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[Impact]
apt crashes (core dump) during cache creation

[Description]
When executing apt-cache several times on a memory constrained system, apt-cache can sometimes crash with a core dump file. This is related to mmap() failing allocations during cache generation, according to an upstream bug report (803417).
There is an upstream patch for this bug (6789e01e do not segfault in cache generation on mmap failure) and it's included in the apt package for other series (see below), so we only need to backport it to Trusty.

Upstream commit: https://salsa.debian.org/apt-
team/apt/commit/6789e01e9370b3b7f65d52138c5657eaa712b4d1

$ git describe --contains 6789e01e9370
1.1_exp16~5

$ rmadison apt
 apt | 1.0.1ubuntu2            | trusty           | source, amd64, arm64, armhf, i386, powerpc, ppc64el
 apt | 1.0.1ubuntu2.19         | trusty-security  | source, amd64, arm64, armhf, i386, powerpc, ppc64el
 apt | 1.0.1ubuntu2.19         | trusty-updates   | source, amd64, arm64, armhf, i386, powerpc, ppc64el
 apt | 1.2.10ubuntu1           | xenial           | source, amd64, arm64, armhf, i386, powerpc, ppc64el, s390x
 apt | 1.6.1                   | bionic           | source, amd64, arm64, armhf, i386, ppc64el, s390x
 apt | 1.7.0                   | cosmic           | source, amd64, arm64, armhf, i386, ppc64el, s390x
 apt | 1.8.0~rc2               | disco            | source, amd64, arm64, armhf, i386, ppc64el, s390x


[Test Case]
Unfortunately, this issue is not easy to reproduce. It seems to trigger randomly when "apt-cache policy" is called on a system under memory pressure, triggering the failure path in the mmap() allocations.

[Regression Potential]
The regression potential for this should be low, as it's a change in the failure path for memory allocations. This shouldn't be triggered during normal usage, and we'll thoroughly test the patched package with autopkgtests and perform scripted runs in memory-constrained systems with stress-ng.
We also have a confirmation from an impacted user that the upstream patch fixes their issue without further problems.

** Affects: apt (Ubuntu)
     Importance: Undecided
         Status: Fix Released

** Affects: apt (Ubuntu Trusty)
     Importance: Medium
     Assignee: Heitor R. Alves de Siqueira (halves)
         Status: In Progress

-- 
apt segfaults when generating cache file
https://bugs.launchpad.net/bugs/1815129
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