Integration test for upgrades

Overview

The goal of this PR is to write an integration test that tests the charm upgrade process. There should be no downtime in the upgrade process, so the way of checking it in this test is to make a request every 0.5 seconds to the endpoint /srv/status (to the ingress endpoint), in a similar way as how it will work in a real environment.

Two issues were found in the test.

Lack of a graceful shutdown. Kubernetes (pebble) sends a SIGTERM signal to Discourse. However, unicorn requires SIGQUIT instead for a graceful shutdown (see https://yhbt.net/unicorn/SIGNALS.html). Besides, it looks that having some time before sending the signal could help (see https://medium.com/@acgs771126/ror-unicorn-graceful-shutdown-in-kubernetes-f66d83e1eb48). This is similar to what is suggested in Heroku (see https://blog.heroku.com/unicorn_rails). For that, a patch has been generated that translates SIGTERM to SIGQUIT, and also a delay before sending SIGQUIT to the unicorn master.

Workload containers are ready even when there is no discourse running. To improve this situation, the pebble check discourse-setup-completed has been moved to the rock, so it always runs. Before that, the check was only added when the discourse service was also added.

Rationale

Juju Events Changes

Module Changes

Library Changes

Checklist

[x] The charm style guide was applied
[x] The contributing guide was applied
[x] The changes are compliant with ISD054 - Manging Charm Complexity
[x] The documentation is generated using src-docs
[x] The documentation for charmhub is updated.
[x] The PR is tagged with appropriate label (urgent, trivial, complex)

Test coverage for fb21dd17a7e2666f0d5d2715e9d005ab63046025

Name              Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------
src/charm.py        321     27     78     12    90%   186, 194-195, 207, 335->343, 376->381, 393, 575-577, 582-583, 595-597, 602-603, 615-617, 640-642, 684->exit, 694->exit, 724-730, 756->exit, 770-771, 781->exit, 795
src/database.py      30      1      8      1    95%   56
-------------------------------------------------------------
TOTAL               351     28     86     13    91%

Static code analysis report

Run started:2024-04-30 13:45:45.343895

Test results:
    No issues identified.

Code scanned:
    Total lines of code: 2042
    Total lines skipped (#nosec): 3
    Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
    Total issues (by severity):
        Undefined: 0
        Low: 0
        Medium: 0
        High: 0
    Total issues (by confidence):
        Undefined: 0
        Low: 0
        Medium: 0
        High: 0
Files skipped (0):

canonical / discourse-k8s-operator