // and throw failed. If an exception is thrown from any of them, then the
     // instance should not be used any further.
     //
+    // Note that when reading from multiple streams in the non-blocking mode,
+    // only the last stream to be destroyed can normally have the skip mode
+    // since in case of an exception, skipping will be blocking.
+    //
     process::pipe
-    open (const char* args0, bool force = false, bool blocking = true);
+    open (const char* args0,
+          bool force = false,
+          fdstream_mode mode = fdstream_mode::skip);
 
     // Read the diagnostics from the parent end of the pipe if one was opened
     // and buffer/stream it as necessary. Return true if there could be more
@@ -82,10 +89,10 @@ namespace build2
     // with the stream mode (blocking or non). If buffering is performed, then
     // depending on the expected diagnostics the custom processing may want to
     // reserve an appropriate initial buffer size to avoid unnecessary
-    // reallocation. As a convenience, in the blocking mode, if the stream
-    // still contains some diagnostics, then it can be handled by calling
-    // read(). This is helpful when needing to process only the inital part of
-    // the diagnostics.
+    // reallocation. As a convenience, in the blocking mode only, if the
+    // stream still contains some diagnostics, then it can be handled by
+    // calling read(). This is useful when needing to process only the inital
+    // part of the diagnostics.
     //
     bool
     read ();
@@ -93,70 +100,47 @@ namespace build2
     // Close the parent end of the pipe if one was opened and write out any
     // buffered diagnostics.
     //
-    // If the verbosity level is between 1 and the specified value and the
-    // child process exited with non-0 code, then print the command line after
-    // the diagnostics. Normally the specified verbosity will be 1 and the
-    // command line args represent the verbosity level 2 (logical) command
-    // line. The semantics os the args/args_size arguments is the same as
-    // in print_process() below.
+    // If the child process exited abnormally or normally with non-0 code,
+    // then print the error diagnostics to this effect. Additionally, if the
+    // verbosity level is between 1 and the specified value, then print the
+    // command line as info after the error. If omit_normall is true, then
+    // don't print either for the normal exit (usually used when process
+    // failure can be tolerated).
+    //
+    // Normally the specified verbosity will be 1 and the command line args
+    // represent the verbosity level 2 (logical) command line. Note that args
+    // should only represent a single command in a pipe (see print_process()
+    // below for details).
     //
     // If the diag_buffer instance is destroyed before calling close(), then
     // any buffered diagnostics is discarded.
     //
+    // Note: see also run_finish(diag_buffer&).
+    //
     // @@ TODO: need overload with process_env (see print_process).
     //
     void
     close (const cstrings& args,
            const process_exit& pe,
            uint16_t verbosity = 1,
-           const location& loc = {})
+           const location& loc = {},
+           bool omit_normall = false)
     {
-      close (args.data (), args.size (), pe, verbosity, loc);
+      close (args.data (), pe, verbosity, loc, omit_normall);
     }
 
     void
     close (const char* const* args,
            const process_exit& pe,
            uint16_t verbosity = 1,
-           const location& loc = {})
-    {
-      close (args, 0, pe, verbosity, loc);
-    }
-
-    void
-    close (const char* const* args, size_t args_size,
-           const process_exit& pe,
-           uint16_t verbosity = 1,
-           const location& loc = {});
+           const location& loc = {},
+           bool omit_normall = false);
 
-
-    // This version calls close() plus it first waits for the process to
-    // finish and later throws failed if it didn't exit with 0 code (so
-    // similar to run_finish ()).
+    // As above but with a custom diag record for the child exit diagnostics,
+    // if any.
     //
     void
-    finish (const cstrings& args,
-            process& pr,
-            uint16_t verbosity = 1,
-            const location& loc = {})
-    {
-      finish (args.data (), args.size (), pr, verbosity, loc);
-    }
-
-    void
-    finish (const char* const* args,
-            process& pr,
-            uint16_t verbosity = 1,
-            const location& loc = {})
-    {
-      finish (args, 0, pr, verbosity, loc);
-    }
-
-    void
-    finish (const char* const* args, size_t args_size,
-            process&,
-            uint16_t verbosity = 1,
-            const location& = {});
+    close (diag_record&&);
 
     // Direct access to the underlying stream and buffer for custom processing
     // (see read() above for details).
diff --git a/libbuild2/dist/operation.cxx b/libbuild2/dist/operation.cxx
index 9a662df..f6d00e4 100644
--- a/libbuild2/dist/operation.cxx
+++ b/libbuild2/dist/operation.cxx
@@ -27,7 +27,7 @@ namespace build2
     // install -d <dir>
     //
     static void
-    install (const process_path&, const dir_path&);
+    install (const process_path&, context&, const dir_path&);
 
     // install <file> <dir>[/<name>]
     //
@@ -504,7 +504,7 @@ namespace build2
         fail << "unable to clean target directory " << td;
 
       auto_rmdir rm_td (td); // Clean it up if things go bad.
-      install (dist_cmd, td);
+      install (dist_cmd, ctx, td);
 
       // Copy over all the files. Apply post-processing callbacks.
       //
@@ -571,7 +571,7 @@ namespace build2
 
         dir_path d (td / dl);
         if (!exists (d))
-          install (dist_cmd, d);
+          install (dist_cmd, ctx, d);
 
         path r (install (dist_cmd, t, d, rn));
 
@@ -727,7 +727,7 @@ namespace build2
     // install -d <dir>
     //
     static void
-    install (const process_path& cmd, const dir_path& d)
+    install (const process_path& cmd, context& ctx, const dir_path& d)
     {
       path reld (relative (d));
 
@@ -741,7 +741,7 @@ namespace build2
       if (verb >= 2)
         print_process (args);
 
-      run (cmd, args);
+      run (ctx, cmd, args);
     }
 
     // install <file> <dir>[/<name>]
@@ -784,7 +784,7 @@ namespace build2
       if (verb >= 2)
         print_process (args);
 
-      run (cmd, args);
+      run (t.ctx, cmd, args);
 
       return d / (n.empty () ? relf.leaf () : n);
     }
@@ -939,12 +939,13 @@ namespace build2
 
       // Change the archiver's working directory to dist_root.
       //
-      apr = run_start (app,
+      // Note: this function is called during serial execution and so no
+      // diagnostics buffering is needed (here and below).
+      //
+      apr = run_start (process_env (app, root),
                        args,
                        0                 /* stdin  */,
-                       (i != 0 ? -1 : 1) /* stdout */,
-                       true              /* error */,
-                       root);
+                       (i != 0 ? -1 : 1) /* stdout */);
 
