zpool iostat: refresh pool list every interval

[robn]

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

Description

When running zpool iostat in interval mode, it would not notice any new pools created or imported, and would forget any destroyed or exported, so would not notice if they came back. This leads to outputting "no pools available" every interval until killed.

It looks like this was at least intended to work; the comment above zpool_do_iostat() indicates that it is expected to "deal with pool creation/destruction" and that pool_list_update() would detect new pools. That call however was removed in 3e43edd, though its unclear if that broke this behaviour and it wasn't noticed, or if it never worked, or if something later broke it. That said, the lack of pool_list_update() is only part of the reason it doesn't work properly.

The fundamental problem is that the various things involved in refreshing or updating the list of pools would aggressively ignore, remove, skip or fail on pools that stop existing, or that already exist. Mostly this meant that once a pool is removed from the list, it will never be seen again. Restoring pool_list_update() to the zpool_do_iostat() loop only partially fixes this - it would find "new" pools again, but only in the "all pools" (no args) mode, and because its iterator callback add_pool() would abort the iterator if it already has a pool listed, it would only add pools if there weren't any already.

So, this commit reworks the structure somewhat. pool_list_update() becomes pool_list_refresh(), and will ensure the state of all pools in the list are updated. In the "all pools" mode, it will also add new pools and remove pools that disappear, but when a fixed list of pools is used, the list doesn't change, only the state of the pools within it.

The rest of the commit is adjusting things for this much simpler structure. Regardless of the mode in use, pool_list_refresh() will always do the right thing, so the driver code can just get on with the display.

Now that pools can appear and disappear, I've made it so the header (if enabled) is re-printed when the list changes, so that its easier to see what's happening if the column widths change.

How Has This Been Tested?

New tests included to confirm the behaviour of both the "all pools" and "list of pools" modes.

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.

[behlendorf]

Works as advertised! This has been a frustration for years, it's so nice to see it finally fixed.