ZTS: Test boundary conditions in alloc_class_012

[ghost]

Motivation and Context

Issue #9142 describes an error in the checks for device removal that
can prevent removal of special allocation class vdevs in some
situations.

Description

Enhance alloc_class/alloc_class_012_pos to check situations where this
bug occurs.

Update zts-report with knowledge of issue #9142.

Also fixed a bug in the test on FreeBSD, using an incorrect flag for stat. Used get_objnum from libtest instead.

How Has This Been Tested?

Ran ZTS for -T alloc_class on FreeBSD.

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.

[behlendorf]

This seems sensible to me as a short term way to mitigate the disruptive ZTS failures. Has anyone looked in to how involved resolving the underlying removal issue would be? That would really be preferable.

[ghost]

I am under the impression that @amotin is looking at how to solve the issue and will create a PR when ready, at which point the tests should start passing and the exceptions can be removed from zts-report.

This PR was prompted by the discussion about the issue between @amotin and @don-brady on Slack.

@don-brady wrote:

There is a ZTS case for this in alloc_class_012_pos.ksh. Does this test pass on FreeBSD? if so and you have a case that fails maybe we can tweak the test to catch the failure?

[behlendorf]

I see, sounds good.