[ThomasWaldmann]

Backport of #10509 to 1.4-maint, fixes #6652.

stat() / lstat() do not trigger an automount (on Linux, they act as if AT_NO_AUTOMOUNT was given), but opening the directory does. borg create stats a directory, then opens it and compares with fstat(): for an automount point that is not mounted yet, these differ, so borg considered it a race condition ("file type or inode changed") and skipped the whole tree. This affects autofs (indirect and direct maps), systemd .automount units and ZFS .zfs/snapshot/* on Linux.

Fix (same as on master)

• platform.on_different_mounts(fd1, fd2): Linux compares the mount ids (from /proc/self/fdinfo/<fd>), other platforms compare st_dev; None if undeterminable (handled like "no").
• archive.is_automount(): accepts the change only if it is a directory before and after and the opened directory is on a different mount than its parent directory. Everything else still raises BackupRaceConditionError.
• In _rec_walk, we then use the fstat() result and redo the skip_inodes and --one-file-system decisions.
• Recursion roots that are directories are opened before taking their stat (stat_dir_mounted), so the mount happens early and --one-file-system uses the device number of the mounted filesystem.

Differences to the master PR

• Adapted to the monolithic archiver.py (no stat_root() on 1.4, so stat_dir_mounted() is called directly for the recursion roots; symlinked recursion roots are not followed on 1.4, so no follow_symlinks variant).
• on_different_mounts is imported once from .base in platform/__init__.py and overridden for Linux, matching how 1.4 structures that module.
• The borg create test is an ArchiverTestCase method (skipped for the binary, as it patches objects).

Testing

Full test suite on macOS (Python 3.14): 1471 passed, 382 skipped. The Linux mount-id code path is identical to master, where it was tested manually on Debian 13 with autofs, systemd automount and ZFS snapshots.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 83.87097% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.14%. Comparing base (e81a51f) to head (21923d0).
⚠️ Report is 12 commits behind head on 1.4-maint.

Files with missing lines	Patch %	Lines
src/borg/archiver.py	77.77%	3 Missing and 1 partial ⚠️
src/borg/platform/base.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint   #10575      +/-   ##
=============================================
+ Coverage      82.11%   82.14%   +0.02%     
=============================================
  Files             38       38              
  Lines          11485    11514      +29     
  Branches        1810     1816       +6     
=============================================
+ Hits            9431     9458      +27     
- Misses          1468     1469       +1     
- Partials         586      587       +1     

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