pgsql: Allow ConditionVariable[PrepareTo]Sleep to auto-switch betweenC

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От Tom Lane
Тема pgsql: Allow ConditionVariable[PrepareTo]Sleep to auto-switch betweenC
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Msg-id E1eYwvr-0004DY-P6@gemulon.postgresql.org
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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

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/4af2190eb04b6a9160f2820e00802eabc78b4b1c

Modified Files
--------------
src/backend/storage/lmgr/condition_variable.c | 22 ++++++++++++++--------
1 file changed, 14 insertions(+), 8 deletions(-)


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