       // Start the compressor if required.
       //
@@ -1022,12 +1023,13 @@ namespace build2
         // Note that to only get the archive name (without the directory) in
         // the output we have to run from the archive's directory.
         //
-        process pr (run_start (pp,
+        // Note: this function is called during serial execution and so no
+        // diagnostics buffering is needed.
+        //
+        process pr (run_start (process_env (pp, ad /* cwd */),
                                args,
-                               0             /* stdin */,
-                               c_fd.get ()   /* stdout */,
-                               true          /* error */,
-                               ad            /* cwd */));
+                               0             /* stdin  */,
+                               c_fd.get ()   /* stdout */));
         run_finish (args, pr);
       }
       else
diff --git a/libbuild2/functions-process.cxx b/libbuild2/functions-process.cxx
index c4e5c24..28b7a99 100644
--- a/libbuild2/functions-process.cxx
+++ b/libbuild2/functions-process.cxx
@@ -4,6 +4,8 @@
 #include <libbutl/regex.hxx>
 #include <libbutl/builtin.hxx>
 
+#include <libbuild2/scope.hxx>
+#include <libbuild2/context.hxx>
 #include <libbuild2/function.hxx>
 #include <libbuild2/variable.hxx>
 
@@ -181,18 +183,32 @@ namespace build2
   }
 
   static inline value
-  run_builtin (builtin_function* bf, const strings& args, const string& bn)
+  run_builtin (const scope* s,
+               builtin_function* bf,
+               const strings& args,
+               const string& bn)
   {
+    // See below.
+    //
+    if (s != nullptr && s->ctx.phase != run_phase::load)
+      fail << "process.run() called during " << s->ctx.phase << " phase";
+
     return run_builtin_impl (bf, args, bn, read);
   }
 
   static inline value
-  run_builtin_regex (builtin_function* bf,
+  run_builtin_regex (const scope* s,
+                     builtin_function* bf,
                      const strings& args,
                      const string& bn,
                      const string& pat,
                      const optional<string>& fmt)
   {
+    // See below.
+    //
+    if (s != nullptr && s->ctx.phase != run_phase::load)
+      fail << "process.run_regex() called during " << s->ctx.phase << " phase";
+
     // Note that we rely on the "small function object" optimization here.
     //
     return run_builtin_impl (bf, args, bn,
@@ -293,6 +309,9 @@ namespace build2
                [] (const string& s) {return s.c_str ();});
     cargs.push_back (nullptr);
 
+    // Note that for now these functions can only be called during the load
+    // phase (see below) and so no diagnostics buffering is needed.
+    //
     return run_start (3               /* verbosity */,
                       pp,
                       cargs,
@@ -352,6 +371,15 @@ namespace build2
   static inline value
   run_process (const scope* s, const process_path& pp, const strings& args)
   {
+    // The only plausible place where these functions can be called outside
+    // the load phase are scripts and there it doesn't make much sense to use
+    // them (the same can be achieved with commands in a uniform manner). Note
+    // that if there is no scope, then this is most likely (certainly?) the
+    // load phase (for example, command line).
+    //
+    if (s != nullptr && s->ctx.phase != run_phase::load)
+      fail << "process.run() called during " << s->ctx.phase << " phase";
+
     return run_process_impl (s, pp, args, read);
   }
 
@@ -362,6 +390,11 @@ namespace build2
                      const string& pat,
                      const optional<string>& fmt)
   {
+    // See above.
+    //
+    if (s != nullptr && s->ctx.phase != run_phase::load)
+      fail << "process.run_regex() called during " << s->ctx.phase << " phase";
+
     // Note that we rely on the "small function object" optimization here.
     //
     return run_process_impl (s, pp, args,
@@ -377,7 +410,7 @@ namespace build2
     if (builtin_function* bf = builtin (args))
     {
       pair<string, strings> ba (builtin_args (bf, move (args), "run"));
-      return run_builtin (bf, ba.second, ba.first);
+      return run_builtin (s, bf, ba.second, ba.first);
     }
     else
     {
@@ -395,7 +428,7 @@ namespace build2
     if (builtin_function* bf = builtin (args))
     {
       pair<string, strings> ba (builtin_args (bf, move (args), "run_regex"));
-      return run_builtin_regex (bf, ba.second, ba.first, pat, fmt);
+      return run_builtin_regex (s, bf, ba.second, ba.first, pat, fmt);
     }
     else
     {
@@ -420,7 +453,8 @@ namespace build2
     // result, then such variables should be reported with the
     // config.environment directive.
     //
-    // Note that this function is not pure.
+    // Note that this function is not pure and can only be called during the
+    // load phase.
     //
     f.insert (".run", false) += [](const scope* s, names args)
     {
@@ -446,7 +480,8 @@ namespace build2
     // result, then such variables should be reported with the
     // config.environment directive.
     //
-    // Note that this function is not pure.
+    // Note that this function is not pure and can only be called during the
+    // load phase.
     //
     {
       auto e (f.insert (".run_regex", false));
diff --git a/libbuild2/install/rule.cxx b/libbuild2/install/rule.cxx
index 9f01578..aa13169 100644
--- a/libbuild2/install/rule.cxx
+++ b/libbuild2/install/rule.cxx
@@ -848,7 +848,7 @@ namespace build2
           text << "install " << chd;
       }
 
-      run (pp, args);
+      run (ctx, pp, args);
     }
 
     void file_rule::
@@ -903,7 +903,7 @@ namespace build2
       }
 
       if (!ctx.dry_run)
-        run (pp, args);
+        run (ctx, pp, args);
     }
 
     void file_rule::
@@ -946,7 +946,7 @@ namespace build2
       }
 
       if (!ctx.dry_run)
-        run (pp, args);
+        run (ctx, pp, args);
 #else
       // The -f part.
       //
@@ -1179,8 +1179,13 @@ namespace build2
               text << "uninstall " << reld;
           }
 
-          process pr (run_start (pp, args));
-          r = run_finish_code (args, pr);
+          diag_buffer dbuf (rs.ctx);
+          process pr (run_start (pp, args,
+                                 0                   /* stdin */,
+                                 1                   /* stdout */,
+                                 dbuf.open (args[0]) /* stderr */));
+          dbuf.read ();
+          r = run_finish_code (dbuf, args, pr);
         }
 
         if (!r)
@@ -1279,7 +1284,7 @@ namespace build2
           print_process (args);
 
         if (!ctx.dry_run)
-          run (pp, args);
+          run (ctx, pp, args);
       }
 
       return true;
diff --git a/libbuild2/parser.cxx b/libbuild2/parser.cxx
index ed3b144..9b0c7a3 100644
--- a/libbuild2/parser.cxx
+++ b/libbuild2/parser.cxx
@@ -3126,13 +3126,16 @@ namespace build2
                [] (const string& s) {return s.c_str ();});
     cargs.push_back (nullptr);
 
