[Reproducible-builds] Minimising work needed for this build-path issue

[Christoph Berg]

Re: Ximin Luo 2016-09-06 <1f0f5aa5-449b-5bca-7e0b-e8dfab756b04 at debian.org>
> Well, we could have dpkg-buildflags also set SOURCE_ROOT somehow. A hacky strawman solution, is to patch:
> 
> 1. dpkg-buildflags --export to also define SOURCE_ROOT
> 2. /usr/share/dpkg/buildflags.mk
> 
> Debhelper and CDBS should pull in SOURCE_ROOT automatically via (1), no changes needed.
> 
> However code search https://codesearch.debian.net/search?q=dpkg-buildflags+--get shows that many packages are doing custom things with specific flags, which we would probably break with the above approach. (Many of these results are also using debhelper but (a) it's unclear how this interacts with their custom flags and (b) I imagine quite a lot of them aren't using it.)
> 
> It would likely be easier/safer to just patch GCC directly.

I've been poking around in the PostgreSQL packages a bit more, and the
problem gets worse :(

I've successfully (modulo an actual upload) patched pg_config to
remove -fdebug-prefix-map from the "pg_config --cflags" output, the
build path issue is also present in all extension modules. In current
sid, the extensions are compiled using the server's -fdebug-prefix-map
setting (which doesn't do anything), but then with my fix, the
extensions would be compiled just without any -fdebug-prefix-map
because "pg_config --cflags" says so.

Not sure what to do here. I could tweak pg_buildext (which most
extensions use) to re-add a new -fdebug-prefix-map flag. But
everything would just be much easier if dpkg-buildflags and gcc would
use -fdebug-prefix-map=SOURCE_ROOT=., in which case I could leave
pg_config unpatched, and the same CFLAGS would also work for all
extension modules, just with a different SOURCE_ROOT setting.

Christoph
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