mount: add strip_components, numeric_ids and archive_dir_format mount options, fixes #4880

[ThomasWaldmann]

fstab and autofs entries (fuse.borgfs) can only give mount options (-o ...), not borg CLI options or environment variables. Until now, strip_components=N in such an entry was passed on to libfuse, which rejects unknown options, so the mount failed. In borg 2, the only workarounds were the mount: section of the config file (applies to all borgfs mounts) or BORG_STRIP_COMPONENTS (no way to set it from autofs / mount(8)).

Three commits:

• strip_components=NUMBER mount option: same as --strip-components NUMBER, also feeds the item filter (skipping too-short paths). The mount option wins if both are given. Fixes mount: support strip-components #4880.
• numeric_ids mount option: same as --numeric-ids; numeric_ids=no overrides the CLI flag. Also clarifies the docs, which said "using numeric-ids" although that was only the CLI flag.
• archive_dir_format=FORMAT mount option: same as BORG_MOUNT_ARCHIVE_DIR_FORMAT; the mount option wins if both are given. Errors name the source (archive_dir_format mount option: ... vs. BORG_MOUNT_ARCHIVE_DIR_FORMAT: ...). As mount options are separated by commas, FORMAT can not contain a comma (documented). borg webdav keeps using the env var.

All three options are removed from the option list before it is passed to libfuse.

Tests: parse_mount_options() unit tests in vfs_test.py (no FUSE needed), plus a mount option case in test_fuse_archive_dir_format (mount option set while the env var is also set). Also checked that borgfs /repo mnt -o ro,allow_other,strip_components=4,numeric_ids hands only ro,allow_other (plus borg's defaults) to libfuse, and ran ArchiveVFS.create_filesystem() on a test repo with env only / mount option + env / invalid formats from either source.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.13%. Comparing base (7e3d305) to head (c7f2f2d).
⚠️ Report is 7 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10505      +/-   ##
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+ Coverage   89.12%   89.13%   +0.01%     
==========================================
  Files         103      103              
  Lines       19783    19793      +10     
  Branches     3099     3102       +3     
==========================================
+ Hits        17631    17642      +11     
+ Misses       1490     1488       -2     
- Partials      662      663       +1     

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[PhrozenByte]

It might be a good addition to also support BORG_MOUNT_ARCHIVE_DIR_FORMAT as a mount option, since fstab and autofs entries don't support env variables either.