Update NGINX configuration to work with horizontal scaling

[amandahla]

Note: Integration test will be done in further story.

Overview

In Synapse, the main unit and workers handle different endpoints. This PR prepares NGINX to handle it while scaling by using template files that the charm will update with the proper main unit address.

Also, this PR changes Mjolnir to be enabled only in the main unit and redirecting abuse reports properly to it.

Flow

• Synapse NGINX Pebble ready event is emitted.
• The charm will call pebble.replan_nginx() with the main unit address.
• The replan_nginx will copy the files main_location.conf.template and abuse_report_location.conf.template to their original places and run the command sed to replace the string "main-unit" to the main unit address received.
• Synapse NGINX Pebble layer is replanned.

If the main unit is changed, the relation-changed event is emitted. The same process will happen there.

Why all this?

Any unit can correctly forward requests for the main unit if this is the case. Especially considering that only the main unit will run Mjolnir.
Otherwise, the unit itself will handle the request.

Rationale

Redirect requests as expected while horizontal scaling Synapse.

Juju Events Changes

NGINX pebble ready event.
_on_relation_changed event.

Module Changes

Library Changes

Checklist

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

Charmhub doc will be updated once horizontal scaling is in place.

[merkata]

looks good, left some comments

[merkata]

That's a new one, I think previously we used some internal _exec function (that wraps container.exec) and I'm seeing a wait() for the first time.

[amandahla]

Well, this is an experiment ;-)
I thought about not catching the error if something happens here.

[amandahla]

Based:
https://discourse.charmhub.io/t/handling-temporary-errors-in-charms/13247

[javierdelapuente]

why this change?

[amandahla]

This test is not using the harness fixture so calling "begin_with_initial_hooks" would generate: "ops.pebble.APIError: execution handler not found, please register one using Harness.handle_exec"

So I changed to only call the config changed event.

[github-actions]

Test coverage for 96c1b67

Name                            Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------------------
src/actions/__init__.py             3      0      0      0   100%
src/actions/register_user.py       22      0      2      0   100%
src/actions/reset_instance.py      21      0      2      0   100%
src/admin_access_token.py          10      0      0      0   100%
src/backup.py                     196      5     28      2    97%   403-404, 473-474, 530->532, 533
src/backup_observer.py            133     16     14      0    89%   130-133, 138-141, 177-180, 209-212
src/charm.py                      288     10     68      7    95%   140->142, 145, 257-258, 285, 292-293, 370->exit, 388-391, 453-454
src/charm_state.py                 94      0     12      0   100%
src/charm_types.py                 27      0      8      0   100%
src/database_client.py             53      1     10      3    94%   35, 47->exit, 69->exit
src/database_observer.py           50      3      6      1    93%   71-73, 89->91
src/exceptions.py                   4      0      0      0   100%
src/irc_bridge.py                  65      9     12      3    82%   75, 77-78, 105-106, 155-165
src/mjolnir.py                     88     12     26      3    83%   60-68, 79, 82->84, 176-177
src/observability.py               10      0      0      0   100%
src/pebble.py                     194     21     28      7    87%   167->exit, 178-182, 201-202, 219-220, 253-254, 256-257, 268, 270, 272, 274, 305-306, 325-326, 345-346
src/redis_observer.py              53      5      6      1    90%   67-70, 90-91
src/saml_observer.py               45      1      8      0    98%   63
src/smtp_observer.py               70      4     16      2    93%   74-78, 81, 100->105
src/synapse/__init__.py             4      0      0      0   100%
src/synapse/admin.py               20      9      2      0    50%   40-41, 63-74
src/synapse/api.py                176      3     24      3    97%   176, 229, 402
src/synapse/workload.py           283     35     57      8    86%   297-298, 310-320, 377->379, 394->exit, 398-399, 419-420, 460-461, 497-498, 511-519, 589, 627, 674->677, 701-702, 720, 728->730, 730->732, 737-738, 755, 764-765
src/user.py                        24      0      4      0   100%
---------------------------------------------------------------------------
TOTAL                            1933    134    333     40    92%

Static code analysis report

Working... ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 0:00:01
Run started:2024-04-01 21:35:20.883453

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 10113
  Total lines skipped (#nosec): 4
  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):

[yanksyoon]

LGTM, minor nitpicky comments regarding file structure! Thank you!