From 0275a8661dce5b89960d2baf6245bf08679fb596 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Tue, 14 Mar 2017 14:12:01 +0200
Subject: Fix scheduler bug

---
 build2/scheduler     | 11 +++++++---
 build2/scheduler.txx | 57 ++++++++++++++++++++++++++++++++++++----------------
 2 files changed, 48 insertions(+), 20 deletions(-)

diff --git a/build2/scheduler b/build2/scheduler
index cfccefe..a8f81cd 100644
--- a/build2/scheduler
+++ b/build2/scheduler
@@ -463,6 +463,7 @@ namespace build2
     {
       size_t& s (tq.size);
       size_t& t (tq.tail);
+      size_t& m (tq.mark);
 
       if (s != task_queue_depth_)
       {
@@ -470,6 +471,10 @@ namespace build2
         //                                         |       |    |
         t = s != 0 ? (t != task_queue_depth_ - 1 ? t + 1 : 0) : t;
         s++;
+
+        if (m == task_queue_depth_) // Enable the mark if first push.
+          m = t;
+
         queued_task_count_.fetch_add (1, std::memory_order_release);
         return &tq.data[t];
       }
@@ -523,11 +528,11 @@ namespace build2
 
       task_data& td (tq.data[t]);
 
-      // Save the old queue mark and set the new one in case the task we are
-      // about to run adds sub-tasks.
+      // Save the old queue mark and disable it in case the task we are about
+      // to run adds sub-tasks. The first push(), if any, will reset it.
       //
       size_t om (m);
-      m = t; // Where next push() will go.
+      m = task_queue_depth_;
 
       //                     normal  wrap                     empty
       //                     |       |                        |
diff --git a/build2/scheduler.txx b/build2/scheduler.txx
index c9d2d14..3410b6f 100644
--- a/build2/scheduler.txx
+++ b/build2/scheduler.txx
@@ -18,23 +18,29 @@ namespace build2
     static_assert (std::is_trivially_destructible<task>::value,
                    "not trivially destructible");
 
-    // Push into the queue unless we are running serially or the queue is
-    // full.
+    // If running serially, then run the task synchronously. In this case
+    // there is no need to mess with task count.
     //
-    task_data* td (nullptr);
-
-    if (max_active_ != 1)
+    if (max_active_ == 1)
     {
-      task_queue* tq (task_queue_); // Single load.
-      if (tq == nullptr)
-        tq = &create_queue ();
+      forward<F> (f) (forward<A> (a)...);
+      return false;
+    }
+
+    // Try to push the task into the queue falling back to running serially
+    // if the queue is full.
+    //
+    task_queue* tq (task_queue_); // Single load.
+    if (tq == nullptr)
+      tq = &create_queue ();
 
+    {
       lock ql (tq->mutex);
 
       if (tq->shutdown)
         throw system_error (ECANCELED, std::system_category ());
 
-      if ((td = push (*tq)) != nullptr)
+      if (task_data* td = push (*tq))
       {
         // Package the task (under lock).
         //
@@ -47,16 +53,33 @@ namespace build2
         td->thunk = &task_thunk<F, A...>;
       }
       else
+      {
         tq->stat_full++;
-    }
 
-    // If serial/full, then run the task synchronously. In this case there is
-    // no need to mess with task count.
-    //
-    if (td == nullptr)
-    {
-      forward<F> (f) (forward<A> (a)...);
-      return false;
+        // We have to perform the same mark adjust/restore as in pop_back()
+        // since the task we are about to execute synchronously may try to
+        // work the queue.
+        //
+        // It would have been cleaner to package all this logic into push()
+        // but that would require dragging function/argument types into it.
+        //
+        size_t& s (tq->size);
+        size_t& t (tq->tail);
+        size_t& m (tq->mark);
+
+        size_t om (m);
+        m = task_queue_depth_;
+
+        {
+          ql.unlock ();
+          forward<F> (f) (forward<A> (a)...); // Should not throw.
+          ql.lock ();
+        }
+
+        m = s == 0 ? t : om;
+
+        return false;
+      }
     }
 
     // Increment the task count.
-- 
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