libzfs: add zfs_mount_at() function

[kevans91]

Motivation and Context

zfs_mount_at will be used by FreeBSD's libbe to transition away from directly using zmount.

Description

This adds the capability for libzfs consumers to mount a dataset at an arbitrary mountpoint without resorting to digging deeper into libzfs internals to use hidden OS-specific mounting interfaces, and to do so without modifying dataset/pool properties to achieve the desired effect.

How Has This Been Tested?

Replaced libbe's zmount() with zfs_mount_at() in our local zfs; diff is similar enough that I wouldn't anticipate any issues in the transition here.

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)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux 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.

I'm not entirely sure where one would find libzfs documentation, given the nature of it. Also unsure of where one would add tests for it, but given that this just pulls the 'grab mountpoint 'part out into zfs_mount() and doesn't functionally change much- I guess the test coverage doesn't drop all that much.

[codecov]

Codecov Report

Merging #9833 into master will increase coverage by <1%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master    #9833    +/-   ##
========================================
+ Coverage      79%      80%   +<1%     
========================================
  Files         385      385            
  Lines      121610   121615     +5     
========================================
+ Hits        96429    96706   +277     
+ Misses      25181    24909   -272

Flag	Coverage Δ
#kernel	80% <ø> (ø)	⬆️
#user	67% <100%> (+1%)	⬆️

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[behlendorf]

Providing such an interface makes sense. Are you aware of anything other than the mountpoint which would be useful to specify?

[kevans91]

I had put some thought into it, but hadn't come up with anything (unless it's feasible to, say, set sync=disabled for just this mount).

Perhaps it would be better to structure this as some kind of zfs_mount_nvl instead to leave it open-ended for future expansion, them for the time being we just reject any property that isn't "mountpoint" -- worst case scenario, we're stuck with a function that's needlessly flexible. We can avoid referring to the extra parameter as "properties" so we aren't tied to 1:1 mapping property names.

[behlendorf]

OK, I couldn't think of anything immediately useful either. Why don't we go ahead with the PR as proposed but keep the idea for a zfs_mount_nvl in our back pocket. We can always implement it if someone comes up with a consumer that needs a more flexible interface.

[ghost]

We have test coverage from existing tests since it's used by zfs_mount(). 👍

[kevans91]

OK, I couldn't think of anything immediately useful either. Why don't we go ahead with the PR as proposed but keep the idea for a zfs_mount_nvl in our back pocket. We can always implement it if someone comes up with a consumer that needs a more flexible interface.

Sounds good to me!