DPE-8980 Support Juju 4: use 'ip' instead of 'private-address' (if available)

[taurus-forever]

Issue

No Juju 4 support by the charm.

Solution

The Juju 4 has removed support databag fiesl private-address, ingress-address and more. The field we should use is ip now. The PG16 charm still have to support Juju 3.6 LTS, so adding support of the ip field with backward compatibility.

We are not going to allow Juju 4 deployments (assume in metadata.yaml),
however users can use the following command to test Juju 4:

juju deploy postgresql --channel 16/edge --force

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.70%. Comparing base (18cf99b) to head (9c0441e).
⚠️ Report is 4 commits behind head on 16/edge.

Files with missing lines	Patch %	Lines
src/relations/tls.py	0.00%	0 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (33.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff            @@
##           16/edge    #1318   +/-   ##
========================================
  Coverage    70.70%   70.70%           
========================================
  Files           16       16           
  Lines         4298     4298           
  Branches       691      691           
========================================
  Hits          3039     3039           
  Misses        1049     1049           
  Partials       210      210           

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[taurus-forever]

No quick win :-(

ERROR juju.worker.uniter.context cannot apply changes: creating secrets: secret with label "database-peers.postgresql.unit" already exists

Need further investigation. No Juju Secret changes spotted in Juju 4 release notes

[taurus-forever]

It looks like Juju 4 doesn't allow the same Juju Secrets label for different units, while it used to work well in Juju 3.6:

> juju version
3.6.12-genericlinux-amd64

> juju secrets
ID                    Name  Owner         Rotation  Revision  Last updated
d4k4mdm05d8c5p9k29r0  -     postgresql    never            1  10 minutes ago  
d4k4mem05d8c5p9k29rg  -     postgresql/0  never            1  9 minutes ago   
d4k4q7u05d8c5p9k29s0  -     postgresql/1  never            1  2 minutes ago   

> juju show-secret d4k4mem05d8c5p9k29rg
d4k4mem05d8c5p9k29rg:
  revision: 1
  owner: postgresql/0
  label: database-peers.postgresql.unit
  created: 2025-11-27T12:59:47Z
  updated: 2025-11-27T12:59:47Z
> juju show-secret d4k4q7u05d8c5p9k29s0
d4k4q7u05d8c5p9k29s0:
  revision: 1
  owner: postgresql/1
  label: database-peers.postgresql.unit
  created: 2025-11-27T13:07:44Z
  updated: 2025-11-27T13:07:44Z

Juju 4 doesn't allow to create secret with label database-peers.postgresql.unit for postgresql/1 as it is already exist for postgresql/0.

Branched to juju/juju#21344

[taurus-forever]

The issue juju/juju#21344 has been fixed, and now PostgreSQL cluster can be deployed.
However no success with the relation to postgresql-test-app nor data-integrator 😢
Juju belives the relation has been established succesfully, but no juju secret created with app credentials => app stays pending for data... the further investigation required.

Reproducer: juju deploy postgresql --channel 16/edge/juju4