Using findmnt in the presence of a stale NFS mount

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Hi,

On Arch Linux, various package install scripts use findmnt to check
the status of /boot or /usr. However, if there exists a stale NFS
mount on the system (for instance on /mnt), findmnt hangs when used in
commands such as:

findmnt -snero fstype --tab-file '/etc/fstab' -T '/usr'
findmnt -s /boot
findmnt -s /usr

(note that on my system, /usr is not a mount point of any kind, while
/boot is ext4. /mnt is the stale NFS mount)

This older Arch bug https://bugs.archlinux.org/task/36629 refers to
the problem, but as far as I can tell the problem still exists in the
same form (at the time, the package maintainer added the -T option to
the command "findmnt -snero fstype --tab-file '/etc/fstab' -T '/usr'",
but I don't believe it changed anything).

In any case, from having a look at the code on the latest git (I can
reproduce the hang with 710ed55dcd), I believe the problem comes from
the following sequence.

When used with the -s|--fstab flag, findmnt goes through the list of
mount points in the tab file, and compares them with the target
specified, say '/usr'.

The comparison is done using mnt_fs_match_target() in
findmnt.c:match_func(). mnt_fs_match_target() uses mnt_resolve_path()
on both target ('/usr') and
fs->target, which uses canonicalize_path(fs->target), which uses
realpath(fs->target). As the fs->target is the stale NFS mount point,
this last call hangs in a lstat().

While it is clear that a system with a stale NFS mount point is a
minefield, would it be possible to avoid the call to
"realpath(fs->target)" (maybe controlled by a command line option)?

As it is, it is impossible to upgrade an Arch Linux install when there
is a stale NFS mount anywhere on the system (and listed in fstab).

Thanks,
Eric
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