[Bug 1782094] Re: pkgstripfiles: symlinking docs and optimizing pngs/trucating changelogs in parallel produces undeterministic results

[Francis Ginther]

** Tags added: id-5b2bdda1b1a060708081efe3

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Title:
  pkgstripfiles: symlinking docs and optimizing pngs/trucating
  changelogs  in parallel produces undeterministic results

Status in pkgbinarymangler package in Ubuntu:
  New

Bug description:
  [Impact]

   * PNG optimization and changelog truncation can happen in parallel with symlinking docs across packages. When checking for duplicates files the duplicate check may compare the processed files with non-processed ones and in that case they are found to be different despite the processed (or the unprocessed) files would be identical. The resulting binary packages would be still valid, but may be different for different architectures since the parallel build process may hit this race condition when processing different files making the arch:any packages not coinstallable on a multiarch-enabled system.
   * One manifestation of the problem can be seen in https://launchpadlibrarian.net/355140796/buildlog_ubuntu-bionic-i386.gpm_1.20.7-5_BUILDING.txt.gz where symlinking's file comparison occurs while truncating the changelog:
  ...
  Searching for duplicated docs in dependency libgpm2...
  pkgstripfiles: Truncating usr/share/doc/libgpm2/changelog.Debian.gz to topmost ten records

  gzip: /<<PKGBUILDDIR>>/debian/libgpm-dev/../libgpm2/usr/share/doc/libgpm2/changelog.Debian.gz: unexpected end of file
  cmp: EOF on /dev/fd/62 which is empty
  ...

   * The fix is protecting png optimization and trucating changelogs
  with the same lock as used to serialize doc symlinking thus those
  steps are serializes as well.

  [Test Case]

   * Build an affected package (like gpm) for i386 and amd64 with fixed pkgbinarymangler and observe the files in /usr/share/doc in the built packages to be identical and there should be no line like the following in the build log:
  gzip: /<<PKGBUILDDIR>>/debian/libgpm-dev/../libgpm2/usr/share/doc/libgpm2/changelog.Debian.gz: unexpected end of file
   * Note that due to the non-deterministic nature of the failure several builds could be necessary to reproduce the original problem and also for verifying that the problem is fixed. You can increase the chances of observing the failure by building the packages with higher level of parallelism (-jX).

  [Regression Potential]

   * Due to serialization the build times may increase for packages where the serialized steps ran in parallel originally. IMO there is not much we can do about that apart from trying to speed up the steps themselves.
   * Since no additional locks were introduced I believe the builds won't break or stall due to this change.

  [Other Info]

   * There are some packages which need to be rebuilt with the updated
  pkgbinarymangler, collecting them is in progress.

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