Add cron service and cleanup script

[nrobinaubertin]

Overview

Synapse can leave empty directories in the media directory after deleting old/unused files. There is currently no option to remove them and they can accumulate, using inodes and space.

This PR adds a cron service inside the workload container to regularly launch a script meant to delete those empty directories.
The PR was structure to simplify the future addition of scripts that should be handled by the cron service. Just add them in the corresponding cron.{hourly,daily,weekly,monthly} directory.

Remarks:

• I tried the cleanup script on a VPS with half a million empty directories: CPU and memory usage were not noticeable and I tweaked the script to lower the IOPS (see the comments inside it for explanations)

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)

[github-actions]

license-eye has totally checked 115 files.

Valid	Invalid	Ignored	Fixed
51	2	62	0

Click to see the invalid file list

• synapse_rock/cron/cron.weekly/remote_content_empty_directory_cleanup.py
• synapse_rock/scripts/run_cron.py

[javierdelapuente]

LGTM

[merkata]

💯

[github-actions]

Test coverage for 5b7a9e8

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/backup.py                     159      5     30      2    96%   322-323, 366-367, 423->425, 426
src/backup_observer.py             80      8      8      0    91%   126-129, 134-137
src/charm.py                      222      8     50      4    96%   165-166, 227-228, 247-248, 289, 303
src/charm_state.py                 63      1     10      1    97%   127
src/charm_types.py                 23      0      6      0   100%
src/database_client.py             53      1     10      3    94%   35, 47->exit, 69->exit
src/database_observer.py           48      4      4      0    92%   69-71, 87
src/exceptions.py                   4      1      0      0    75%   22
src/mjolnir.py                     76      6     20      1    91%   60-64, 73
src/observability.py                9      0      0      0   100%
src/pebble.py                     102     12     16      6    85%   95-96, 98-99, 107, 109, 111, 115, 136-137, 152-153
src/saml_observer.py               45      1      8      0    98%   64
src/smtp_observer.py               70      3     14      1    95%   70-74, 96->101
src/synapse/__init__.py             3      0      0      0   100%
src/synapse/api.py                161      2     22      2    98%   207, 376
src/synapse/workload.py           275      8     42      6    96%   421->exit, 450-451, 503-504, 540-541, 557, 606->609, 655, 663->665, 665->667
src/user.py                        24      0      4      0   100%
---------------------------------------------------------------------------
TOTAL                            1463     60    248     26    95%

Static code analysis report

Run started:2024-02-08 08:54:51.323672

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 7895
  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):