+    // Note: we are in the serial load phase and so no diagnostics buffering
+    // is needed.
+    //
     process pr (run_start (3            /* verbosity */,
                            cargs,
                            0            /* stdin  */,
                            -1           /* stdout */,
-                           true         /* error  */,
-                           dir_path ()  /* cwd    */,
+                           {-1, 2}      /* stderr */,
                            nullptr      /* env    */,
+                           dir_path ()  /* cwd    */,
                            l));
     try
     {
diff --git a/libbuild2/script/run.cxx b/libbuild2/script/run.cxx
index 1971bd1..0d9e472 100644
--- a/libbuild2/script/run.cxx
+++ b/libbuild2/script/run.cxx
@@ -1015,6 +1015,8 @@ namespace build2
       //
       // Set the empty_ flag to false after the first character is read.
       //
+      // @@ PERF: reading one character at a time is not ideal.
+      //
       auto get = [this] () -> optional<char>
       {
         char r;
diff --git a/libbuild2/types.hxx b/libbuild2/types.hxx
index d93da6d..bf412a3 100644
--- a/libbuild2/types.hxx
+++ b/libbuild2/types.hxx
@@ -429,6 +429,7 @@ namespace build2
 
   // <libbutl/fdstream.hxx>
   //
+  using butl::nullfd;
   using butl::auto_fd;
   using butl::fdpipe;
   using butl::ifdstream;
diff --git a/libbuild2/utility.cxx b/libbuild2/utility.cxx
index e73cd61..aa67c92 100644
--- a/libbuild2/utility.cxx
+++ b/libbuild2/utility.cxx
@@ -218,8 +218,7 @@ namespace build2
              const char* args[],
              int in,
              int out,
-             bool err,
-             const dir_path& cwd,
+             process::pipe err,
              const location& l)
   try
   {
@@ -233,10 +232,8 @@ namespace build2
       args,
       in,
       out,
-      (err ? 2 : 1),
-      (!cwd.empty ()
-       ? cwd.string ().c_str ()
-       : pe.cwd != nullptr ? pe.cwd->string ().c_str () : nullptr),
+      move (err),
+      pe.cwd != nullptr ? pe.cwd->string ().c_str () : nullptr,
       pe.vars);
   }
   catch (const process_error& e)
@@ -258,7 +255,7 @@ namespace build2
   }
 
   bool
-  run_wait (const char* args[], process& pr, const location& loc)
+  run_wait (const char* const* args, process& pr, const location& loc)
   try
   {
     return pr.wait ();
@@ -269,17 +266,23 @@ namespace build2
   }
 
   bool
-  run_finish_impl (const char* args[],
+  run_finish_impl (const char* const* args,
                    process& pr,
-                   bool err,
+                   bool f,
                    const string& l,
                    const location& loc)
-  try
   {
     tracer trace ("run_finish");
 
-    if (pr.wait ())
-      return true;
+    try
+    {
+      if (pr.wait ())
+        return true;
+    }
+    catch (const process_error& e)
+    {
+      fail (loc) << "unable to execute " << args[0] << ": " << e << endf;
+    }
 
     // Note: see similar code in diag_buffer::close/finish().
     //
@@ -290,16 +293,6 @@ namespace build2
 
     // Normall but non-zero exit status.
     //
-    if (err)
-    {
-      // While we assuming diagnostics has already been issued (to STDERR), if
-      // that's not the case, it's a real pain to debug. So trace it.
-      //
-      l4 ([&]{trace << "process " << args[0] << " " << e;});
-
-      throw failed ();
-    }
-
     // Even if the user asked to suppress diagnostiscs, one error that we
     // want to let through is the inability to execute the program itself.
     // We cannot reserve a special exit status to signal this so we will
@@ -307,19 +300,281 @@ namespace build2
     // result in a single error line printed by run_start() above.
     //
     if (l.compare (0, 18, "unable to execute ") == 0)
-      fail (loc) << l;
+      error (loc) << l;
+
+    if (f)
+    {
+      // While we assume diagnostics has already been issued (to stderr), if
+      // that's not the case, it's a real pain to debug. So trace it. (And
+      // if you think that doesn't happen in sensible programs, check GCC
+      // bug #107448).
+      //
+      l4 ([&]{trace << "process " << args[0] << " " << e;});
+
+      throw failed ();
+    }
 
     return false;
   }
-  catch (const process_error& e)
+
+  bool
+  run_finish_impl (diag_buffer& dbuf,
+                   const char* const* args,
+                   process& pr,
+                   bool f,
+                   uint16_t v,
+                   const location& loc)
   {
-    fail (loc) << "unable to execute " << args[0] << ": " << e << endf;
+    try
+    {
+      pr.wait ();
+    }
+    catch (const process_error& e)
+    {
+      fail (loc) << "unable to execute " << args[0] << ": " << e << endf;
+    }
+
+    const process_exit& pe (*pr.exit);
+
+    dbuf.close (args, pe, v, loc, !f /* omit_normall */);
+
+    if (pe)
+      return true;
+
+    if (f)
+      throw failed ();
+
+    return false;
+  }
+
+  void
+  run (context& ctx, const process_env& pe, const char* args[])
+  {
+    if (ctx.phase == run_phase::load)
+    {
+      process pr (run_start (pe, args));
+      run_finish (args, pr);
+    }
+    else
+    {
+      diag_buffer dbuf (ctx);
+      run (dbuf, pe, args);
+    }
   }
 
