[DPE-3380] Handle S3 relation in primary non-leader unit

[marceloneppel]

Issue

The stanza can only be initialised in the PostgreSQL primary unit if it's the Juju leader unit.

Otherwise, the unit fails with an error, as shown in canonical/postgresql-k8s-operator#370.

Solution

Ported from canonical/postgresql-k8s-operator#375.

Allow the non-leader unit (when it's the PostgreSQL primary) to initialise the stanza.

For that, all checks for the unit being the Juju leader unit were changed to check whether the unit is the PostgreSQL primary.

Also, some coordination is now made in the relation databags to share the stanza name and the init-pgbackrest flag.

The current flow doesn't change if the PostgreSQL primary unit is also the Juju leader unit.

If they are different units, then:

• The stanza name and the init-pgbackres flag are set in the primary unit relation databag.
• The leader unit copies those values to the application relation databag (to be used later for other actions in backups).
• The pgbackrest check command is executed in the primary unit, which clears the init-pgbackrest flag from the unit relation databag.
• The leader unit clears the init-pgbackrest flag from the application relation databag.
• The primary unit clears the stanza name from the unit relation databag.

Both unit and integration tests were added/updated to ensure we keep this flow intact.

Also, the fix from canonical/postgresql-k8s-operator#273 was added through this PR, as it was not applied to the VM charm.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (acc0d26) 80.00% compared to head (548fbde) 80.05%.

❗ Current head 548fbde differs from pull request most recent head 641562e. Consider uploading reports for the commit 641562e to get more accurate results

Files	Patch %	Lines
src/charm.py	16.66%	4 Missing and 1 partial ⚠️
src/backups.py	96.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #340      +/-   ##
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+ Coverage   80.00%   80.05%   +0.04%     
==========================================
  Files          10       10              
  Lines        2176     2206      +30     
  Branches      344      354      +10     
==========================================
+ Hits         1741     1766      +25     
- Misses        368      371       +3     
- Partials       67       69       +2     

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[taurus-forever]

Great!