txg: generalise txg_wait_synced_sig() to txg_wait_synced_flags()

[robn]

[Sponsors: Klara, Inc., Wasabi Technology, Inc.]

Motivation and Context

We have work in progress that will require breaking out of a txg_wait_synced() call in some situations. These are still in development, and may change, but are:

• To make fsync() return error when the pool suspends, txg_wait_synced() needs to return when the pool suspends
• A variation of forced export (a new update in Support forced export of ZFS pools #11082) may want to signal a condvar to throw out everything blocked in txg_wait_synced().

This is the supporting API change to allow these kind of things to be added in the future.

Description

txg_wait_synced_sig() is "wait for txg, unless a signal arrives". We expect that future development will require similar "wait unless X" behaviour.

This generalises the API as txg_wait_synced_flags(), where the provided flags describe the events that should cause the call to return.

Instead of a boolean, the return is now an error code, which the caller can use to know which event caused the call to return.

The existing call to txg_wait_synced_sig() is now txg_wait_synced_flags(TXG_WAIT_SIGNAL).

How Has This Been Tested?

Compile checked on Linux and FreeBSD. Basic sanity checks done.

(Tested a lot on internal projects too).

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.

[amotin]

I have no objections, primarily a worry whether we'll be able to watch for few things same time. They'd need to have similar locking. But that's later. Meanwhile just couple things to consider.