Set builtin result while holding mutex

While the original code was probably correct, there is suspicion this
is causing a TSAN false-positive.

3 files changed, 14 insertions, 13 deletions

diff --git a/libbutl/builtin.cxx b/libbutl/builtin.cxx
index b13a59a..6f451bf 100644
--- a/libbutl/builtin.cxx
+++ b/libbutl/builtin.cxx
@@ -2177,17 +2177,17 @@ namespace butl
   {
     unique_ptr<builtin::async_state> s (
       new builtin::async_state (
+        r,
         [fn,
-         &r,
          &args,
          in = move (in), out = move (out), err = move (err),
          &cwd,
-         &cbs] () mutable noexcept
+         &cbs] () mutable noexcept -> uint8_t
         {
-          r = fn (args,
-                  move (in), move (out), move (err),
-                  cwd,
-                  cbs);
+          return fn (args,
+                     move (in), move (out), move (err),
+                     cwd,
+                     cbs);
         }));
 
     return builtin (r, move (s));
diff --git a/libbutl/builtin.hxx b/libbutl/builtin.hxx
index b8546be..b301f8a 100644
--- a/libbutl/builtin.hxx
+++ b/libbutl/builtin.hxx
@@ -90,8 +90,7 @@ namespace butl
       // be able to capture auto_fd by value in a lambda, etc).
       //
       template <typename F>
-      explicit
-      async_state (F);
+      async_state (uint8_t&, F);
     };
 
     builtin (std::uint8_t& r, std::unique_ptr<async_state>&& s = nullptr)
diff --git a/libbutl/builtin.ixx b/libbutl/builtin.ixx
index 24fbae3..d77590b 100644
--- a/libbutl/builtin.ixx
+++ b/libbutl/builtin.ixx
@@ -47,13 +47,14 @@ namespace butl
   //
   template <typename F>
   inline builtin::async_state::
-  async_state (F f)
-      : thread ([f = std::move (f), this] () mutable noexcept
+  async_state (uint8_t& r, F f)
+      : thread ([this, &r, f = std::move (f)] () mutable noexcept
                 {
-                  f ();
+                  uint8_t t (f ());
 
                   {
                     unique_lock l (this->mutex);
+                    r = t;
                     finished = true;
                   }
 
@@ -68,9 +69,10 @@ namespace butl
   {
     std::unique_ptr<builtin::async_state> s (
       new builtin::async_state (
-        [f = std::move (f), &r] () mutable noexcept
+        r,
+        [f = std::move (f)] () mutable noexcept -> uint8_t
         {
-          r = f ();
+          return f ();
         }));
 
     return builtin (r, move (s));