Forward received condition variable signals on cancel.

After a process decides not to wait for a condition variable, it can
still consume a signal before it reaches ConditionVariableCancelSleep().
In that case, pass the signal on to another waiter if possible, so that
a signal doesn't go missing when there is another process ready to
receive it.

Author: Thomas Munro
Reviewed-by: Shawn Debnath
Discussion: https://postgr.es/m/CA%2BhUKGLQ_RW%2BXs8znDn36e-%2Bmq2--zrPemBqTQ8eKT-VO1OF4Q%40mail.gmail.com

src/backend/storage/lmgr/condition_variable.c

@@ -245,6 +245,7 @@ void
 ConditionVariableCancelSleep(void)
 {
 	ConditionVariable *cv = cv_sleep_target;
+	bool		signaled = false;
 
 	if (cv == NULL)
 		return;
@@ -252,8 +253,18 @@ ConditionVariableCancelSleep(void)
 	SpinLockAcquire(&cv->mutex);
 	if (proclist_contains(&cv->wakeup, MyProc->pgprocno, cvWaitLink))
 		proclist_delete(&cv->wakeup, MyProc->pgprocno, cvWaitLink);
+	else
+		signaled = true;
 	SpinLockRelease(&cv->mutex);
 
+	/*
+	 * If we've received a signal, pass it on to another waiting process, if
+	 * there is one.  Otherwise a call to ConditionVariableSignal() might get
+	 * lost, despite there being another process ready to handle it.
+	 */
+	if (signaled)
+		ConditionVariableSignal(cv);
+
 	cv_sleep_target = NULL;
 }