Re: ci: namespace ccache by PostgreSQL major version

Hi,

On 2026-07-06 09:52:19 +0200, Peter Eisentraut wrote:
> On 03.07.26 16:26, Nazir Bilal Yavuz wrote:
> > Hi,
> > 
> > When the Postgres major version is updated, CI fails with [1]:
> > 
> > 2026-07-02 17:01:01.938 UTC [828] FATAL:  incompatible library
> > "D:/a/postgresql/postgresql/build/tmp_install/usr/local/pgsql/lib/utf8_and_win.dll":
> > version mismatch
> > 2026-07-02 17:01:01.938 UTC [828] DETAIL:  Server is version 20,
> > library is version 19.
> > 
> > This happens because the GitHub Actions ccache is restored across a
> > version bump, so objects built against the previous version can be
> > reused, producing libraries that no longer match the server version.
> > 
> > Here is an attempt to solve this problem by namespacing ccache by
> > Postgres major version. I added a 'PG_MAJOR_VERSION' variable to the
> > CI file and used that as a prefix to ccache key. I made this
> > 'PG_MAJOR_VERSION' variable automatically updated by
> > 'version_stamp.pl' script.
> > 
> > Another solution could be reading the version from build files (e.g
> > meson.build), but then this read needs to be done at each CI run.
> 
> I'm suspicious about this direction.  The major version is not the only
> piece of data that determines ABI compatibility between the server and
> extensions.  This would only be a partial information.  The ABI information
> exists in the code, and so changes should be visible to ccache.  Maybe we
> are using ccache in the wrong mode or something (see "depend mode", "direct
> mode", etc.).

Yea, I don't think that's the right answer. I'm reasonably sure I debugged
this issue a while ago:

https://postgr.es/m/phsrssp75npoyalqsolcd7fmnmlbzbmquc2p7w7mqjlw7432jk%40bzskz3luyjvb

Not a lot has happened on the ccache front, so I suspect we should start
adding the -fpch-deps flag I mentioned in that thread.

We could make adding -fpch-deps conditional on pch support being enabled, but
given that it doesn't do anything when pch is not used, I'd just add it when
supported by the compiler.

Greetings,

Andres Freund