From 048bcc1604a7e8aca666e219c0fdc3cd5517029e Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Fri, 2 Jun 2023 06:10:34 +0200
Subject: Fix data race in library metadata protocol logic

---
 libbuild2/cc/common.cxx        | 5 +++--
 libbuild2/cc/windows-rpath.cxx | 8 ++++++--
 libbuild2/target.hxx           | 6 +++++-
 3 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/libbuild2/cc/common.cxx b/libbuild2/cc/common.cxx
index c423c37..07a4d48 100644
--- a/libbuild2/cc/common.cxx
+++ b/libbuild2/cc/common.cxx
@@ -382,9 +382,10 @@ namespace build2
           for (const prerequisite_target& pt: l.prerequisite_targets[a])
           {
             // Note: adhoc prerequisites are not part of the library metadata
-            // protocol (and we should check for adhoc first to avoid races).
+            // protocol (and we should check for adhoc first to avoid races
+            // during execute).
             //
-            if (pt == nullptr || pt.adhoc ())
+            if (pt.adhoc () || pt == nullptr)
               continue;
 
             if (marked (pt))
diff --git a/libbuild2/cc/windows-rpath.cxx b/libbuild2/cc/windows-rpath.cxx
index 9387078..bd5a928 100644
--- a/libbuild2/cc/windows-rpath.cxx
+++ b/libbuild2/cc/windows-rpath.cxx
@@ -128,7 +128,9 @@ namespace build2
       library_cache lib_cache;
       for (const prerequisite_target& pt: t.prerequisite_targets[a])
       {
-        if (pt == nullptr || pt.adhoc ())
+        // Note: during execute so check for ad hoc first to avoid data races.
+        //
+        if (pt.adhoc () || pt == nullptr)
           continue;
 
         bool la;
@@ -255,7 +257,9 @@ namespace build2
       library_cache lib_cache;
       for (const prerequisite_target& pt: t.prerequisite_targets[a])
       {
-        if (pt == nullptr || pt.adhoc ())
+        // Note: during execute so check for ad hoc first to avoid data races.
+        //
+        if (pt.adhoc () || pt == nullptr)
           continue;
 
         bool la;
diff --git a/libbuild2/target.hxx b/libbuild2/target.hxx
index d4ad780..69714fc 100644
--- a/libbuild2/target.hxx
+++ b/libbuild2/target.hxx
@@ -940,7 +940,11 @@ namespace build2
     // (e.g., in match data) with a few well-known execeptions (see
     // group_recipe and inner_recipe).
     //
-    // Note that the recipe may modify this list. @@ TMP TSAN issue
+    // Note that the recipe may modify this list during execute. Normally this
+    // would be just blanking out of ad hoc prerequisites, in which case check
+    // for ad hoc first and for not NULL second if accessing prerequisites of
+    // targets that you did not execute (see the library metadata protocol in
+    // cc for an example).
     //
     mutable action_state<build2::prerequisite_targets> prerequisite_targets;
 
-- 
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