[Internal] PPAF: Fixes Hub region header never sent due to premature NoRetry on single-master 404/1002.

[aavasthy]

Pull Request Template

Description

PR Description

Summary

Fixes a bug in ClientRetryPolicy.ShouldRetryOnSessionNotAvailable where the hub region processing header (x-ms-cosmos-hub-region-processing-only) was set but never actually sent to the backend on single-master accounts. The retry threshold sessionTokenRetryCount > 1 caused NoRetry immediately after the header flag was set, preventing the retry that would carry the header.

Fix

Changed sessionTokenRetryCount > 1 → sessionTokenRetryCount > 2.

Corrected Flow

Attempt	count before method	Guard sets header?	count++	Check count > 2	Result	Header on request?
1st request	—	—	—	—	Sent to preferred read region	❌ No
1st 404/1002	0	No (0 >= 1 = false)	→ 1	1 > 2 = false	✅ Retry to write region (index=0)	❌ No
2nd 404/1002	1	Yes (1 >= 1 = true)	→ 2	2 > 2 = false	✅ Retry to write region (index=0)	✅ Yes
3rd 404/1002	2	Yes (already true)	→ 3	3 > 2 = true	⛔ NoRetry — hub confirmed not available	✅ Yes

End-to-End Hub Region Discovery (Happy Path with 403/3)

Step	Request	Response	Hub Header?	What happens
1	Read to preferred region	404/1002	❌	Session not available → retry to write region
2	Read to write region	404/1002	❌	Write region also returns session not available → header flag set → retry
3	Read to write region	403/3	✅	Non-hub region rejects hub-only request → PPAF marks endpoint unavailable → retry to next region
4	Read to next region	200 OK	✅	Hub region found, request succeeds, hub cached for partition

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Closing issues

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[kundadebdatta]

LGTM.

[kushagraThapar]

Since this is specifically fixed for single master, I wonder if we can add some tests for multi-master account? making sure the retries are same in multi-master and are correct.
@jeet1995 can you please review this PR as well, thanks!

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[aavasthy]

Since this is specifically fixed for single master, I wonder if we can add some tests for multi-master account? making sure the retries are same in multi-master and are correct. @jeet1995 can you please review this PR as well, thanks!

We have multi-master tests already added ensuring that this header is not there
https://github.com/Azure/azure-cosmos-dotnet-v3/blob/f8f93e55c65a435fec2de7b7a566ecdca8d80dee/Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/ClientRetryPolicyTests.cs?plain=1#L415C23-L415C23

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[NaluTripician]

@sdkReviewAgent-2

[kundadebdatta]

LGTM Now.

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[xinlian12]

✅ Review complete (22:21)

Posted 2 inline comment(s).

_{Steps: ✓ context, correctness, cross-sdk, design, history, past-prs, synthesis, test-coverage}