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

[Ian Johnson]

Dan, note that although we set the SNAP_REEXEC environment variable in
that integration test, that environment variable is actually not obeyed
when it comes to the mksquashfs command which is called here:

https://github.com/snapcore/snapd/blob/dfba7de59a41bc22786d87f53b20deea14240713/snap/squashfs/squashfs.go#L524

and the function snapdtool.CommandFromSystemSnap does not observe the
SNAP_REEXEC environment variable at all, instead just checking if the
snapd or core snaps exist at all, and using the mksquashfs from those
snaps instead of what's on the host on $PATH (though we do fallback and
if there is no mksquashfs found in the core or snapd snaps (somehow???),
then we use $PATH).

As I mentioned on IRC, we could potentially someday add a workaround
where if mksquashfs from the system snap specifically segfaults then we
will fall back to using the host's mksquashfs, but we have chosen for
now not to work on that patch.

Hope this helps explain things.

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https://bugs.launchpad.net/bugs/1943077

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

Status in snapd:
  Fix Committed
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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