   void
-  run_io_error (const char* args[], const io_error& e)
+  run (diag_buffer& dbuf, const process_env& pe, const char* args[])
   {
-    fail << "io error reading " << args[0] << " output: " << e << endf;
+    process pr (run_start (pe,
+                           args,
+                           0                   /* stdin  */,
+                           1                   /* stdout */,
+                           dbuf.open (args[0]) /* stderr */));
+    dbuf.read ();
+    run_finish (dbuf, args, pr);
+  }
+
+  bool
+  run (context& ctx,
+       uint16_t verbosity,
+       const process_env& pe,
+       const char* args[],
+       const function<bool (string&, bool)>& f,
+       bool tr,
+       bool err,
+       bool ignore_exit,
+       sha256* checksum)
+  {
+    if (err && ctx.phase != run_phase::load)
+    {
+      diag_buffer dbuf (ctx);
+      return run (dbuf, verbosity, pe, args, f, tr, ignore_exit, checksum);
+    }
+
+    process pr (run_start (verbosity,
+                           pe,
+                           args,
+                           0  /* stdin */,
+                           -1 /* stdout */,
+                           {-1, err ? 2 : 1}));
+
+    string l; // Last line of output.
+    try
+    {
+      ifdstream is (move (pr.in_ofd), fdstream_mode::skip);
+
+      bool empty (true);
+
+      // Make sure we keep the last line.
+      //
+      for (bool last (is.peek () == ifdstream::traits_type::eof ());
+           !last && getline (is, l); )
+      {
+        last = (is.peek () == ifdstream::traits_type::eof ());
+
+        if (tr)
+          trim (l);
+
+        if (checksum != nullptr)
+          checksum->append (l);
+
+        if (empty)
+        {
+          empty = f (l, last);
+
+          if (!empty && checksum == nullptr)
+            break;
+        }
+      }
+
+      is.close ();
+    }
+    catch (const io_error& e)
+    {
+      if (run_wait (args, pr))
+        fail << "io error reading " << args[0] << " output: " << e << endf;
+
+      // If the child process has failed then assume the io error was
+      // caused by that and let run_finish() deal with it.
+    }
+
+    if (!(run_finish_impl (args, pr, err, l) || ignore_exit))
+      return false;
+
+    return true;
+  }
+
+  bool
+  run (diag_buffer& dbuf,
+       uint16_t verbosity,
+       const process_env& pe,
+       const char* args[],
+       const function<bool (string&, bool)>& f,
+       bool tr,
+       bool ignore_exit,
+       sha256* checksum)
+  {
+    // We have to use the non-blocking setup since we have to read from stdout
+    // and stderr simultaneously.
+    //
+    process pr (
+      run_start (verbosity,
+                 pe,
+                 args,
+                 0                                /* stdin */,
+                 -1                               /* stdout */,
+                 dbuf.open (args[0],
+                            false /* force */,
+                            fdstream_mode::non_blocking |
+                            fdstream_mode::skip)  /* stderr */));
+
+    try
+    {
+      // Note that while we read both streams until eof in the normal
+      // circumstances, we cannot use fdstream_mode::skip for the exception
+      // case on both of them: we may end up being blocked trying to read one
+      // stream while the process may be blocked writing to the other. So in
+      // case of an exception we only skip the diagnostics and close stdout
+      // hard. The latter should happen first so the order of the dbuf/is
+      // variables is important.
+      //
+      ifdstream is (move (pr.in_ofd),
+                    fdstream_mode::non_blocking,
+                    ifdstream::badbit);
+
+      bool empty (true);
+
+      // Read until we reach EOF on all streams.
+      //
+      // Note that if dbuf is not opened, then we automatically get inactive
+      // nullfd entry.
+      //
+      fdselect_set fds {is.fd (), dbuf.is.fd ()};
+      fdselect_state& ist (fds[0]);
+      fdselect_state& dst (fds[1]);
+
+      // To detect the last line we are going keep the previous line and
+      // only call the function once we've read the next.
+      //
+      optional<string> pl;
+
+      for (string l; ist.fd != nullfd || dst.fd != nullfd; )
+      {
+        if (ist.fd != nullfd && getline_non_blocking (is, l))
+        {
+          if (eof (is))
+          {
+            if (pl && empty)
+              f (*pl, true /* last */);
+
+            ist.fd = nullfd;
+          }
+          else
+          {
+            if (checksum != nullptr || empty)
+            {
+              if (tr)
+                trim (l);
+
+              if (checksum != nullptr)
+                checksum->append (l);
+
+              if (empty)
+              {
+                if (pl)
+                {
+                  if ((empty = f (*pl, false /* last */)))
+                      swap (l, *pl);
+
+                  // Note that we cannot bail out like in the other version
+                  // since we don't have the skip mode on is. Plus, we might
+                  // still have the diagnostics.
+                }
+                else
+                  pl = move (l);
+              }
+            }
+
+            l.clear ();
+          }
+
+          continue;
+        }
+
+        ifdselect (fds);
+
+        if (dst.ready)
+        {
+          if (!dbuf.read ())
+            dst.fd = nullfd;
+        }
+      }
+
+      is.close ();
+    }
+    catch (const io_error& e)
+    {
+      if (run_wait (args, pr))
+      {
+        // Note that we will drop the diagnostics in this case since reading
+        // it could have been the cause of this error.
+        //
+        fail << "io error reading " << args[0] << " output: " << e << endf;
+      }
+
+      // If the child process has failed then assume the io error was
+      // caused by that and let run_finish() deal with it.
+    }
+
+    if (ignore_exit)
+    {
+      if (!run_finish_code (dbuf, args, pr, verbosity - 1))
+        return false;
+    }
+    else
+      run_finish (dbuf, args, pr, verbosity - 1);
+
+    return true;
   }
 
   fdpipe
diff --git a/libbuild2/utility.hxx b/libbuild2/utility.hxx
index 88ea43d..a2259df 100644
--- a/libbuild2/utility.hxx
+++ b/libbuild2/utility.hxx
@@ -261,46 +261,13 @@ namespace build2
   [[noreturn]] LIBBUILD2_SYMEXPORT void
   run_search_fail (const path&, const location& = location ());
 
-  // Wait for process termination returning true if the process exited
-  // normally with a zero code and false otherwise. The latter case is
-  // normally followed up with a call to run_finish().
-  //
-  LIBBUILD2_SYMEXPORT bool
-  run_wait (const char* args[], process&, const location& = location ());
-
-  bool
-  run_wait (cstrings& args, process&, const location& = location ());
-
-  // Wait for process termination. Issue diagnostics and throw failed in case
-  // of abnormal termination. If the process has terminated normally but with
-  // a non-zero exit status, then assume the diagnostics has already been
-  // issued and just throw failed. The last argument is used in cooperation
-  // with run_start() in case STDERR is redirected to STDOUT.
-  //
-  void
-  run_finish (const char* args[],
-              process&,
-              const string& = string (),
-              const location& = location ());
-
-  void
-  run_finish (cstrings& args, process& pr, const location& l = location ());
-
-  // As above but if the process has exited normally with a non-zero code,
-  // then return false rather than throwing.
-  //
-  bool
-  run_finish_code (const char* args[],
-                   process&,
-                   const string& = string (),
-                   const location& = location ());
-
-  // Start a process with the specified arguments. If in is -1, then redirect
-  // STDIN to a pipe (can also be -2 to redirect to /dev/null or equivalent).
-  // If out is -1, redirect STDOUT to a pipe. If error is false, then
-  // redirecting STDERR to STDOUT (this can be used to suppress diagnostics
-  // from the child process). Issue diagnostics and throw failed in case of an
-  // error.
+  // Start a process with the specified arguments. Issue diagnostics and throw
+  // failed in case of an error. If in is -1, then redirect stdin to a pipe
+  // (can also be -2 to redirect it to /dev/null or equivalent). If out is -1,
+  // then redirect stdout to a pipe. If stderr is redirected to stdout (can
+  // be used to analyze diagnostics from the child process), then, in case of
+  // an error, the last line read from stdout must be passed to run_finish()
+  // below.
   //
   LIBBUILD2_SYMEXPORT process
   run_start (uint16_t verbosity,
@@ -308,9 +275,8 @@ namespace build2
              const char* args[],
              int in = 0,
              int out = 1,
-             bool error = true,
-             const dir_path& cwd = dir_path (),
-             const location& = location ());
+             process::pipe = {-1, 2},
+             const location& = {});
 
