fix releasing the global read lock when mysqlshell backup fails

[rvrangel]

Description

This fixes an issue we noticed during testing where if the backup process (mysqlsh) fails, be it because of incorrect parameters, missing binary, etc, it won't release the global read lock we are holding on the instance. This ensures we are releasing the lock even if we hit an error early on.

I also added unit tests to capture when those particular scenarios happen.

Related Issue(s)

Fixes #17011, an issue introduced that only affects the new mysqlshell backup engine introduced in #16295

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

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[frouioui]

I am adding the Backport to: release-21.0 label as the issue was introduced in v21.0.0-rc1 and the fix should be part of v21.0.0-rc2.

[codecov]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.09%. Comparing base (0fa7e58) to head (cfcd511).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/mysqlctl/fakemysqldaemon.go	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17000      +/-   ##
==========================================
- Coverage   69.34%   67.09%   -2.25%     
==========================================
  Files        1571     1571              
  Lines      204180   251763   +47583     
==========================================
+ Hits       141582   168915   +27333     
- Misses      62598    82848   +20250     

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[mattlord]

Nice work, @rvrangel ! I only had some minor comments. Let me know what you think and I'll come back to this quickly.

[mattlord]

Nit, but any reason for this not to remain a const?

Looks like it's only changed for the test, the value used is passed in as a parameter to generate the new file:

generateTestFile(t, mysqlShellBackupBinaryName, "#!/bin/bash\nexit 1")

And we've told the engine to use this other file instead of mysqlsh. This is OK, but couldn't we instead set and unset a shell alias for mysqlsh so that the shell calls our test script? It feels a little less bad than allowing the binary name to be changed although it's the same effect.

[rvrangel]

Indeed, this is only changed for the test. I am not sure setting an alias would work, as the exec documentation says:

Unlike the "system" library call from C and other languages, the os/exec package intentionally does not invoke the system shell and does not expand any glob patterns or handle other expansions, pipelines, or redirections typically done by shells.

I would imagine the above also means that setting an alias wouldn't work

[mattlord]

Thank you, @rvrangel ! ❤️