snap: use the -no-fragments mksquashfs option

[tyhicks]

Use of the Squashfs fragments feature results in unpredictable snap
composition due to how fragments are gathered and compressed using a
separate thread in mksquashfs. Disable fragments entirely in snap pack
and tests that call mksquashfs directly so that the review tools can
perform validation of snaps and to be consistent with what snapcraft is
doing.

Fixes: https://launchpad.net/bugs/1576763
Signed-off-by: Tyler Hicks tyhicks@canonical.com

This PR corresponds to the following:

snapcore/snapcraft#1805
https://forum.snapcraft.io/t/proposal-to-disable-squashfs-fragments-in-snaps/3103

[codecov-io]

Codecov Report

Merging #4396 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4396      +/-   ##
==========================================
+ Coverage   78.03%   78.04%   +<.01%     
==========================================
  Files         449      449              
  Lines       30906    30907       +1     
==========================================
+ Hits        24118    24121       +3     
+ Misses       4775     4774       -1     
+ Partials     2013     2012       -1

Impacted Files	Coverage Δ
snap/squashfs/squashfs.go	65% <100%> (+0.59%)	⬆️
overlord/hookstate/hookmgr.go	72.83% <0%> (+1.15%)	⬆️

---

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[bboozzoo]

I've restarted Travis job since it hit a timeout.

[chipaca]

Gosh. We shouldn't have all those mksquashfs lying around now that we have snap pack. You mind if I piggyback on this PR to move those over?

[jdstrand]

Thanks for the PR! :)

[jdstrand]

Not blocking on this, but -no-fragments is only interesting for store uploads, and anything using -comp gzip is not going to be uploaded to the store anyway (since the resquash tests would fail).

[tyhicks]

I paused before adding it to that mksquashfs command precisely because of what you mentioned. Even though it isn't going to be uploaded to the store, I think it is still work testing with similar arguments to what all other snaps will be generated with.

[jdstrand]

@chipaca should comment on this part, but iirc gzip is being used to help address travis timeouts because it is faster. Is -no-fragments slower than without it?

[tyhicks]

No, in my testing, the -no-fragments option actually speeds up mksquashfs ever so slightly.

[jdstrand]

I've restarted travis since it timed out.

[tyhicks]

@chipaca Yes, please feel free to push a commit to use snap pack everywhere.

[tyhicks]

The test failures are unrelated to the changes in this PR. Three test runs have failed due to Travis timeouts and another due to HTTP requests to the store timing out.

[tyhicks]

Is there anything preventing this change from being merged? The corresponding change to snapcraft has been merged (snapcore/snapcraft@9a131cb) so I'd hate for this PR to linger for too long since we'd like to change the review-tools to require -no-fragments soon.

[chipaca]

@tyhicks just that testing infra is a bit broken, as soon as it's passing things this'll get merged