boundTaskExiting: don't set task->stopped unless this is the last call (#4850)

The bug in this case was that we had a worker thread making a foreign
call which invoked a callback (in this case it was performGC, I
think).  When the callback ended, boundTaskExiting() was setting
task->stopped, but the Task is now per-OS-thread, so it is shared by
the worker that made the original foreign call.  When the foreign call
returned, because task->stopped was set, the worker was not placed on
the queue of spare workers.  Somehow the worker woke up again, and
found the spare_workers queue empty, which lead to a crash.

Two bugs here: task->stopped should not have been set by
boundTaskExiting (this broke when I split the Task and InCall structs,
in 6.12.2), and releaseCapabilityAndQueueWorker() should not be
testing task->stopped anyway, because it should only ever be called
when task->stopped is false (this is now an assertion).

1 files changed, 8 insertions, 2 deletions

diff --git a/rts/Task.c b/rts/Task.c
index a15758c4f1..a5de804418 100644
--- a/rts/Task.c
+++ b/rts/Task.c
@@ -270,8 +270,6 @@ newBoundTask (void)
 void
 boundTaskExiting (Task *task)
 {
-    task->stopped = rtsTrue;
-
 #if defined(THREADED_RTS)
     ASSERT(osThreadId() == task->id);
 #endif
@@ -279,6 +277,14 @@ boundTaskExiting (Task *task)
 
     endInCall(task);
 
+    // Set task->stopped, but only if this is the last call (#4850).
+    // Remember that we might have a worker Task that makes a foreign
+    // call and then a callback, so it can transform into a bound
+    // Task for the duration of the callback.
+    if (task->incall == NULL) {
+        task->stopped = rtsTrue;
+    }
+
     debugTrace(DEBUG_sched, "task exiting");
 }