Re: Loaded footgun open_datasync on Windows

From: Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
To: Noah Misch <noah(at)leadboat(dot)com>
Cc: Michael Paquier <michael(at)paquier(dot)xyz>, Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Loaded footgun open_datasync on Windows
Date: 2018-09-10 14:46:40
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Noah Misch wrote:
> On Wed, Jun 27, 2018 at 12:09:24PM +0200, Laurenz Albe wrote:
> > Michael Paquier wrote:
> > > > I have added it to the July commitfest.
> > >
> > > Have you looked at the possibility of removing the log file constraints
> > > in pg_upgrade with the change you are doing here so as things would be
> > > more consistent with non-Windows platforms, simplifying some code on the
> > > way?
> >
> > Can you explain what the "log file constraints" are?
> > If it is in any way related, and I can handle it, sure.
> See this comment in pg_upgrade.h:
> /*
> * WIN32 files do not accept writes from multiple processes
> *
> * On Win32, we can't send both pg_upgrade output and command output to the
> * same file because we get the error: "The process cannot access the file
> * because it is being used by another process." so send the pg_ctl
> * command-line output to a new file, rather than into the server log file.
> * Ideally we could use UTILITY_LOG_FILE for this, but some Windows platforms
> * keep the pg_ctl output file open by the running postmaster, even after
> * pg_ctl exits.
> *
> * We could use the Windows pgwin32_open() flags to allow shared file
> * writes but is unclear how all other tools would use those flags, so
> * we just avoid it and log a little differently on Windows; we adjust
> * the error message appropriately.
> */
> If you grep src/bin/pg_upgrade for WIN32, roughly a third of the hits are
> workarounds for this problem. I agree with Michael that removing those
> workarounds would be a good test of frontend pgwin32_open() and worth
> including in the initial commit.

Thank you for the explanation.

I didn't get pg_upgrade to work without the log file hacks; I suspect that there
is more than just log file locking going on, but my Windows skills are limited.

How shall I proceed?
I think that it is important to get pg_test_fsync to work correctly on Windows,
and if my patch does not break the buildfarm, that's what it does.

I have attached a new version, the previous one was bit-rotted.

Laurenz Albe

Attachment Content-Type Size
0001-Use-pgwin32_open-in-frontend-code-on-Windows_V3.patch text/x-patch 2.4 KB

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