Fix panic in schema tracker in presence of keyspace routing rules

[GuptaManan100]

Description

This PR fixes the panic found in #16382.

This PR reproduced the panic as a unit test and then it fixes it. The problem is that we are using FindRoutedTable function to find the table which can fail to find the table if keyspace routing rules are present.

The fix is to not go through the FindRoutedTable function at all. In the updateTableInfo function, we already go over all the tables and add them to the keyspace map if the table doesn't exist when we populate the columns in setColumns.
We should use this table map directly.

The use of FindRoutedTable would be justified if we were supporting cross-keyspace foreign keys. Since we don't support them, all the foreign key references are guaranteed to be to tables that exist in the tables of the same keyspace. So it is safe to find both the table and the table it is foreign key related to in the table map that we populate right before this loop.

Related Issue(s)

• Fixes Bug Report: Panic in vtgate schema manager because of keyspace rules #16382

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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[harshit-gangal]

The use of FindRoutedTable would be justified if we were supporting cross-keyspace foreign keys. Since we don't support them, all the foreign key references are guaranteed to be to tables that exist in the tables of the same keyspace. So it is safe to find both the table and the table it is foreign key related to in the table map that we populate right before this loop.

I do not agree with this completely. The decision to not support cross-shard foreign key is taken at the planning level, it should not be restricted for schema tracking.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 68.67%. Comparing base (fd6411d) to head (bed5869).
Report is 5 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/vschema_manager.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16383      +/-   ##
==========================================
- Coverage   68.68%   68.67%   -0.01%     
==========================================
  Files        1548     1548              
  Lines      199063   199084      +21     
==========================================
- Hits       136721   136717       -4     
- Misses      62342    62367      +25     

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[GuptaManan100]

We're split on how to handle foreign keys for tables that have routing rules on it. We'll discuss this further and do a separate PR for the fix. For now, we're just going to fix the panic for them.

[GuptaManan100]

i have backported the changes to v20 and v19. it is not needed in v18 because we don't set the primary key values in that release.