Fix: Separate Lock for Keyspace to Update Controller Mapping in Schema Tracking

[harshit-gangal]

Description

Problem

Previously, schema tracking update calls to VTTablet via GetSchema RPC and access to the keyspace-to-update-controller mapping (tracked map) were using the same lock. These operations are unrelated, but GetSchema RPC calls can be slow due to network-related reasons.

As a result, fetching the update controller from the map was blocked, leading to:

• Slow processing of health updates
• Channel overflow, causing dropped health check updates

Solution

Introduced a separate mutex for managing access to the tracked map, ensuring that schema tracking operations are not blocked by RPC calls. This prevents delays in processing health updates and improves overall stability.

Backport Reason: Health check misses can lead to missing schema changes.

Related Issue(s)

Checklist

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Deployment Notes

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[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.47%. Comparing base (b05df12) to head (816df9e).

Files with missing lines	Patch %	Lines
go/vt/vtgate/schema/tracker.go	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17873      +/-   ##
==========================================
+ Coverage   67.45%   67.47%   +0.01%     
==========================================
  Files        1594     1594              
  Lines      259064   259075      +11     
==========================================
+ Hits       174760   174813      +53     
+ Misses      84304    84262      -42     

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[GuptaManan100]

While this works, should we get rid of the locking altogether in the codepath that is reading data from vttablets?