zfs test send-c_volume is flaky

[pcd1193182]

Motivation and Context

During some automated test runs, especially with github workers, this test fails spuriously, with a message about a checksum mismatch. This happens because the test works by reading in the raw zvol device and taking the checksum of the data; however, the way it's structured, the return value of that dd call isn't checked. If the file isn't present, we just checksum the empty string, which does not compare equal to the normal value. This makes it look like corruption is happening, when in fact the device is not present yet.

Description

We use block_device_wait to wait for the zvol block device to actually appear, and we log the result of the dd calls by using an intermediate file.

How Has This Been Tested?

Several runs of the test in question, and the rest of the test suite.

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.

[gmelikov]

LGTM

[behlendorf]

Thanks for running this down!