Fix vttablet not being marked as not serving due when MySQL stalls

[arthurschreiber]

Description

The heartbeat reader uses an interval to check the timestamp of the last received heartbeat write on replica tablets.

See:

https://github.com/github/vitess-gh/blob/976f228e12c07d78091678b428038175bc46a60c/go/vt/vttablet/tabletserver/repltracker/reader.go#L80-L81
https://github.com/github/vitess-gh/blob/976f228e12c07d78091678b428038175bc46a60c/go/vt/vttablet/tabletserver/repltracker/reader.go#L107-L108

The *heartbeatReader.readHeartbeat func then stores the result of the last heartbeat read in two fields of heartbeatReader, namely lastKnownLag and lastKnownError.

Unfortunately, there's a problem with context /timeout handling in the implementation, which can cause readHeartbeat to stall if MySQL is stalling too. This can be reproduced by sending SIGSTOP to the MySQL process, and then observing that the vttablet process will actually keep reporting MySQL as healthy.

There's a Context with a timeout passed down to the *heartbeatReader.fetchMostRecentHeartbeat func, but I think the timeout handling inside of *Conn.Exec is badly implemented. I wasn't able to trace down exactly where the problem is, but dbc.env.CheckMySQL might be a culprit, because it tries to query MySQL without any context handling.

The workaround I'm proposing is to change *heartbeatReader.Status func to just disregard any previously read heartbeat data if the last time the heartbeat was successfully read is older than two times the configured interval. It feels a bit hacky, but works reliably in identifying that MySQL is unhealthy and very quickly signals to vtgate that the tablet is unhealthy.

Related Issue(s)

Fixes #11884

Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.50%. Comparing base (2c10c95) to head (00d227b).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/tabletserver/repltracker/reader.go	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17883      +/-   ##
==========================================
- Coverage   67.52%   67.50%   -0.03%     
==========================================
  Files        1594     1595       +1     
  Lines      259564   259653      +89     
==========================================
+ Hits       175266   175271       +5     
- Misses      84298    84382      +84     

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