From af5ed67002b7031e1735968674bacb5ee868165f Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Fri, 30 Aug 2019 16:03:07 +0200
Subject: Cutoff diagnostics stack when switching to nested context

---
 libbuild2/algorithm.cxx |  4 ++++
 libbuild2/context.hxx   | 13 +++++++++--
 libbuild2/module.cxx    | 60 +++++++++++++++++++++++++++++--------------------
 3 files changed, 51 insertions(+), 26 deletions(-)

diff --git a/libbuild2/algorithm.cxx b/libbuild2/algorithm.cxx
index 7a616a5..511850d 100644
--- a/libbuild2/algorithm.cxx
+++ b/libbuild2/algorithm.cxx
@@ -142,6 +142,10 @@ namespace build2
 
   // target_lock
   //
+  // Note that the stack may contain locks for targets from multiple nested
+  // contexts. This should be harmless (deadlock detection-wise) since
+  // contexts are assumed non-overlapping.
+  //
   static
 #ifdef __cpp_thread_local
   thread_local
diff --git a/libbuild2/context.hxx b/libbuild2/context.hxx
index c2c191e..d2e38e7 100644
--- a/libbuild2/context.hxx
+++ b/libbuild2/context.hxx
@@ -102,9 +102,18 @@ namespace build2
   // @@ CTX: document (backlinks, non-overlap etc). RW story.
   //
   // A context can be preempted to execute another context (we do this, for
-  // example, to update build system modules).
+  // example, to update build system modules). When switching to such a nested
+  // context you may want to cutoff the diagnostics stack (and maybe insert
+  // your own entry), for example:
   //
-  // @@ Progress could clash.
+  //   diag_frame::stack_guard diag_cutoff (nullptr);
+  //
+  // As well as suppress progress which would otherwise clash (maybe in the
+  // future we can do save/restore but then we would need some indication that
+  // we have switched to another task).
+  //
+  // Note also that any given thread should not participate in multiple
+  // schedulers at the same time (see scheduler::join/leave() for details).
   //
   class LIBBUILD2_SYMEXPORT context
   {
diff --git a/libbuild2/module.cxx b/libbuild2/module.cxx
index 5cfb1ec..b5914cd 100644
--- a/libbuild2/module.cxx
+++ b/libbuild2/module.cxx
@@ -209,30 +209,42 @@ namespace build2
         action_targets tgs;
         action a (perform_id, update_id);
 
-        // Note that for now we suppress progress since it would clash with
-        // the progress of what we are already doing (maybe in the future we
-        // can do save/restore but then we would need some sort of diagnostics
-        // that we have switched to another task).
-        //
-        mo_perform.search  ({},      /* parameters */
-                            rs,      /* root scope */
-                            rs,      /* base scope */
-                            path (), /* buildfile */
-                            rs.find_target_key (lr.first, loc),
-                            loc,
-                            tgs);
-
-        mo_perform.match   ({},      /* parameters */
-                            a,
-                            tgs,
-                            1,       /* diag (failures only) */
-                            false    /* progress */);
-
-        mo_perform.execute ({},      /* parameters */
-                            a,
-                            tgs,
-                            1,       /* diag (failures only) */
-                            false    /* progress */);
+        {
+          // Cutoff the existing diagnostics stack and push our own entry.
+          //
+          diag_frame::stack_guard diag_cutoff (nullptr);
+
+          auto df = make_diag_frame (
+            [&loc, &mod](const diag_record& dr)
+            {
+              dr << info (loc) << "while loading build system module " << mod;
+            });
+
+          // Note that for now we suppress progress since it would clash with
+          // the progress of what we are already doing (maybe in the future we
+          // can do save/restore but then we would need some sort of
+          // diagnostics that we have switched to another task).
+          //
+          mo_perform.search  ({},      /* parameters */
+                              rs,      /* root scope */
+                              rs,      /* base scope */
+                              path (), /* buildfile */
+                              rs.find_target_key (lr.first, loc),
+                              loc,
+                              tgs);
+
+          mo_perform.match   ({},      /* parameters */
+                              a,
+                              tgs,
+                              1,       /* diag (failures only) */
+                              false    /* progress */);
+
+          mo_perform.execute ({},      /* parameters */
+                              a,
+                              tgs,
+                              1,       /* diag (failures only) */
+                              false    /* progress */);
+        }
 
         assert (tgs.size () == 1);
         const target& l (tgs[0].as_target ());
-- 
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