[MM-63462] Use local time for backup log filenames

[hanzei]

Summary

Backup log filenames were formatted in UTC (lumberjack default) while log entry timestamps used the server's local timezone. This caused a confusing mismatch for admins trying to correlate a rotation event with something that happened at a specific local time — the filename would show e.g. T09-14-56 (UTC) while the last entry inside the file said 14:44:55 +05:30.

Setting LocalTime: true on the lumberjack logger aligns backup filenames with log entry timestamps. Admins who prefer UTC can achieve consistent UTC output by running the server process in the UTC timezone (TZ=UTC).

Testing

A test for this was explored and then removed. The only meaningful approach was to mutate the global time.Local to a non-UTC timezone, trigger rotation, and verify the backup filename reflected local time. However, lumberjack's internal millRun goroutine reads time.Local via time.Parse even after the logger is shut down (lumberjack's Close() does not drain that goroutine), so restoring time.Local in a defer races with it. There is no way to wait for the mill goroutine from outside the library.

The fix is a one-liner against a well-understood lumberjack flag; the behavior is covered by lumberjack's own test suite (TestLocalTime).

Ticket

https://mattermost.atlassian.net/browse/MM-63462

🤖 Generated with Claude Code

[github-actions]

Test Results

353 tests  ±0   353 ✅ ±0   1m 31s ⏱️ -2s
 10 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 33e2ccb. ± Comparison against base commit 34e81f8.

♻️ This comment has been updated with latest results.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

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📥 Commits

Reviewing files that changed from the base of the PR and between 34e81f8 and 33e2ccb.

📒 Files selected for processing (1)

• targets/file.go

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📝 Walkthrough

Walkthrough

NewFileTarget now configures lumberjack to use local time in rotated log filenames.

Changes

File target logging

Layer / File(s)	Summary
Configure local-time rotation targets/file.go	NewFileTarget sets lumberjack's LocalTime option to true.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to 33e2c

Rotated file-log names now use the server’s local timezone, matching log-entry timestamps. No merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: configuring backup log filenames to use local time.
Description check	✅ Passed	The description explains the local-time filename change, its purpose, testing considerations, and related ticket. It is directly related to the changeset.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch MM-63462-local-time-backup-filenames

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