[Bug 1921626] Autopkgtest regression report (apt/2.0.6)

[Ubuntu SRU Bot]

All autopkgtests for the newly accepted apt (2.0.6) for focal have finished running.
The following regressions have been reported in tests triggered by the package:

reprotest/0.7.14 (ppc64el)
apport/2.20.11-0ubuntu27.18 (amd64)


Please visit the excuses page listed below and investigate the failures, proceeding afterwards as per the StableReleaseUpdates policy regarding autopkgtest regressions [1].

https://people.canonical.com/~ubuntu-archive/proposed-
migration/focal/update_excuses.html#apt

[1] https://wiki.ubuntu.com/StableReleaseUpdates#Autopkgtest_Regressions

Thank you!

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https://bugs.launchpad.net/bugs/1921626

Title:
  size mismatch error if request of unknown size is larger than others

Status in apt package in Ubuntu:
  Fix Released
Status in apt source package in Bionic:
  Fix Committed
Status in apt source package in Focal:
  Fix Committed
Status in apt source package in Groovy:
  New
Status in apt source package in Hirsute:
  Fix Released

Bug description:
  [Impact]

  Downloads fail if:

  - there is at least one package each with Size and no size on a mirror
  - a package without a Size field is larger than a package with a Size field that's currently in the pipeline

  Also, this was silent, we need to add an error so people fix repos.
  For hirsute and earlier, this is a warning; for hirsute+1 it's an
  error so people's CI fails on them and doesn't "succeed with warnings"

  [Test plan]

  We have included a test case in the apt integration tests, which
  downloads three packages a, b, c where b is largest and has no Size
  field. With 2.2.2, it fails; with 2.2.3 it succceeds.

  We have also added a test case that a warning is shown.

  [Where problems could occur]
  Problems can only occur if you try to download packages without a Size field, as that is the only place code changes (adding code guarded with if ... Size ... == 0; 0 being unknown size).

  [Other changes]
  2.2.3 includes the same change as 2.2.2ubuntu1

  [Original bug report]

  1) Ubuntu 18.04.5 LTS

  2) apt 1.6.12ubuntu0.2

  3) What you expected to happen

  I set a custom set of repositories in /etc/apt/sources.list and then I
  run "apt install <list of packages>". I expect the command to download
  and install the packages.

  4) What happened instead

  "apt install ..." fails during the download phase with "File has
  unexpected size ...."

  5) What I've established trying to debug the issue:

  - Disabling http pipelining resolves the issue: "apt -oAcquire::http::Pipeline-Depth=0 install ..."
  - All the packages, and repo metadata in the referenced repositories is correct
  - The issue is easily reproducible in my setup with different repositories
  - tcpdump shows that requests and responses are in the correct order, and contain the correct data

  More details about the issue:
  https://projects.theforeman.org/issues/32178

  With all the above in mind, it appears that this must be a bug in apt's http pipeline handling.
  It seem that apt is trying to match a request to do wrong response, and size doesn't match.

  I've attached an example log, where the error pops up for multiple
  packages, and they all appear to be compared to one size (86464
  bytes). That size is correct for one of the package being downloaded,
  but somehow apt is trying to match to multiple other packages.

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