Allow block cloning to be interrupted by a signal.

[pjd]

Even though block cloning is much faster than regular copying, it is not instantaneous - the file might be large and the recordsize small. It would be nice to be able to interrupt it with a signal (e.g., SIGINFO on FreeBSD to see the progress).

Motivation and Context

Description

How Has This Been Tested?

With those changes I can interrupt cloning large files, which was impossible before.

This allows to only to stop cloning in the middle, but also to track progress with SIGINFO.
Also see: https://reviews.freebsd.org/D45243

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)
• 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.

[johnpyp]

Does this warrant a test case or two?

Also, does this cancel the block clone cleanly without anything leftover? (I'm not familiar with the flow/error handling)

[pjd]

Does this warrant a test case or two?

I think it might be unstable to try to send signal before the operation finishes. It will be easier once ratelimiting is merged - then we will be able to slow down block cloning and make it predictable when sending a signal.

Also, does this cancel the block clone cleanly without anything leftover? (I'm not familiar with the flow/error handling)

At least on FreeBSD, the only interface to use block cloning currently is through copy_file_range(2). copy_file_range(2) can be interrupted before the whole file is copied and the caller should just continue and keep at it until copy_file_range(2) returns <= 0. So when block cloning is interrupted we simply did a partial clone and the application should continue to finish the operation.