Backport #15468 to 2.2: Fix nfs_truncate_shares without /etc/exports.d

[alex-moch]

Calling nfs_reset_shares on Linux prints a warning: failed to lock /etc/exports.d/zfs.exports.lock: No such file or directory
when /etc/exports.d does not exist. The directory gets created, when a filesystem is actually exported through nfs_toggle_share and nfs_init_share. The truncation of /etc/exports.d/zfs.exports happens unconditionally when calling zfs mount -a (via zfs_do_mount and share_mount in cmd/zfs/zfs_main.c).

Fixing the issue only in the Linux part, since the exports file on freebsd is in /etc/zfs/, which seems present on 2 FreeBSD systems I have access to (through /etc/zfs/compatibility.d/), while a Debian box does not have the directory even if /usr/sbin/exportfs is present through the nfs-kernel-server package.

The code for exports_available is copied from nfs_available above.

Fixes: ede037c
("Make zfs-share service resilient to stale exports")

Reviewed-by: Brian Atkinson [email protected]
Reviewed-by: Brian Behlendorf [email protected]

Closes #15369
Closes #15468

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Motivation and Context

Backport of #15468 to the zfs-2.2.10-staging branch.

Fixes #15369 and #17967

Description

Clean cherry-pick of commit 41e55b4 from master to zfs-2.2.10-staging.

The fix adds an exports_available() check before attempting to truncate the exports file, preventing the segfault when the directory doesn't exist.

How Has This Been Tested?

• Original fix was tested and merged to master in Fix nfs_truncate_shares without /etc/exports.d #15468
• Patch applies cleanly to zfs-2.2.10-staging
• Tested on musl-based Gentoo system - builds cleanly and zfs share -a no longer segfault

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.