[Bug 1943077] Re: snapd fails to autopkgtest on mksquashfs, which is looking for libgcc_s

[Michael Hudson-Doyle]

https://github.com/snapcore/snapd/pull/10757

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Title:
  snapd fails to autopkgtest on mksquashfs, which is looking for
  libgcc_s

Status in snapd:
  New
Status in openssh package in Ubuntu:
  New
Status in squashfs-tools package in Ubuntu:
  New

Bug description:
  During current snapd autopkgtest, a failure can be observed:

  https://autopkgtest.ubuntu.com/results/autopkgtest-
  impish/impish/amd64/s/snapd/20210907_175258_5451b@/log.gz

  
  error: cannot pack "/home/gopath/src/github.com/snapcore/snapd/tests/smoke/sandbox/test-snapd-sandbox": mksquashfs call failed: 
  -----
  libgcc_s.so.1 must be installed for pthread_cancel to work
  Parallel mksquashfs: Using 1 processor
  Creating 4.0 filesystem on /home/gopath/src/github.com/snapcore/snapd/tests/smoke/sandbox/test-snapd-sandbox/test-snapd-sandbox_1.0_all.snap, block size 131072.
  -----
  error: cannot read snap file: "/var/lib/snapd/snaps/.local-install-136125945"
         is not a snap or snapdir

  
  I traced the underlying call to the following:
  "/snap/snapd/current/lib/x86_64-linux-gnu/ld-2.23.so" "--library-path"
  "/snap/snapd/current/usr/local/lib:/snap/snapd/current/lib/x86_64-linux-gnu:/snap/snapd/current/usr/lib/x86_64-linux-gnu"
  "/snap/snapd/current/usr/bin/mksquashfs" asdf asdf.squashfs

  (the real call has more arguments, this is a simplified version that
  produces the failure)

  To observe this, one can create a VM based a cloud image from
  https://cloud-images.ubuntu.com/impish/current, then run the above command in
  the resulting environment.
  Doing so will test against snapd v2.51.4 (12883).
  Retesting with edge 2.52+git635.gada2d87 (13323) produces an equivalent result.

  Running a more bland `/snap/snapd/current/usr/bin/mksquashfs asdf
  asdf.squashfs` without messing with library paths has mksquashfs behaving as
  expected.  Also, getting a v2.51.3 snapd seems to behave itself.  Updating from
  2.51.3 -> 2.51.4 also works.

  One can see a passing mksquashfs by taking libgcc_s.so.1 from hirsute
  and placing it in the library path for the above call.

  In the working scenario, it appears that libgcc_s is being obtained
  from outside of /snap (and also libz).  In the failing scenario, this
  external copy of libgcc_s is not being loaded (per gdb info sharedlibrary),
  but libz still is.

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