[VTAdmin API] Fix schema cache flag, add documentation

[notfelineit]

Description

Currently, when a user passes any schema cache flags, but does not pass backfill-queue-size, vtadmin-api crashes because backfill-queue-size is defaulted to -1 and crashes with:

panic: makechan: size out of range

goroutine 1 [running]:
vitess.io/vitess/go/vt/vtadmin/cache.New[...](0xc000375570, {0xdf8475800, 0xffffffffffffffff, 0x5f5e100, 0xffffffffffffffff, 0xffffffffffffffff, 0x2faf080})
	vitess.io/vitess/go/vt/vtadmin/cache/cache.go:131 +0x20d

at https://github.com/vitessio/vitess/blob/main/go/vt/vtadmin/cache/cache.go#L131

This PR makes it so that backfill-queue-size is defaulted to 0 if the value is negative. It also adds the schema-cache-* flags to the example cluster.yml, and adds some logs.

A subsequent PR will make it so that the debug endpoint returns the cache values as they are actually set. (currently shows -1 if the user didn't pass configuration, even if defaults are different).

Backport reason

This is a bug that breaks vtadmin-api usage.

Related Issue(s)

#15717

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

None

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 68.39%. Comparing base (f118ba2) to head (eb0f7b7).
Report is 5 commits behind head on main.

Files	Patch %	Lines
go/vt/vtadmin/cluster/cluster.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15704      +/-   ##
==========================================
- Coverage   68.40%   68.39%   -0.02%     
==========================================
  Files        1556     1556              
  Lines      195121   195275     +154     
==========================================
+ Hits       133479   133564      +85     
- Misses      61642    61711      +69     

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[mattlord]

LGTM.

I think that saying “this is the default value” can be removed from the descriptions. I also think that we could better describe what a specific value means — sometimes 0 means none, other times disabled, and other times unlimited.

We should also create a quick issue that describes the bug we’re fixing. Largely a cut and paste from the PR description. Then we can add a line that says Fixing: XXXX in the related issues section of the description.

thanks!

[mattlord]

What does 0 mean? Unlimited?

[notfelineit]

I hope this answers the question:
#15704 (comment)

[mattlord]

0 means that we don’t backfill?

[notfelineit]

I think it means that the underlying channel will have a size of 0, and every send to the backfill queue will block until the queue is "empty" again. If that's non-ideal, we can change it to 1. I had it at 1 but that broke an existing test assumption.