Write config.ini atomically

[Sloth-on-meth]

Problem

save_config() opened config.ini in "w" mode (truncating it) and then streamed config.write() into it. A crash, kill or full disk mid-write leaves a truncated file, and that file holds the HA token and admin password. It is written by /admin/notice, /admin/test-mode and the migrate endpoints.

Changes

• New atomic_io.atomic_write_text(path, content), extracted from the same strategy users_store._save_atomic already uses:
  • temp file next to the target, fsync, os.replace (atomic);
  • falls back to the system temp dir when the app directory can't take new files (/app is root-owned while the container runs as appuser);
  • when os.replace fails because config.ini is a single-file Docker bind mount (as in docker-compose.yml), backs the file up, overwrites in place, and restores the backup if that write fails.
• save_config() renders to a string first, then calls it.
• users_store.py is intentionally untouched to keep this PR small; it could adopt the helper later.

Tests

tests/test_atomic_io.py: content + no leftover temp files, missing file/dir, bind-mount fallback, original restored when the fallback write fails, tmp-dir fallback, and save_config() uses it. Full suite passes (114), ruff clean.

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https://claude.ai/code/session_01PemRAtiBKjoZV6HkBCGDBC

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Summary by CodeRabbit

• Bug Fixes
  • Configuration changes are now saved more reliably, reducing the risk of a partially written settings file.
  • If a save cannot be completed, existing settings are preserved when possible, helping prevent configuration loss.
  • Saving settings can now recover from certain file replacement problems, while keeping the existing configuration intact when recovery is possible.

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📒 Files selected for processing (3)

• app.py
• atomic_io.py
• tests/test_atomic_io.py

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📝 Walkthrough

Walkthrough

Configuration saving now serializes settings in memory and writes them through a new atomic text-writing helper. The helper uses temporary files, replacement, and an in-place fallback. Added tests cover successful writes, fallback errors, restoration, and configuration persistence.

Changes

Configuration Write

Layer / File(s)	Summary
Atomic text writer atomic_io.py, tests/test_atomic_io.py	The new atomic_write_text() helper creates and syncs a temporary file before replacement. If replacement fails, it backs up an existing target and writes in place, restoring the backup if that write fails. Tests cover replacement, file creation, fallback, restoration, and temporary-file placement.
Configuration save integration app.py, tests/test_atomic_io.py	save_config() serializes configuration to memory and passes the text to atomic_write_text(). A test checks the saved content and verifies that no extra files remain.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 30.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: writing config.ini atomically.
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.

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Actionable comments posted: 2

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🤖 Prompt to fix review comments

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Inline comments:
Review comments at @atomic_io.py:
- Around line 40-43: Create the backup for an existing target beside the
accepted temporary file, rather than at path + ".bak", so backup creation works
when the target directory is not writable; clean up the backup as appropriate
after the write. Add a test that makes temporary-file creation fall back to a
writable directory and makes os.replace fail, then verifies the save succeeds
through the in-place write fallback.
- Around line 48-65: Update the backup cleanup in the atomic write flow so
`.bak` is removed only after the write or restoration to `path` succeeds. If
`shutil.copy2(backup_path, path)` fails during restoration, preserve the backup;
leave the existing cleanup behavior for temporary files unchanged.

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[Sloth-on-meth]

Two commits on this PR:

• 51b8bae: same hardening CodeRabbit flagged on Fix #41: users.json.bak PermissionError when /app is read-only #53, applied here since this backs up config.ini (HA token, admin password): copyfile instead of copy2 so the 0600 backup doesn't inherit the file's mode, and the restore-failure error names the retained backup. Tests added (the permission test fails with copy2).
• Merge-conflict fix: atomic_write_text is now imported inside save_config() so this no longer conflicts with Accept only ASCII digits in PINs and compare them as bytes #49 on the users_store import line (no behaviour change).

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