Don't reset latch in ConditionVariablePrepareToSleep().

It's not OK to do that without calling CHECK_FOR_INTERRUPTS().
Let the next wait loop deal with it, following the usual pattern.

One consequence of this bug was that a SIGTERM delivered in a very
narrow timing window could leave a parallel worker process waiting
forever for a condition variable that will never be signaled, after
an error was raised in other process.

The code is a bit different in the stable branches due to commit
1321509f, making problems less likely there.  No back-patch for now,
but we may finish up deciding to make a similar change after more
discussion.

Author: Thomas Munro
Reviewed-by: Shawn Debnath
Reported-by: Tomas Vondra
Discussion: https://postgr.es/m/CA%2BhUKGJOm8zZHjVA8svoNT3tHY0XdqmaC_kHitmgXDQM49m1dA%40mail.gmail.com

diff --git a/src/backend/storage/lmgr/condition_variable.c b/src/backend/storage/lmgr/condition_variable.c
index 0e7f07961eedcadcab9ec64412ca7c7814042933..37b6a4eecdb68a4a97de33a2c225cc673abec550 100644 (file)
--- a/src/backend/storage/lmgr/condition_variable.c
+++ b/src/backend/storage/lmgr/condition_variable.c
@@ -93,12 +93,6 @@ ConditionVariablePrepareToSleep(ConditionVariable *cv)
    /* Record the condition variable on which we will sleep. */
    cv_sleep_target = cv;
 
-   /*
-    * Reset my latch before adding myself to the queue, to ensure that we
-    * don't miss a wakeup that occurs immediately.
-    */
-   ResetLatch(MyLatch);
-
    /* Add myself to the wait queue. */
    SpinLockAcquire(&cv->mutex);
    proclist_push_tail(&cv->wakeup, pgprocno, cvWaitLink);