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Commit4af2190

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Allow ConditionVariable[PrepareTo]Sleep to auto-switch between CVs.
The original coding here insisted that callers manually cancel any preparedsleep 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 biteus in future if the use of condition variables increases; anything we cando to make the use of this API simpler and more robust is attractive.Hence, allow these functions to automatically switch their attention toa different CV when required. This is safe for the same reason it was OKfor commitaced5a9 to let a broadcast operation cancel any prepared CVsleep: whenever we return to the other test-and-sleep loop, we willautomatically re-prepare that CV, paying at most an extra test of thatloop's exit condition.Back-patch to v10 where condition variables were introduced. Ordinarilywe would probably not back-patch a change like this, but since it does notinvalidate any coding pattern that was legal before, it seems safe enough.Furthermore, there's an open bug in replorigin_drop() for which thesimplest fix requires this. Even if we chose to fix that in some morecomplicated way, the hazard would remain that we might back-patch someother bug fix that requires this behavior.Patch by me, reviewed by Thomas Munro.Discussion:https://postgr.es/m/2437.1515368316@sss.pgh.pa.us
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‎src/backend/storage/lmgr/condition_variable.c

Lines changed: 14 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -70,10 +70,15 @@ ConditionVariablePrepareToSleep(ConditionVariable *cv)
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}
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/*
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* It's not legal to prepare a sleep until the previous sleep has been
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* completed or canceled.
73+
* If some other sleep is already prepared, cancel it; this is necessary
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* because we have just one static variable tracking the prepared sleep,
75+
* and also only one cvWaitLink in our PGPROC. It's okay to do this
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* because whenever control does return to the other test-and-sleep loop,
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* its ConditionVariableSleep call will just re-establish that sleep as
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* the prepared one.
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*/
76-
Assert(cv_sleep_target==NULL);
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if (cv_sleep_target!=NULL)
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ConditionVariableCancelSleep();
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/* Record the condition variable on which we will sleep. */
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cv_sleep_target=cv;
@@ -121,16 +126,16 @@ ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info)
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* allows you to skip manipulation of the wait list, or not met initially,
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* in which case preparing first allows you to skip a spurious test of the
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* caller's exit condition.
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*
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* If we are currently prepared to sleep on some other CV, we just cancel
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* that and prepare this one; see ConditionVariablePrepareToSleep.
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*/
125-
if (cv_sleep_target==NULL)
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if (cv_sleep_target!=cv)
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{
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ConditionVariablePrepareToSleep(cv);
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return;
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}
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/* Any earlier condition variable sleep must have been canceled. */
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Assert(cv_sleep_target==cv);
133-
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while (!done)
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{
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CHECK_FOR_INTERRUPTS();
@@ -246,7 +251,8 @@ ConditionVariableBroadcast(ConditionVariable *cv)
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* prepared CV sleep. The next call to ConditionVariableSleep will take
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* care of re-establishing the lost state.
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*/
249-
ConditionVariableCancelSleep();
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if (cv_sleep_target!=NULL)
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ConditionVariableCancelSleep();
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/*
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* Inspect the state of the queue. If it's empty, we have nothing to do.

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