platform: use F_FULLSYNC on macOS for SyncFile data durability, fixes #9383

[mr-raj12]

• SyncFile.sync() calls platform.fdatasync(self.fd), which resolves to os.fdatasync or os.fsync - On macOS, fsync() only flushes data to the drive's write cache, not to persistent storage

  • True durability requires fcntl(fd, F_FULLFSYNC) — without it, a power loss could lose data that SyncFile believed was safely persisted
  • This was acknowledged with a TODO at src/borg/platform/base.py:154: TODO: Use F_FULLSYNC on macOS.
  • Implements a platform-specific sync wrapper as suggested in platform: use F_FULLSYNC on macOS for SyncFile data durability #9383

  Fixes platform: use F_FULLSYNC on macOS for SyncFile data durability #9383

  Changes

  File	Change
  src/borg/platform/darwin.pyx	Add fdatasync() using fcntl F_FULLFSYNC with os.fsync() fallback; add sync_dir() using the same
  src/borg/platform/__init__.py	Import fdatasync, sync_dir from .darwin in the darwin block
  src/borg/platform/base.py	Remove resolved TODO comment
  src/borg/testsuite/platform/darwin_test.py	Add 4 tests: F_FULLFSYNC usage, fsync fallback, fdatasync integration, sync_dir integration

  Checklist

  • PR is against master
  • New code has tests
  • Tests pass
  • Commit message references related issue
  • Code formatted with black

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.43%. Comparing base (cace06f) to head (0354697).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/platform/__init__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9385      +/-   ##
==========================================
- Coverage   76.44%   76.43%   -0.01%     
==========================================
  Files          85       85              
  Lines       14799    14800       +1     
  Branches     2212     2212              
==========================================
  Hits        11313    11313              
- Misses       2808     2809       +1     
  Partials      678      678              

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[ThomasWaldmann]

For performance reasons, we don't do macOS tests for PRs, so we have to check this when it is merged.

Also a backport to 1.4-maint is likely needed.

[mr-raj12]

For performance reasons, we don't do macOS tests for PRs, so we have to check this when it is merged.

Also a backport to 1.4-maint is likely needed.

sure and shall I prepare the backport PR for 1.4-maint now or after this is merged to master?

[github-actions]

Backport failed for 1.4-maint, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin 1.4-maint
git worktree add -d .worktree/backport-9385-to-1.4-maint origin/1.4-maint
cd .worktree/backport-9385-to-1.4-maint
git switch --create backport-9385-to-1.4-maint
git cherry-pick -x 0354697c56e4ea047b4ca6550622fab5e5e505f4

[ThomasWaldmann]

Wait until the macOS tests there are finished and successful:

https://github.com/borgbackup/borg/actions/runs/22236158222