Avoid deadlock in freeTask (called by forkProcess)

Summary: Documented in more detail inline with the change.

Test Plan: validate

Reviewers: austin, simonmar, duncan

Reviewed By: austin, simonmar, duncan

Subscribers: simonmar, relrod, carter

Differential Revision: https://phabricator.haskell.org/D59

rts/Task.c

@@ -350,6 +350,20 @@ discardTasksExcept (Task *keep)
         next = task->all_next;
         if (task != keep) {
             debugTrace(DEBUG_sched, "discarding task %" FMT_SizeT "", (size_t)TASK_ID(task));
+#if defined(THREADED_RTS)
+            // It is possible that some of these tasks are currently blocked
+            // (in the parent process) either on their condition variable
+            // `cond` or on their mutex `lock`. If they are we may deadlock
+            // when `freeTask` attempts to call `closeCondition` or
+            // `closeMutex` (the behaviour of these functions is documented to
+            // be undefined in the case that there are threads blocked on
+            // them). To avoid this, we re-initialize both the condition
+            // variable and the mutex before calling `freeTask` (we do
+            // precisely the same for all global locks in `forkProcess`).
+            initCondition(&task->cond);
+            initMutex(&task->lock);
+#endif
+
             // Note that we do not traceTaskDelete here because
             // we are not really deleting a task.
             // The OS threads for all these tasks do not exist in