   inline process
   run_start (uint16_t verbosity,
@@ -318,11 +284,10 @@ namespace build2
              cstrings& args,
              int in = 0,
              int out = 1,
-             bool error = true,
-             const dir_path& cwd = dir_path (),
-             const location& l = location ())
+             process::pipe err = {-1, 2},
+             const location& l = {})
   {
-    return run_start (verbosity, pe, args.data (), in, out, error, cwd, l);
+    return run_start (verbosity, pe, args.data (), in, out, move (err), l);
   }
 
   inline process
@@ -330,11 +295,10 @@ namespace build2
              const char* args[],
              int in = 0,
              int out = 1,
-             bool error = true,
-             const dir_path& cwd = dir_path (),
-             const location& l = location ())
+             process::pipe err = {-1, 2},
+             const location& l = {})
   {
-    return run_start (verb_never, pe, args, in, out, error, cwd, l);
+    return run_start (verb_never, pe, args, in, out, move (err), l);
   }
 
   inline process
@@ -342,45 +306,10 @@ namespace build2
              cstrings& args,
              int in = 0,
              int out = 1,
-             bool error = true,
-             const dir_path& cwd = dir_path (),
-             const location& l = location ())
+             process::pipe err = {-1, 2},
+             const location& l = {})
   {
-    return run_start (pe, args.data (), in, out, error, cwd, l);
-  }
-
-  inline void
-  run (const process_env& pe, // Implicit-constructible from process_path.
-       const char* args[])
-  {
-    process pr (run_start (pe, args));
-    run_finish (args, pr);
-  }
-
-  inline void
-  run (const process_env& pe,  // Implicit-constructible from process_path.
-       cstrings& args)
-  {
-    run (pe, args.data ());
-  }
-
-  inline void
-  run (const process_path& p,
-       const char* args[],
-       const dir_path& cwd,
-       const char* const* env = nullptr)
-  {
-    process pr (run_start (process_env (p, env), args, 0, 1, true, cwd));
-    run_finish (args, pr);
-  }
-
-  inline void
-  run (const process_path& p,
-       cstrings& args,
-       const dir_path& cwd,
-       const char* const* env = nullptr)
-  {
-    run (p, args.data (), cwd, env);
+    return run_start (pe, args.data (), in, out, move (err), l);
   }
 
   // As above, but search for the process (including updating args[0]) and
@@ -391,16 +320,16 @@ namespace build2
              const char* args[],
              int in = 0,
              int out = 1,
-             bool error = true,
-             const dir_path& cwd = dir_path (),
+             process::pipe err = {-1, 2},
              const char* const* env = nullptr,
-             const location& l = location ())
+             const dir_path& cwd = {},
+             const location& l = {})
   {
     process_path pp (run_search (args[0], l));
     return run_start (verbosity,
-                      process_env (pp, env), args,
-                      in, out, error,
-                      cwd, l);
+                      process_env (pp, cwd, env), args,
+                      in, out, move (err),
+                      l);
   }
 
   inline process
@@ -408,37 +337,184 @@ namespace build2
              cstrings& args,
              int in = 0,
              int out = 1,
-             bool error = true,
-             const dir_path& cwd = dir_path (),
+             process::pipe err = {-1, 2},
              const char* const* env = nullptr,
-             const location& l = location ())
+             const dir_path& cwd = {},
+             const location& l = {})
   {
-    return run_start (verbosity, args.data (), in, out, error, cwd, env, l);
+    return run_start (verbosity,
+                      args.data (),
+                      in, out, move (err),
+                      env, cwd, l);
+  }
+
+  // Wait for process termination returning true if the process exited
+  // normally with a zero code and false otherwise. The latter case is
+  // normally followed up with a call to run_finish().
+  //
+  LIBBUILD2_SYMEXPORT bool
+  run_wait (const char* const* args, process&, const location& = location ());
+
+  bool
+  run_wait (const cstrings& args, process&, const location& = location ());
+
+  // Wait for process termination. Issue diagnostics and throw failed in case
+  // of abnormal termination. If the process has terminated normally but with
+  // a non-zero exit status, then assume the diagnostics has already been
+  // issued and just throw failed. The line argument is used in cooperation
+  // with run_start() in case stderr is redirected to stdout (see the
+  // implementation for details).
+  //
+  void
+  run_finish (const char* const* args,
+              process&,
+              const string& line = string (),
+              const location& = location ());
+
+  void
+  run_finish (const cstrings& args,
+              process&,
+              const location& = location ());
+
+  // As above but if the process has exited normally with a non-zero code,
+  // then return false rather than throwing.
+  //
+  bool
+  run_finish_code (const char* const* args,
+                   process&,
+                   const string& = string (),
+                   const location& = location ());
+
+  bool
+  run_finish_code (const cstrings& args,
+                   process&,
+                   const location& = location ());
+
+  // As above but with diagnostics buffering.
+  //
+  // Specifically, this version first waits for the process termination, then
+  // calls diag_buffer::close(verbosity), and finally throws failed if the
+  // process didn't exit with 0 code. Note that what gets printed in case of
+  // normal termination with non-0 code is different compared to the above
+  // versions (see diag_buffer::close() for details).
+  //
+  class diag_buffer;
+
+  void
+  run_finish (diag_buffer&,
+              const char* const* args,
+              process&,
+              uint16_t verbosity = 1,
+              const location& = location ());
+
+  void
+  run_finish (diag_buffer&,
+              const cstrings& args,
+              process&,
+              uint16_t verbosity = 1,
+              const location& = location ());
+
+  // As above but if the process has exited normally with a non-zero code,
+  // then return false rather than throwing. Note: diag_buffer::close() is
+  // called with omit_normall=true assuming appropriate custom diagnostics
+  // will be issued, if required.
+  //
+  bool
+  run_finish_code (diag_buffer&,
+                   const char* const* args,
+                   process&,
+                   uint16_t verbosity = 1,
+                   const location& = location ());
+
+  bool
+  run_finish_code (diag_buffer&,
+                   const cstrings& args,
+                   process&,
+                   uint16_t verbosity = 1,
+                   const location& = location ());
+
+  // Run the process with the specified arguments by calling the above start
+  // and finish functions. Buffer diagnostics unless in the load phase.
+  //
+  LIBBUILD2_SYMEXPORT void
+  run (context&,
+       const process_env& pe, // Implicit-constructible from process_path.
+       const char* args[]);
+
+  LIBBUILD2_SYMEXPORT void
+  run (diag_buffer&,
+       const process_env& pe,
+       const char* args[]);
+
+  inline void
+  run (context& ctx,
+       const process_env& pe,
+       cstrings& args)
+  {
+    run (ctx, pe, args.data ());
   }
 
