VDiff: fix race when a vdiff resumes on vttablet restart

[rohit-nayak-ps]

Description

There is a race when vttablet starts and the vdiff engine opens. It starts vdiffs which need to be resumed where we restart vdiffs in error state concurrently with the vdiff engine init. It is possible that globalStats map is not initialized when a new controller gets added and the controller adds it self to the map.

This PR starts the goroutine that runs the controller thread only after the inits have been done.

if err := vde.initControllers(rows); err != nil {
		return err
	}
	vde.updateStats()

	// At this point we've fully and successfully opened so begin
	// retrying error'd VDiffs until the engine is closed.
	vde.wg.Add(1)
	go func() {
		defer vde.wg.Done()
		if vde.fortests {
			return
		}
		vde.retryErroredVDiffs()
	}()

This was added way back, so we backport to all supported versions: https://github.com/vitessio/vitess/pull/10639/files#diff-931d120fc70d7007ffd77176f54c526f678c1a6c4e703dbf54b3086c967dbd3dR323

Related Issue(s)

• Fixes: Bug Report: Segfault and panic when restarting tablet with active vdiff #17637

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 90.47619% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.77%. Comparing base (9c6c380) to head (2cdeec5).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/tabletmanager/shard_sync.go	77.77%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17638   +/-   ##
=======================================
  Coverage   67.76%   67.77%           
=======================================
  Files        1586     1587    +1     
  Lines      255763   255801   +38     
=======================================
+ Hits       173315   173361   +46     
+ Misses      82448    82440    -8     

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[rohit-nayak-ps]

Added this because we also saw a panic here when the vdiff panic occurred. Possibly a different root cause (race?) is causing that since we should never get a nil event, but adding a guard here so vttablet doesn't panic in such a case but logs it.

[mattlord]

Thanks, @rohit-nayak-ps !

[mattlord]

@rohit-nayak-ps the stats were only added in v20: fa59f9d