Re: Re: [PATCH] Atomic pgrename on Windows

From: David Steele <david(at)pgmasters(dot)net>
To: Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>
Cc: Magnus Hagander <magnus(at)hagander(dot)net>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Re: [PATCH] Atomic pgrename on Windows
Date: 2018-03-06 14:04:59
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Hi Alexander,

On 1/20/18 10:13 AM, Magnus Hagander wrote:
> On Tue, Nov 28, 2017 at 2:47 AM, Michael Paquier
> <michael(dot)paquier(at)gmail(dot)com <mailto:michael(dot)paquier(at)gmail(dot)com>> wrote:
> On Mon, Nov 27, 2017 at 3:28 PM, Alexander Korotkov
> <a(dot)korotkov(at)postgrespro(dot)ru <mailto:a(dot)korotkov(at)postgrespro(dot)ru>> wrote:
> > Attached patch atomic-pgrename-windows-1.patch fixes this problem.  It
> > appears to be possible to atomically replace file on Windows – ReplaceFile()
> > does that.  ReplaceFiles() requires target file to exist, this is why we
> > still need to call MoveFileEx() when it doesn't exist.
> Do you think that it could be safer to unlink the target file first
> with pgunlink()? This way you make sure that the target file is
> removed and not locked. This change makes me worrying about the
> introduction of more race conditions.
> Unlinking it first seems dangerous, as pointed out by Andres.
> What about first trying ReplaceFile() and then if it fails with "target
> doesn't exist", then call MoveFileEx().
> Or the other way around -- try MoveFileEx() first since that seems to
> work most of the time today (if it mostly broke we'd be in trouble
> already), and if it fails with a sharing violation, try ReplaceFile()?
> And perhaps end up doing it something similar to what we do with shared
> memory which is just to loop over it and try  each a couple of time,
> before giving up and failing?

This patch was mistakenly left as Needs Review during the last
commitfest but it's pretty clear that a new patch is required.

This certainly sounds like a non-trivial change. Perhaps it should be
submitted for PG12?


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