Fix VTOrc holding locks after shutdown

[GuptaManan100]

Description

This PR fixes the issue described in #11441. On further investigation it was found that this problem was introduced when we started using the servenv package in VTOrc.

The proposed fix is to run the code that wait for the shards to be let go as part of the synchronous on termination hooks.
Unit tests validating the wait function have also been added.

Related Issue(s)

• Fixes Bug Report: VTOrc does not let go of the shards it is holding during shutdown #11441

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[deepthi]

I think this is good practice anyway, but I do wonder if we have a bug in the lock code where we are not checking for a canceled context.

[deepthi]

IIUC, we already had the code for this, we just needed to register it properly with servenv, correct?

[GuptaManan100]

@deepthi Yes, we had code for this already. The thing is that when we started using servenv, it also came with its own handling of SIGTERM. So this code and servenv code would both see the SIGTERM, this code would block until all the locks were let go, but in the meantime servenv would finish, which would cause the main go routine to finish and the program would stop killing all the other go-routines including this one.

Now, we register this function as a on-termination hook to be run, so SIGTERM is only caught by servenv, but as part of the closing of the servenv, this function is called and this blocks until all the locks are released.