   inline void
-  run (uint16_t verbosity,
+  run (diag_buffer& dbuf,
+       const process_env& pe,
+       cstrings& args)
+  {
+    run (dbuf, pe, args.data ());
+  }
+
+  // As above but pass cwd/env vars as arguments rather than as part of
+  // process_env.
+  //
+  inline void
+  run (context& ctx,
+       const process_path& p,
+       const char* args[],
+       const char* const* env,
+       const dir_path& cwd = {})
+  {
+    run (ctx, process_env (p, cwd, env), args);
+  }
+
+  inline void
+  run (diag_buffer& dbuf,
+       const process_path& p,
        const char* args[],
-       const dir_path& cwd = dir_path (),
-       const char* const* env = nullptr)
+       const char* const* env,
+       const dir_path& cwd = {})
   {
-    process pr (run_start (verbosity, args, 0, 1, true, cwd, env));
-    run_finish (args, pr);
+    run (dbuf, process_env (p, cwd, env), args);
   }
 
   inline void
-  run (uint16_t verbosity,
+  run (context& ctx,
+       const process_path& p,
        cstrings& args,
-       const dir_path& cwd = dir_path (),
-       const char* const* env = nullptr)
+       const char* const* env,
+       const dir_path& cwd = {})
   {
-    run (verbosity, args.data (), cwd, env);
+    run (ctx, p, args.data (), env, cwd);
+  }
+
+  inline void
+  run (diag_buffer& dbuf,
+       const process_path& p,
+       cstrings& args,
+       const char* const* env,
+       const dir_path& cwd = {})
+  {
+    run (dbuf, p, args.data (), env, cwd);
   }
 
   // Start the process as above and then call the specified function on each
   // trimmed line of the output until it returns a non-empty object T (tested
   // with T::empty()) which is then returned to the caller.
   //
+  // If verbosity is specified, print the process commands line at that level.
+  //
+  // If error is false, then redirecting stderr to stdout (can be used to
+  // suppress and/or analyze diagnostics from the child process). Otherwise,
+  // buffer diagnostics unless in the load phase.
+  //
   // The predicate can move the value out of the passed string but, if error
   // is false, only in case of a "content match" (so that any diagnostics
   // lines are left intact). The function signature should be:
@@ -454,7 +530,8 @@ namespace build2
   //
   template <typename T, typename F>
   T
-  run (uint16_t verbosity,
+  run (context&,
+       uint16_t verbosity,
        const process_env&, // Implicit-constructible from process_path.
        const char* args[],
        F&&,
@@ -464,20 +541,117 @@ namespace build2
 
   template <typename T, typename F>
   inline T
-  run (const process_env& pe, // Implicit-constructible from process_path.
+  run (context& ctx,
+       uint16_t verbosity,
+       const process_env& pe,
+       cstrings& args,
+       F&& f,
+       bool error = true,
+       bool ignore_exit = false,
+       sha256* checksum = nullptr)
+  {
+    return run<T> (ctx,
+                   verbosity,
+                   pe, args.data (),
+                   forward<F> (f),
+                   error, ignore_exit, checksum);
+  }
+
+  template <typename T, typename F>
+  T
+  run (diag_buffer&,
+       uint16_t verbosity,
+       const process_env&,
+       const char* args[],
+       F&&,
+       bool ignore_exit = false,
+       sha256* checksum = nullptr);
+
+  template <typename T, typename F>
+  inline T
+  run (diag_buffer& dbuf,
+       uint16_t verbosity,
+       const process_env& pe,
+       cstrings& args,
+       F&& f,
+       bool ignore_exit = false,
+       sha256* checksum = nullptr)
+  {
+    return run<T> (dbuf,
+                   verbosity,
+                   pe, args.data (),
+                   forward<F> (f),
+                   ignore_exit, checksum);
+  }
+
+  template <typename T, typename F>
+  inline T
+  run (context& ctx,
+       const process_env& pe,
        const char* args[],
        F&& f,
        bool error = true,
        bool ignore_exit = false,
        sha256* checksum = nullptr)
   {
-    return run<T> (
-      verb_never, pe, args, forward<F> (f), error, ignore_exit, checksum);
+    return run<T> (ctx,
+                   verb_never,
+                   pe, args,
+                   forward<F> (f),
+                   error, ignore_exit, checksum);
+  }
+
+  template <typename T, typename F>
+  inline T
+  run (context& ctx,
+       const process_env& pe,
+       cstrings& args,
+       F&& f,
+       bool error = true,
+       bool ignore_exit = false,
+       sha256* checksum = nullptr)
+  {
+    return run<T> (ctx,
+                   pe, args.data (),
+                   forward<F> (f),
+                   error, ignore_exit, checksum);
+  }
+
+  template <typename T, typename F>
+  inline T
+  run (diag_buffer& dbuf,
+       const process_env& pe,
+       const char* args[],
+       F&& f,
+       bool ignore_exit = false,
+       sha256* checksum = nullptr)
+  {
+    return run<T> (dbuf,
+                   verb_never,
+                   pe, args,
+                   forward<F> (f),
+                   ignore_exit, checksum);
+  }
+
+  template <typename T, typename F>
+  inline T
+  run (diag_buffer& dbuf,
+       const process_env& pe,
+       cstrings& args,
+       F&& f,
+       bool ignore_exit = false,
+       sha256* checksum = nullptr)
+  {
+    return run<T> (dbuf,
+                   pe, args.data (),
+                   forward<F> (f),
+                   ignore_exit, checksum);
   }
 
   template <typename T, typename F>
   inline T
-  run (uint16_t verbosity,
+  run (context& ctx,
+       uint16_t verbosity,
        const char* args[],
        F&& f,
        bool error = true,
@@ -485,15 +659,71 @@ namespace build2
        sha256* checksum = nullptr)
   {
     process_path pp (run_search (args[0]));
-    return run<T> (
-      verbosity, pp, args, forward<F> (f), error, ignore_exit, checksum);
+    return run<T> (ctx,
+                   verbosity,
+                   pp, args,
+                   forward<F> (f),
+                   error, ignore_exit, checksum);
+  }
+
+  template <typename T, typename F>
+  inline T
+  run (context& ctx,
+       uint16_t verbosity,
+       cstrings& args,
+       F&& f,
+       bool error = true,
+       bool ignore_exit = false,
+       sha256* checksum = nullptr)
+  {
+    return run<T> (ctx,
+                   verbosity,
+                   args.data (),
+                   forward<F> (f),
+                   error, ignore_exit, checksum);
+  }
+
+  template <typename T, typename F>
+  inline T
+  run (diag_buffer& dbuf,
+       uint16_t verbosity,
+       const char* args[],
+       F&& f,
+       bool ignore_exit = false,
+       sha256* checksum = nullptr)
+  {
+    process_path pp (run_search (args[0]));
+    return run<T> (dbuf,
+                   verbosity,
+                   pp, args,
+                   forward<F> (f),
+                   ignore_exit, checksum);
+  }
+
+  template <typename T, typename F>
+  inline T
+  run (diag_buffer& dbuf,
+       uint16_t verbosity,
+       cstrings& args,
+       F&& f,
+       bool ignore_exit = false,
+       sha256* checksum = nullptr)
+  {
+    return run<T> (dbuf,
+                   verbosity,
+                   args.data (),
+                   forward<F> (f),
+                   ignore_exit, checksum);
   }
 
