Allow zfs to send replication streams with missing snapshots

[pabloyoyoista]

Motivation and Context

A similar change was proposed previously in #5285 by @loli10K. In the pull request, it was discussed that the integration of OpenZFS 6111 (6635624) reduced the utility of sending replication streams. If there are any datasets in the hierarchy which are not snapshoted, a replication send of any parent will fail due to the missing snapshots. The rationale for that decision is to avoid administrators to think that they've sent all the data when in reality that is not the case. According to #5285, the right way to do this would be to add an extra flag (I chose -f|--force --skip-missing|-s) which modifies that behavior for administrators which request it.

Also, this is my first contribution to the OpenZFS project, so I apologize for any possible newbie mistakes. I have not added documentation on purpose, because I would like to be sure that the ideas here make sense before putting extra effort into it.

Description

This patch adds an extra option (--skip-missing|-s) to zfs send. The skip flag can only be used in conjunction with -R. In the regular replication behavior, if zfs encounters a dataset without the corresponding snapshot and that was created before the parent snapshot, then an error will occur and the send will be aborted. This MR modifies that behavior, so that if skip flag is also provided, then the dataset and its children will be ignored. Instead of an error, a warning text will be written to stderr and the send will continue.

How Has This Been Tested?

One small test has been added to the testsuite, which I could extend if required and some local testing worked without issues. I have not done any further tests in a real system, but I could also possibly do if needed. I have placed the test under functional/cli_root/zfs_send because it is just a small feature... But I hesitated whether it would fit best under functional/rsend with a bit extra testing. Any suggestions?

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.

[bghira]

one comment is that the -f flag name is very vague and overused. it might become the norm to use -f without remembering why. can you make it more specific to skipping missing snapshot?

[pabloyoyoista]

Yes, that is totally true. Maybe -s|--skip-missing? -S|--saved is already used to send partially received datasets, but I'd guess the use cases are different enough not to get confusing due to the capitalization. Any other suggestions otherwise?

[pabloyoyoista]

Replaced to --skip-missing|-s now

[pabloyoyoista]

As there were not any other comments, I have finished and pushed the documentation. This should be ready for review.

[behlendorf]

Thanks for tackling this! I don't have any objection to adding a command line option to allow this.