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 aced5a92b 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

diff --git a/src/backend/storage/lmgr/condition_variable.c b/src/backend/storage/lmgr/condition_variable.c
index 552bdaaf809c65b07cf848a72c80d0909474c643..730adf1c57ff3da5ba01b700b022af85955a1f62 100644 (file)
--- a/src/backend/storage/lmgr/condition_variable.c
+++ b/src/backend/storage/lmgr/condition_variable.c
@@ -70,10 +70,15 @@ 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;
@@ -121,16 +126,16 @@ ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info)
         * allows you to skip manipulation of the wait list, or not met initially,
         * in which case preparing first allows you to skip a spurious test of the
         * caller's 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);
-
        while (!done)
        {
                CHECK_FOR_INTERRUPTS();
@@ -246,7 +251,8 @@ 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.