De-pessimize ConditionVariableCancelSleep().

Commit b91dd9de was concerned with a theoretical problem with our
non-atomic condition variable operations.  If you stop sleeping, and
then cancel the sleep in a separate step, you might be signaled in
between, and that could be lost.  That doesn't matter for callers of
ConditionVariableBroadcast(), but callers of ConditionVariableSignal()
might be upset if a signal went missing like this.

Commit bc971f4025 interacted badly with that logic, because it doesn't
use ConditionVariableSleep(), which would normally put us back in the
wait list.  ConditionVariableCancelSleep() would be confused and think
we'd received an extra signal, and try to forward it to another backend,
resulting in wakeup storms.

New idea: ConditionVariableCancelSleep() can just return true if we've
been signaled.  Hypothetical users of ConditionVariableSignal() would
then still have a way to deal with rare lost signals if they are
concerned about that problem.

Back-patch to 16, where bc971f4025 arrived.

Reported-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/2840876b-4cfe-240f-0a7e-29ffd66711e7%40enterprisedb.com

src/backend/storage/lmgr/condition_variable.c

@@ -223,15 +223,17 @@ ConditionVariableTimedSleep(ConditionVariable *cv, long timeout,
  *
  * Do nothing if nothing is pending; this allows this function to be called
  * during transaction abort to clean up any unfinished CV sleep.
+ *
+ * Return true if we've been signaled.
  */
-void
+bool
 ConditionVariableCancelSleep(void)
 {
 	ConditionVariable *cv = cv_sleep_target;
 	bool		signaled = false;
 
 	if (cv == NULL)
-		return;
+		return false;
 
 	SpinLockAcquire(&cv->mutex);
 	if (proclist_contains(&cv->wakeup, MyProc->pgprocno, cvWaitLink))
@@ -240,15 +242,9 @@ ConditionVariableCancelSleep(void)
 		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;
+
+	return signaled;
 }
 
 /*

src/include/storage/condition_variable.h

@@ -56,7 +56,7 @@ extern void ConditionVariableInit(ConditionVariable *cv);
 extern void ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info);
 extern bool ConditionVariableTimedSleep(ConditionVariable *cv, long timeout,
 										uint32 wait_event_info);
-extern void ConditionVariableCancelSleep(void);
+extern bool ConditionVariableCancelSleep(void);
 
 /*
  * Optionally, ConditionVariablePrepareToSleep can be called before entering