Allow ConditionVariable[PrepareTo]Sleep to auto-switch between CVs.

The original coding here insisted that callers manually cancel any prepared
sleep for one condition variable before starting a sleep on another one.
While that's not a huge burden today, it seems like a gotcha that will bite
us in future if the use of condition variables increases; anything we can
do to make the use of this API simpler and more robust is attractive.
Hence, allow these functions to automatically switch their attention to
a different CV when required.  This is safe for the same reason it was OK
for commit aced5a92 to let a broadcast operation cancel any prepared CV
sleep: whenever we return to the other test-and-sleep loop, we will
automatically re-prepare that CV, paying at most an extra test of that
loop's exit condition.

Back-patch to v10 where condition variables were introduced.  Ordinarily
we would probably not back-patch a change like this, but since it does not
invalidate any coding pattern that was legal before, it seems safe enough.
Furthermore, there's an open bug in replorigin_drop() for which the
simplest fix requires this.  Even if we chose to fix that in some more
complicated way, the hazard would remain that we might back-patch some
other bug fix that requires this behavior.

Patch by me, reviewed by Thomas Munro.

Discussion: https://postgr.es/m/2437.1515368316@sss.pgh.pa.us

src/backend/storage/lmgr/condition_variable.c

@@ -55,10 +55,6 @@ ConditionVariableInit(ConditionVariable *cv)
  * condition between calling ConditionVariablePrepareToSleep and calling
  * ConditionVariableSleep.  If that is inconvenient, omit calling
  * ConditionVariablePrepareToSleep.
- *
- * Only one condition variable can be used at a time, ie,
- * ConditionVariableCancelSleep must be called before any attempt is made
- * to sleep on a different condition variable.
  */
 void
 ConditionVariablePrepareToSleep(ConditionVariable *cv)
@@ -81,10 +77,15 @@ ConditionVariablePrepareToSleep(ConditionVariable *cv)
 	}
 
 	/*
-	 * It's not legal to prepare a sleep until the previous sleep has been
-	 * completed or canceled.
+	 * If some other sleep is already prepared, cancel it; this is necessary
+	 * because we have just one static variable tracking the prepared sleep,
+	 * and also only one cvWaitLink in our PGPROC.  It's okay to do this
+	 * because whenever control does return to the other test-and-sleep loop,
+	 * its ConditionVariableSleep call will just re-establish that sleep as
+	 * the prepared one.
 	 */
-	Assert(cv_sleep_target == NULL);
+	if (cv_sleep_target != NULL)
+		ConditionVariableCancelSleep();
 
 	/* Record the condition variable on which we will sleep. */
 	cv_sleep_target = cv;
@@ -133,16 +134,16 @@ ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info)
 	 * is recommended because it avoids manipulations of the wait list, or not
 	 * met initially, in which case preparing first is better because it
 	 * avoids one extra test of the exit condition.
+	 *
+	 * If we are currently prepared to sleep on some other CV, we just cancel
+	 * that and prepare this one; see ConditionVariablePrepareToSleep.
 	 */
-	if (cv_sleep_target == NULL)
+	if (cv_sleep_target != cv)
 	{
 		ConditionVariablePrepareToSleep(cv);
 		return;
 	}
 
-	/* Any earlier condition variable sleep must have been canceled. */
-	Assert(cv_sleep_target == cv);
-
 	do
 	{
 		CHECK_FOR_INTERRUPTS();
@@ -265,7 +266,8 @@ ConditionVariableBroadcast(ConditionVariable *cv)
 	 * prepared CV sleep.  The next call to ConditionVariableSleep will take
 	 * care of re-establishing the lost state.
 	 */
-	ConditionVariableCancelSleep();
+	if (cv_sleep_target != NULL)
+		ConditionVariableCancelSleep();
 
 	/*
 	 * Inspect the state of the queue.  If it's empty, we have nothing to do.

src/include/storage/condition_variable.h

@@ -40,9 +40,7 @@ extern void ConditionVariableInit(ConditionVariable *cv);
  * ConditionVariableSleep.  Spurious wakeups are possible, but should be
  * infrequent.  After exiting the loop, ConditionVariableCancelSleep must
  * be called to ensure that the process is no longer in the wait list for
- * the condition variable.  Only one condition variable can be used at a
- * time, ie, ConditionVariableCancelSleep must be called before any attempt
- * is made to sleep on a different condition variable.
+ * the condition variable.
  */
 extern void ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info);
 extern void ConditionVariableCancelSleep(void);