[Bug 1837871] [NEW] Add retry logic to snap-tool to make downloads more resilient

[Launchpad Bug Tracker]

You have been subscribed to a public bug by Tobias Koch (tobijk):

[Impact]

* livecd-rootfs builds will fail immediately when a snap-tool invocation
fails to contact the snapstore because of ephemeral connection problems
or a transient error on the server side.

* The snap-tool script included with livecd-rootfs in Eoan has been
enhanced to retry on connection errors and 5xx server errors reducing
the likelihood of image builds breaking due to a flaky connection or a
server hiccup.

[Test Cases]

* Download core, core18 snaps using both `snap download <snap-name>` and
`snap-tool download <snap-name>` and make sure the downloads are
identical.

* Invoke `snap-tool info <snap-name>` for a few snaps, e.g. review-
gator, lpshipit, azure-cli and verify that all fields carry correct
information.

* Test the backoff/retry logic using the following procedure:

Make netcat listen on port 12345

    netcat -l -p 12345

Create a symlink from snaptool.py to snap-tool and import the
ExpBackoffHTTPClient class from a Python session:

    ln -s snap-tool snaptool.py

    python3
    from snaptool import ExpBackoffHTTPClient
    http_client = ExpBackoffHTTPClient()
    request = http_client.get("http://127.0.0.1:12345/")
    request.text()

Go back to the terminal where you invoked netcat and stop it. snap-tool
should print the following and then fail:

WARNING: failed to open URL 'http://127.0.0.1:12345/': Remote end closed connection without response
Retrying HTTP request in 2 seconds...
WARNING: failed to open URL 'http://127.0.0.1:12345/': <urlopen error [Errno 111] Connection refused>
Retrying HTTP request in 4 seconds...

Repeat the procedure above but instead of stopping netcat, paste the
following response:

HTTP/1.1 503 Error

and hit enter twice. You should see

WARNING: failed to open URL 'http://127.0.0.1:12345/': HTTP Error 503: Error
Retrying HTTP request in 2 seconds...

Repeat the above pasting "HTTP/1.1 404 Not found" instead. The snap-tool
should fail immediately.

[Regression Potential]

 * Tool logic and http request headers/body are unchanged, only the way
connections are built has been modified. The expectation is that this
will be more robust and testing in devel hasn't surfaced any bugs, but
there is a slight risk that the tool's behavior has changed in unobvious
corner cases that were missed during testing.

** Affects: livecd-rootfs (Ubuntu)
     Importance: Undecided
         Status: New

-- 
Add retry logic to snap-tool to make downloads more resilient
https://bugs.launchpad.net/bugs/1837871
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