+  // As above but run a program without any arguments or with one argument.
+  //
   // run <prog>
   //
   template <typename T, typename F>
   inline T
-  run (uint16_t verbosity,
+  run (context& ctx,
+       uint16_t verbosity,
        const path& prog,
        F&& f,
        bool error = true,
@@ -501,13 +731,17 @@ namespace build2
        sha256* checksum = nullptr)
   {
     const char* args[] = {prog.string ().c_str (), nullptr};
-    return run<T> (
-      verbosity, args, forward<F> (f), error, ignore_exit, checksum);
+    return run<T> (ctx,
+                   verbosity,
+                   args,
+                   forward<F> (f),
+                   error, ignore_exit, checksum);
   }
 
   template <typename T, typename F>
   inline T
-  run (uint16_t verbosity,
+  run (context& ctx,
+       uint16_t verbosity,
        const process_env& pe, // Implicit-constructible from process_path.
        F&& f,
        bool error = true,
@@ -515,15 +749,19 @@ namespace build2
        sha256* checksum = nullptr)
   {
     const char* args[] = {pe.path->recall_string (), nullptr};
-    return run<T> (
-      verbosity, pe, args, forward<F> (f), error, ignore_exit, checksum);
+    return run<T> (ctx,
+                   verbosity,
+                   pe, args,
+                   forward<F> (f),
+                   error, ignore_exit, checksum);
   }
 
   // run <prog> <arg>
   //
   template <typename T, typename F>
   inline T
-  run (uint16_t verbosity,
+  run (context& ctx,
+       uint16_t verbosity,
        const path& prog,
        const char* arg,
        F&& f,
@@ -532,13 +770,17 @@ namespace build2
        sha256* checksum = nullptr)
   {
     const char* args[] = {prog.string ().c_str (), arg, nullptr};
-    return run<T> (
-      verbosity, args, forward<F> (f), error, ignore_exit, checksum);
+    return run<T> (ctx,
+                   verbosity,
+                   args,
+                   forward<F> (f),
+                   error, ignore_exit, checksum);
   }
 
   template <typename T, typename F>
   inline T
-  run (uint16_t verbosity,
+  run (context& ctx,
+       uint16_t verbosity,
        const process_env& pe, // Implicit-constructible from process_path.
        const char* arg,
        F&& f,
@@ -547,10 +789,45 @@ namespace build2
        sha256* checksum = nullptr)
   {
     const char* args[] = {pe.path->recall_string (), arg, nullptr};
-    return run<T> (
-      verbosity, pe, args, forward<F> (f), error, ignore_exit, checksum);
+    return run<T> (ctx,
+                   verbosity,
+                   pe, args,
+                   forward<F> (f),
+                   error, ignore_exit, checksum);
   }
 
+  // As above but a lower-level interface that erases T and F and can also be
+  // used to suppress trimming.
+  //
+  // The passed function should return true if it should be called again
+  // (i.e., the object is still empty in the T & F interface) and false
+  // otherwise.
+  //
+  // Ruturn true on success and false on failure (only if ignore_exit is
+  // true). (In the latter case, the T & F interface makes the resulting
+  // object empty).
+  //
+  LIBBUILD2_SYMEXPORT bool
+  run (context&,
+       uint16_t verbosity,
+       const process_env&,
+       const char* args[],
+       const function<bool (string& line, bool last)>&,
+       bool trim = true,
+       bool err = true,
+       bool ignore_exit = false,
+       sha256* checksum = nullptr);
+
+  LIBBUILD2_SYMEXPORT bool
+  run (diag_buffer& dbuf,
+       uint16_t verbosity,
+       const process_env&,
+       const char* args[],
+       const function<bool (string& line, bool last)>&,
+       bool trim = true,
+       bool ignore_exit = false,
+       sha256* checksum = nullptr);
+
   // File descriptor streams.
   //
   fdpipe
diff --git a/libbuild2/utility.ixx b/libbuild2/utility.ixx
index aedfc94..f4b8152 100644
--- a/libbuild2/utility.ixx
+++ b/libbuild2/utility.ixx
@@ -6,42 +6,152 @@
 namespace build2
 {
   inline bool
-  run_wait (cstrings& args, process& pr, const location& loc)
+  run_wait (const cstrings& args, process& pr, const location& loc)
   {
     return run_wait (args.data (), pr, loc);
   }
 
-  // Note: currently this function is also used in a run() implementations.
+  // Note: currently this function is also used in run() implementations.
   //
   LIBBUILD2_SYMEXPORT bool
-  run_finish_impl (const char*[],
+  run_finish_impl (const char* const*,
                    process&,
-                   bool error,
+                   bool fail,
                    const string&,
                    const location& = location ());
 
+  LIBBUILD2_SYMEXPORT bool
+  run_finish_impl (diag_buffer&,
+                   const char* const*,
+                   process&,
+                   bool fail,
+                   uint16_t,
+                   const location&);
+
   inline void
-  run_finish (const char* args[],
+  run_finish (const char* const* args,
               process& pr,
               const string& l,
               const location& loc)
   {
-    run_finish_impl (args, pr, true /* error */, l, loc);
+    run_finish_impl (args, pr, true /* fail */, l, loc);
   }
 
   inline void
-  run_finish (cstrings& args, process& pr, const location& loc)
+  run_finish (const cstrings& args, process& pr, const location& loc)
   {
     run_finish (args.data (), pr, string (), loc);
   }
 
