Flaky TestMoveTables(Un)sharded: Handle race condition

[rohit-nayak-ps]

Description

There is a goroutine that runs which starts the vplayer thread for each workflow which is triggered by updating the _vt.vreplication table using VExec . Now most of the time the test ends before it starts doing anything significant. The failure happens when it runs for a bit.

#17166 has added a new query to that path and this was not expected by the db mock layer, which was the reported failure. But the solution is a bit more involved than just adding that query as an expected query. When the vplayer runs it requires some more mocking support. Needed to add an additional query and update the mock object for the VStream() API to return rather than panic. It doesn't matter what this mock returns since the test doesn't make use of the response from that API.

Not sure if there are other flakiness causes here, since this has been flaky for a while but local tests which were erroring out earlier about 5% of the time are passing now.

Tested by running TestMoveTablesSharded several 100s of times, both MoveTables tests 100 times and all test in the directory 50 times.

This happens on v21 as well, so we backport there.

Related Issues

Previous flaky test fix: #17343

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

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[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.67%. Comparing base (059d01a) to head (e072939).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/tabletconntest/fakequeryservice.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17440      +/-   ##
==========================================
- Coverage   67.68%   67.67%   -0.02%     
==========================================
  Files        1583     1584       +1     
  Lines      254321   254360      +39     
==========================================
- Hits       172131   172130       -1     
- Misses      82190    82230      +40     

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[mattlord]

Thanks, @rohit-nayak-ps ! ❤️ Makes total sense. It probably failed this way N times during the development of my PR but I didn't notice because I'd gotten too accustomed to it being flaky — my fault.