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Spurious standby query cancellations

From: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
To: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Spurious standby query cancellations
Date: 2015-08-27 21:55:18
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
In ResolveRecoveryConflictWithLock, there is the comment:

     * If blowing away everybody with conflicting locks doesn't work, after
     * the first two attempts then we just start blowing everybody away
     * it does work.

But what it does is something different than that.

At least under some conditions, it will wait patiently for all initially
conflicting locks to go away.  If that doesn't work twice (because new
lockers joined while we were waiting for the old ones to go away), then it
will wait patiently for all transactions throughout the system to go away
even if they don't conflict, perhaps not even realizing that the lock has
become free in the mean time.

Then when its patience runs out, it kills everything on the system.  But it
never tried to blow away just the conflicting locks, instead it just tried
to wait them out.

The fact that trying to wait them out didn't work (twice) doesn't mean that
killing them wouldn't have worked.

I think that it was intended that num_attempts would get incremented only
once WaitExceedsMaxStandbyDelay becomes true, but that is not what happens
with the current code.

Currently WaitExceedsMaxStandbyDelay only has one caller.  I think it we
should take the sleep code out of that function and move it into the
existing call site, and then have ResolveRecoveryConflictWithLock check
with WaitExceedsMaxStandbyDelay before incrementing num_attempts.



Attachment: standby_lock_cancel_v1.patch
Description: application/octet-stream (2.1 KB)


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