   inline bool
-  run_finish_code (const char* args[],
+  run_finish_code (const char* const* args,
                    process& pr,
                    const string& l,
                    const location& loc)
   {
-    return run_finish_impl (args, pr, false /* error */, l, loc);
+    return run_finish_impl (args, pr, false /* fail */, l, loc);
+  }
+
+  inline bool
+  run_finish_code (const cstrings& args, process& pr, const location& loc)
+  {
+    return run_finish_code (args.data (), pr, string (), loc);
+  }
+
+  inline void
+  run_finish (diag_buffer& dbuf,
+              const char* const* args,
+              process& pr,
+              uint16_t v,
+              const location& loc)
+  {
+    run_finish_impl (dbuf, args, pr, true /* fail */, v, loc);
+  }
+
+  inline void
+  run_finish (diag_buffer& dbuf,
+              const cstrings& args,
+              process& pr,
+              uint16_t v,
+              const location& loc)
+  {
+    run_finish_impl (dbuf, args.data (), pr, true /* fail */, v, loc);
+  }
+
+  inline bool
+  run_finish_code (diag_buffer& dbuf,
+                   const char* const* args,
+                   process& pr,
+                   uint16_t v,
+                   const location& loc)
+  {
+    return run_finish_impl (dbuf, args, pr, false /* fail */, v, loc);
+  }
+
+  inline bool
+  run_finish_code (diag_buffer& dbuf,
+                   const cstrings& args,
+                   process& pr,
+                   uint16_t v,
+                   const location& loc)
+  {
+    return run_finish_impl (dbuf, args.data (), pr, false /* fail */, v, loc);
+  }
+
+  template <typename T, typename F>
+  inline T
+  run (context& ctx,
+       uint16_t verbosity,
+       const process_env& pe,
+       const char* args[],
+       F&& f,
+       bool err,
+       bool ignore_exit,
+       sha256* checksum)
+  {
+    T r;
+    if (!run (ctx,
+              verbosity,
+              pe, args,
+              [&r, &f] (string& l, bool last) // Small function optimmization.
+              {
+                r = f (l, last);
+                return r.empty ();
+              },
+              true /* trim */,
+              err,
+              ignore_exit,
+              checksum))
+      r = T ();
+
+    return r;
+  }
+
+  template <typename T, typename F>
+  inline T
+  run (diag_buffer& dbuf,
+       uint16_t verbosity,
+       const process_env& pe,
+       const char* args[],
+       F&& f,
+       bool ignore_exit,
+       sha256* checksum)
+  {
+    T r;
+    if (!run (dbuf,
+              verbosity,
+              pe, args,
+              [&r, &f] (string& l, bool last) // Small function optimmization.
+              {
+                r = f (l, last);
+                return r.empty ();
+              },
+              true /* trim */,
+              ignore_exit,
+              checksum))
+      r = T ();
+
+    return r;
   }
 
   inline void
diff --git a/libbuild2/utility.txx b/libbuild2/utility.txx
index bb25288..d2fc29c 100644
--- a/libbuild2/utility.txx
+++ b/libbuild2/utility.txx
@@ -54,68 +54,4 @@ namespace build2
 
     return p;
   }
-
-  [[noreturn]] LIBBUILD2_SYMEXPORT void
-  run_io_error (const char*[], const io_error&);
-
-  template <typename T, typename F>
-  T
-  run (uint16_t verbosity,
-       const process_env& pe,
-       const char* args[],
-       F&& f,
-       bool err,
-       bool ignore_exit,
-       sha256* checksum)
-  {
-    process pr (run_start (verbosity,
-                           pe,
-                           args,
-                           0  /* stdin */,
-                           -1 /* stdout */,
-                           err));
-    T r;
-    string l; // Last line of output.
-
-    try
-    {
-      ifdstream is (move (pr.in_ofd), butl::fdstream_mode::skip);
-
-      // Make sure we keep the last line.
-      //
-      for (bool last (is.peek () == ifdstream::traits_type::eof ());
-           !last && getline (is, l); )
-      {
-        last = (is.peek () == ifdstream::traits_type::eof ());
-
-        trim (l);
-
-        if (checksum != nullptr)
-          checksum->append (l);
-
-        if (r.empty ())
-        {
-          r = f (l, last);
-
-          if (!r.empty () && checksum == nullptr)
-            break;
-        }
-      }
-
-      is.close ();
-    }
-    catch (const io_error& e)
-    {
-      if (run_wait (args, pr))
-        run_io_error (args, e);
-
-      // If the child process has failed then assume the io error was
-      // caused by that and let run_finish() deal with it.
-    }
-
-    if (!(run_finish_impl (args, pr, err, l) || ignore_exit))
-      r = T ();
-
-    return r;
-  }
 }
diff --git a/libbuild2/version/snapshot-git.cxx b/libbuild2/version/snapshot-git.cxx
index 2ae3f5b..c4168bf 100644
--- a/libbuild2/version/snapshot-git.cxx
+++ b/libbuild2/version/snapshot-git.cxx
@@ -21,7 +21,7 @@ namespace build2
     static global_cache<snapshot, dir_path> cache;
 
     snapshot
-    extract_snapshot_git (dir_path rep_root)
+    extract_snapshot_git (context& ctx, dir_path rep_root)
     {
       if (const snapshot* r = cache.find (rep_root))
         return *r;
@@ -82,7 +82,11 @@ namespace build2
       args[args_i + 1] = "--porcelain";
       args[args_i + 2] = nullptr;
 
+      // @@ PERF: redo with custom stream reading code (then could also
+      //    get rid of context).
+      //
       r.committed = run<string> (
+        ctx,
         3 /* verbosity */,
         pp,
         args,
@@ -108,7 +112,8 @@ namespace build2
       // (reluctantly) assume that the only reason git cat-file fails is if
       // there is no HEAD (that we equal with the "new repository" condition
       // which is, strictly speaking, might not be the case either). So we
-      // suppress any diagnostics, and handle non-zero exit code.
+      // suppress any diagnostics, and handle non-zero exit code (and so no
+      // diagnostics buffering is needed, plus we are in the load phase).
       //
       string data;
 
@@ -117,12 +122,12 @@ namespace build2
       args[args_i + 2] = "HEAD";
       args[args_i + 3] = nullptr;
 
-      process pr (run_start (3     /* verbosity */,
+      process pr (run_start (3       /* verbosity */,
                              pp,
                              args,
-                             0     /* stdin  */,
-                             -1    /* stdout */,
-                             false /* error  */));
+                             0       /* stdin  */,
+                             -1      /* stdout */,
+                             {-1, 1} /* stderr (to stdout) */));
 
       string l;
       try
diff --git a/libbuild2/version/snapshot.cxx b/libbuild2/version/snapshot.cxx
index d20e633..000bcba 100644
--- a/libbuild2/version/snapshot.cxx
+++ b/libbuild2/version/snapshot.cxx
@@ -12,7 +12,7 @@ namespace build2
   namespace version
   {
     snapshot
-    extract_snapshot_git (dir_path);
+    extract_snapshot_git (context&, dir_path);
 
     static const path git (".git");
 
@@ -46,7 +46,7 @@ namespace build2
         if (butl::entry_exists (d / git,
                                 true /* follow_symlinks */,
                                 true /* ignore_errors */))
-          return extract_snapshot_git (move (d));
+          return extract_snapshot_git (rs.ctx, move (d));
       }
 
       return snapshot ();