mount: list the current borg mounts when no MOUNTPOINT is given, fixes #2579

[ThomasWaldmann]

Fixes #2579.

borg mount without a MOUNTPOINT now prints the mountpoints of all file systems mounted by borg mount, one per line; --json gives a JSON list with mountpoint, source and fstype. This needs neither a repository nor FUSE (no passphrase prompt, no FUSE binding import). borgfs still requires the mountpoint.

Per-OS mount table findings (measured on the test boxes)

How a FUSE file system mounted with fsname=borgfs shows up, and which API enumerates it:

OS	source	fstype	enumerate via
Linux	borgfs	fuse (now fuse.borgfs, see below)	/proc/self/mountinfo
macOS (macFUSE)	borgfs	macfuse	getmntinfo() via ctypes (getmntinfo$INODE64 on x86_64, 2168-byte struct statfs)
FreeBSD	borgfs	fusefs	getmntinfo() via ctypes
NetBSD (librefuse/puffs)	/dev/puffs	puffs|borgfs (the fsname lands in the fstype)	__getvfsstat90() via ctypes (the plain getvfsstat symbol is the pre-9 compat version and returns nothing)
illumos (OmniOS)	borgfs	fuse	/etc/mnttab (5 tab-separated fields)
Windows (WinFsp)	volume label (from volname)	FUSE	GetLogicalDriveStringsW + GetVolumeInformationW

Changes

• new platform API platform.list_mounts() returning the mount table as MountEntry(source, fstype, mountpoint) tuples, implemented in the per-OS platform modules: /proc/self/mountinfo parser in linux.pyx, /etc/mnttab parser in solaris.py, getmntinfo() / getvfsstat() as Cython externs in darwin.pyx, freebsd.pyx and netbsd.pyx (the C headers provide the struct layouts and the getmntinfo$INODE64 / __getvfsstat90 symbol mapping), drive letter enumeration in windows.pyx; other platforms get an empty list with a warning.
• vfs.is_borgfs_mount() holds the recognition rule in one place: source == "borgfs" or fstype.endswith("borgfs"); on Windows: fs name FUSE and a volume label containing borgfs.
• Linux: parse_mount_options adds subtype=borgfs to the libfuse options, so the file system type is fuse.borgfs (matching the fstab examples in the docs) and a borg mount is recognized even with a user-supplied -o fsname=. Verified with llfuse on Debian 13; the llfuse / pyfuse3 / mfusepy CI jobs cover the three bindings. Not added on other platforms (FreeBSD ignores it).
• Linux: list_mounts() raises a clear OSError ("cannot read /proc/self/mountinfo (is /proc mounted?)") when the file is unreadable, and the listing reports any enumerator OSError as a runtime error rather than a traceback.
• tests: parser unit tests with captured mountinfo / mnttab lines, a live list_mounts() test, rule tests, output/JSON tests with a faked mount table, and a FUSE test that mounts a repo and checks it is listed (and no longer listed after unmounting).
• docs: mount epilog, usage example, changelog.

Limitations

• Windows: only mounts on a drive letter are listed. A directory mountpoint is a junction created by WinFsp and there is no API to enumerate those (documented in the epilog and the changelog).
• NetBSD: a user-supplied -o fsname=other makes the fstype puffs|other, which is then not recognized (macOS and FreeBSD ignore such an override, Linux is covered by the subtype).
• OpenBSD: not supported (warning + empty list). The kernel sets the mount source of every FUSE file system to the literal fusefs and ignores fsname, so a borg mount cannot be told apart from other FUSE mounts; borg mount is not usable there with packaged software anyway (no FUSE binding in the port or the ports tree).

The enumerators were run on the Debian 13, OmniOS r151058, FreeBSD 15.1 and NetBSD 11 test boxes (the FreeBSD and NetBSD extensions compiled there against the real headers) and on macOS 27 (where the end-to-end listing of a real borg mount, including a mountpoint with a space, was checked).

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 94.54545% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.34%. Comparing base (4c563dc) to head (2155628).
⚠️ Report is 37 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/archiver/mount_cmds.py	89.47%	2 Missing ⚠️
src/borg/platform/__init__.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10514      +/-   ##
==========================================
+ Coverage   89.30%   89.34%   +0.03%     
==========================================
  Files         104      104              
  Lines       20027    20079      +52     
  Branches     3144     3152       +8     
==========================================
+ Hits        17886    17940      +54     
+ Misses       1482     1481       -1     
+ Partials      659      658       -1     

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