Fix PromoteReplica call in ERS

[GuptaManan100]

Description

As described in #15935, running PromoteReplica before calling SetReplicationSource on the replicas is an issue as it can cause PromoteReplica to indefinitely hang.

This PR fixes 2 issues -

1. We now run PromoteReplica in parallel with SetReplicationSource. This also allows us to not run the PrimaryPosition RPC separately, since PromoteReplica also returns the position of the primary.
2. We should be using a context that has a timeout when running any RPC calls. Previously PromoteReplica and InitPrimary both were being called without adding a context timeout. This has also been fixed now.

Related Issue(s)

• Fixes Bug Report: ERS fails if PromoteReplica runs a write #15935

Checklist

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• Tests were added or are not required
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.46%. Comparing base (3b09eb2) to head (392845f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15934      +/-   ##
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- Coverage   68.47%   68.46%   -0.02%     
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  Files        1562     1562              
  Lines      197083   197080       -3     
==========================================
- Hits       134962   134936      -26     
- Misses      62121    62144      +23     

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[deepthi]

I have a few comments that I think are important to resolve before this can be merged.
Also, can we create a test case for the context fix? We'd have to write the test without this PR's changes first and make sure it fails, and that it passes with this PR.

[GuptaManan100]

@deepthi Added the test as well, which I ensured failed before the changes.

[mattlord]

Where did this change? It was not apparent to me:

We now run PromoteReplica in parallel with SetReplicationSource

I wanted to understand and confirm that part (sorry if I just missed something obvious). Other than that, just minor comments.

[GuptaManan100]

Thank you for the review @mattlord. I'll work on addressing them 👍.

As far as the changes for running PromoteReplica in parallel, earlier we would call PromoteReplica and then the reparentReplicas function. Now, we call PromoteReplica as part of handlePrimary in reparentReplicas. handlePrimary runs in parallel with handleReplica which runs SetReplicationSource.

[mattlord]

Thank you for the review @mattlord. I'll work on addressing them 👍.

As far as the changes for running PromoteReplica in parallel, earlier we would call PromoteReplica and then the reparentReplicas function. Now, we call PromoteReplica as part of handlePrimary in reparentReplicas. handlePrimary runs in parallel with handleReplica which runs SetReplicationSource.

OK, I see. So here: https://github.com/vitessio/vitess/pull/15934/files#diff-aace4cbbbebad37f5072421e16ca464c0f0b4eb98ad287380c2c764d11e7b836R310-R311

We're calling the reparentReplicas function — which does the promotion of the new primary and the replica reparents — instead of doing those two things serially. Now that I've looked at that reparentReplicas function I can see what you meant (it uses goroutines for each higher level task of promotion and reparent). Thanks!

[mattlord]

I only had minor comments/requests left so approving so that you can merge when you've addressed those as you feel is best.

[GuptaManan100]

Thank you @mattlord! I've addressed all the review comments, and just waiting for the tests now! 🚀