Re: Re: [PATCH] Atomic pgrename on Windows

On 1/20/18 10:13 AM, Magnus Hagander wrote:
>
> On Tue, Nov 28, 2017 at 2:47 AM, Michael Paquier
> <michael.paquier@gmail.com <mailto:michael.paquier@gmail.com>> wrote:
>
>     On Mon, Nov 27, 2017 at 3:28 PM, Alexander Korotkov
>     <a.korotkov@postgrespro.ru <mailto:a.korotkov@postgrespro.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.