Fix for premature buffering

[GuptaManan100]

Description

Related Issue(s)

• Bug Report: Premature buffering stop during concurrent reparenting can lead to query failures #16438

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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Deployment Notes

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.15%. Comparing base (e881b9f) to head (c597c44).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17013      +/-   ##
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+ Coverage   67.04%   67.15%   +0.11%     
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  Files        1571     1571              
  Lines      251677   251804     +127     
==========================================
+ Hits       168729   169105     +376     
+ Misses      82948    82699     -249     

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[arthurschreiber]

So this is only running SELECT queries during the failover? SELECT queries don't get buffered, and don't fail if the server is put into read_only mode during the failover. You'll need to run DML queries to actually see any impact. 🤔

[GuptaManan100]

Replica reads aren't buffered, but any query including SELECTs that is target to a PRIMARY tablet are buffered. But yes, the read_only wouldn't really fail for SELECTs. I'll try DMLs instead 👍

[arthurschreiber]

You might have to remove this time.Sleep here. I don't have a good idea of how long the failovers take in the tests, but I'd expect them to take around a second or so, which means it's very unlikely to happen at the same time.

[GuptaManan100]

I had tried with and without the sleep. I actually made a few more changes here and there, but nothing seemed to work. I'll try what you suggested in 👇

[arthurschreiber]

It might be easier to reproduce this via an "external" failover, as it's simpler to get the sequencing right:

• Set up a cluster with 2 shards (-80, 80-)
• Switch -80 and 80- primaries into read only mode to start buffering.
• Start running a DML query that's hitting shard -80 and one DML query that's hitting shard 80-. Both of these queries should get buffered and "block".
• Perform external promotion of a replica in shard -80 to primary. Then run TabletExternallyReparented for the externally promoted replica to mark it as primary in the vitess topology.
• This will stop buffering for both shards -80 and 80-. The query that was buffered for shard -80 will be retried and will be successful, the query that was buffered for shard 80- will fail because buffering was ended prematurely.

[GuptaManan100]

Thanks Arthur! I was able to reproduce the problem now!