Enabling -ffile-prefix-map by default

[Vagrant Cascadian]

Hi Lucas!

I've appreciated all the work you've been doing with archive-wide
rebuilds of Debian over the years!

I'm looking into proposing to enable dpkg's
DEB_BUILD_OPTIONS=reproducible=+fixfilepath by default, further
background in this thread:

  https://alioth-lists.debian.net/pipermail/reproducible-builds/Week-of-Mon-20200601/012375.html

We would like to figure out how many packages it causes to FTBFS
(usually test suites expecting full paths); we've identified a small
handful through tests.reproducible-builds.org:

  https://tests.reproducible-builds.org/debian/issues/bullseye/ftbfs_due_to_f-file-prefix-map_issue.html
  https://tests.reproducible-builds.org/debian/issues/bullseye/ffile_prefix_map_passed_to_clang_issue.html

Those issues were identified through manually checking the build logs,
and our reproducible builds infrastructure is not great for isolating
out the effects of specific changes as each build has several variations
tested.

It would be nice to try an archive-wide rebuild with +fixfilepath
enabled and on failure, automatically retry with -fixfilepath, so that
we can flag which packages build fine without too much worry about false
positives and in an automated fashion.


I've never experimented with anything on an archive-wide scale before,
so would appreciate any ideas, tooling or other help you might be able
to offer. Thanks!


live well,
  vagrant
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