[Bug 1764220] Re: [SRU] dpkg zstd support

[Balint Reczey]

** Merge proposal linked:
   https://code.launchpad.net/~rbalint/britney/+git/hints-ubuntu/+merge/401632

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Title:
  [SRU] dpkg zstd support

Status in dpkg:
  Incomplete
Status in dpkg package in Ubuntu:
  Fix Released
Status in dpkg source package in Xenial:
  Fix Committed

Bug description:
  [Impact]

  * Xenial's dpkg can't decompress zstd-compressed binary packages
  preventing some systems of Launchpad from processing packages with
  such compression. This blocks publishing zstd-compressed binary
  packages through Launchpad for later Ubuntu releases as well.

  [Test Plan]

  * https://people.canonical.com/~rbalint/zstd-debs/ contains a .deb built on Hirsute having both data and control members of the .deb being compressed with zstd.
  * Download and unpack it. With unfixed dpkg an error should be shown.

  $ wget https://people.canonical.com/~rbalint/zstd-debs/glibc-doc-
  reference_2.33-0ubuntu2~zstd1_all.deb

  # unfixed:
  $ dpkg-deb -R glibc-doc-reference_2.33-0ubuntu2~zstd1_all.deb  glibc-doc-extracted
  dpkg-deb: error: archive 'glibc-doc-reference_2.33-0ubuntu2~zstd1_all.deb' uses unknown compression for member 'control.tar.zst', giving up

  # fixed
  $ time dpkg-deb -R glibc-doc-reference_2.33-0ubuntu2~zstd1_all.deb  glibc-doc-extracted

  real	0m0.148s
  user	0m0.041s
  sys	0m0.124s

  * Also install the package:

  root at x-zstd:~# dpkg -i glibc-doc-reference_2.33-0ubuntu2~zstd1_all.deb
  Selecting previously unselected package glibc-doc-reference.
  (Reading database ... 25816 files and directories currently installed.)
  Preparing to unpack glibc-doc-reference_2.33-0ubuntu2~zstd1_all.deb ...
  Unpacking glibc-doc-reference (2.33-0ubuntu2~zstd1) ...
  Setting up glibc-doc-reference (2.33-0ubuntu2~zstd1) ...
  Processing triggers for install-info (6.1.0.dfsg.1-5) ...
  root at x-zstd:~#

  * Build the hello package, it should work
  * Build the hello package overriding the compression to zstd, this should fail:

  $ cat debian/rules
  ...

  override_dh_builddeb:
   dh_builddeb -- -Zzstd

  ...
  make[1]: Entering directory '/root/hello-2.10'
  dh_builddeb -- -Zzstd
  dpkg-deb: error: only decompression is supported for 'zstd'!

  Type dpkg-deb --help for help about manipulating *.deb files;
  Type dpkg --help for help about installing and deinstalling packages.
  dh_builddeb: dpkg-deb -Zzstd --build debian/hello .. returned exit code 2
  debian/rules:12: recipe for target 'override_dh_builddeb' failed

  [Where problems could occur]

  * The fix is isolated and is a backport from Bionic with the compression part omitted. Crashes could happen due to coding errors should they exist.
  Only decompression should be supported and this is verified in the test plan.

  * Incompabilities between libzstd present in Xenial and present in
  later releases could prevent dpkg running on Xenial from successfully
  processing packages built on later releases. The package for Xenial is
  built with libzstd1 (build-depending on libzstd1-dev) which is at
  version 1.3.1+dfsg-1~ubuntu0.16.04.1. Bionic's version is 1.3.3+dfsg-
  2ubuntu1.2 and there were no format breaking changes between those
  versions, not in any later version. See
  https://github.com/facebook/zstd/issues/999#issuecomment-359538229 .

  [Original Bug Text]

  As discussed previously, we want to have zstd support in 18.04 to
  evaluate and potentially enable it in later releases.

  The zstd support adds a dependency on libzstd1 to dpkg. This should
  not have any effect on live images, since libzstd1 is part of the
  various live tasks, as btrfs-progs need it. For installed systems,
  this might be a new dependency (if they do not use btrfs, tor, or some
  other tools), so an increase of ~520 KB, as that's the size of the
  library and the library only depends on libc6.

  The change is isolated, it adds the compressor and decompressor to
  dpkg, please see the attached patch for the details.

  The change is being discussed here:
  https://lists.ubuntu.com/archives/ubuntu-devel/2018-March/040211.html
  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=892664

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