Re: win_flex.exe (and likely win_bison.exe) isn't concurrency safe

On 2022-09-02 Fr 18:43, Andres Freund wrote:
> Hi,
>
> building PG with meson on windows I occasionally got weird errors around
> scanners. Sometimes scanner generation would fail with
>
> win_flex.exe: error deleting file C:\Users\myadmin\AppData\Local\Temp\~flex_out_main_2
>
> sometimes the generated scanner would just be corrupted.
>
>
> I was confused by only hitting this in local VM, not in CI, but after finally
> hunting it down it made more sense:
> https://github.com/lexxmark/winflexbison/issues/86
>
> https://github.com/lexxmark/winflexbison/blob/master/flex/src/main.c#L1051
>
> It uses a temporary file name without any concurrency protection. Looks like
> windows' _tempnam is pretty darn awful and returns a predictable name as long
> as no conflicting file exists.
>
> Our documentation doesn't point to winflexbison, but recommends using
> flex/bison from msys. But I've certainly read about others using winflexbison,
> e.g. [1] [2]. The flex/bison in 'chocolatey', which I've also seen referenced,
> is afaics winflexbison.
>
>
> Afaict the issue also exists in our traditional windows build - but I've not
> seen anybody report this as an issue.
>
>
> I started this thread to document the issue, in case developers using visual
> studio are hitting this today.
>
>
> It looks like a similar issue exists for the windows bison port:
> https://github.com/lexxmark/winflexbison/blob/b2a94ad5fd82cf4a54690a879e14ff9511b77273/bison/src/output.c#L816
>
> I've not observed that failure, presumably because the window for it is much
> shorter.
>
>
> For the meson build it is trivial to "address" this by setting FLEX_TMP_DIR to
> a private directory (which we already need to deal with lex.backup), something
> similar could be done for src/tools/msvc.
>
>
> Unless we think it'd be better to just refuse to work with winflexbison?

No, I think your workaround is better if it works. I don't want to
require installation of msys to build with MSVC if that's avoidable.

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com