Re: prevent immature WAL streaming

On 2021-Aug-24, Bossart, Nathan wrote:

> I wonder if we need to move the call to RegisterSegmentBoundary() to
> somewhere before WALInsertLockRelease() for this to work as intended.
> Right now, boundary registration could take place after the flush
> pointer has been advanced, which means GetSafeFlushRecPtr() could
> still return an unsafe position.

Yeah, you're right -- that's a definite risk.  I didn't try to reproduce
a problem with that, but it is seems pretty obvious that it can happen.

I didn't have a lot of luck with a reliable reproducer script.  I was
able to reproduce the problem starting with Ryo Matsumura's script and
attaching a replica; most of the time the replica would recover by
restarting from a streaming position earlier than where the problem
occurred; but a few times it would just get stuck with a WAL segment
containing a bogus record.  Then, after patch, the problem no longer
occurs.

I attach the patch with the change you suggested.

-- 
Álvaro Herrera              Valdivia, Chile  —  https://www.EnterpriseDB.com/
Tom: There seems to be something broken here.
Teodor: I'm in sackcloth and ashes...  Fixed.
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