fix(resolve): share response headers with singleflight followers

[jensneuse]

Summary by CodeRabbit

• New Features

  • Added public APIs to retrieve per-request response context and to get/set deduplication/shared-data callbacks.
  • Enhanced request deduplication to propagate leader HTTP metadata (status, headers) and typed shared data to followers.

• Tests

  • Added tests validating shared-data and HTTP metadata propagation across deduplicated concurrent requests.

When singleflight deduplication is active, follower requests previously received only the serialized response body from the leader — subgraph response headers (e.g. Cache-Control, custom propagated headers) were silently dropped.

This fix addresses both singleflight mechanisms:

• Subgraph request singleflight: SingleFlightItem now carries responseHeaders and statusCode from the leader's HTTP call. Followers populate their ResponseContext from these fields, so LoaderHooks.OnFinished receives the correct ResponseInfo.ResponseHeaders for every request.
• Inbound request singleflight: Two opt-in callbacks (GetDeduplicationData / SetDeduplicationData) on Context allow the cosmo router to extract arbitrary state (e.g. accumulated response headers) from the leader and apply it to each follower before writing the response.

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📝 Walkthrough

Walkthrough

Adds leader-to-follower deduplication state propagation for single-flight requests: new context callbacks for typed get/set, storage of leader HTTP metadata (status and headers) on single-flight items, propagation of that metadata to follower contexts, and supporting tests. Also exposes a ResponseContext getter from the HTTP client package.

Changes

Cohort / File(s)	Summary
HTTP Response Context Retrieval v2/pkg/engine/datasource/httpclient/nethttpclient.go	Added GetResponseContext(ctx context.Context) *ResponseContext to read an injected ResponseContext from context.
Deduplication Data Callbacks v2/pkg/engine/resolve/context.go	Added public callback fields GetDeduplicationData func(ctx context.Context) any and SetDeduplicationData func(ctx context.Context, data any), a generic helper SetDeduplicationCallbacks[T any](...) for type-safe wiring, and cleared callbacks in Free().
Inbound single-flight state v2/pkg/engine/resolve/inbound_request_singleflight.go	Added SharedData any to InflightRequest for leader→follower opaque state; adjusted cancellation return path in GetOrCreate.
Subgraph single-flight state v2/pkg/engine/resolve/subgraph_request_singleflight.go	Added responseHeaders http.Header and statusCode int fields to SingleFlightItem to carry leader HTTP metadata.
Loader: capture/propagate HTTP metadata v2/pkg/engine/resolve/loader.go	Leader captures HTTP response metadata (status code and cloned headers) into the leader item after load; followers receive and apply that metadata from the shared item.
Resolver: propagate deduplication data v2/pkg/engine/resolve/resolve.go	When followers reuse single-flight data, code now calls SetDeduplicationData on follower contexts with leader SharedData, and leaders capture SharedData via GetDeduplicationData before finishing single-flight.
Tests: single-flight header/state propagation v2/pkg/engine/resolve/resolve_test.go, v2/pkg/engine/resolve/subgraph_request_singleflight_test.go, v2/pkg/engine/resolve/inbound_request_singleflight_test.go	Added tests verifying leader→follower propagation of deduplication shared data and HTTP response body/status/headers; added concurrent follower timeout and leader-error follower tests.

Sequence Diagram(s)

sequenceDiagram
    participant Client
    participant Resolver
    participant Context as RequestContext
    participant InflightMgr as Inflight Manager
    participant HTTP as HTTP Client

    Client->>Resolver: Resolve request (may be follower)
    alt No existing inflight (Leader)
        Resolver->>HTTP: Execute HTTP request
        HTTP-->>Resolver: Response + ResponseContext
        Resolver->>Context: Capture ResponseContext (status, headers)
        Resolver->>InflightMgr: Store leader SharedData/status/headers
    end

    Note over Resolver,InflightMgr: Followers arriving while leader is active

    alt Follower uses single-flight result
        Client->>Resolver: Resolve (follower)
        Resolver->>InflightMgr: Retrieve leader SharedData/status/headers
        InflightMgr-->>Resolver: Return SharedData
        Resolver->>Context: Inject SharedData via SetDeduplicationData
        Resolver-->>Client: Return response using leader's HTTP metadata
    end

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Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'fix(resolve): share response headers with singleflight followers' directly describes the main objective of the PR: implementing header sharing for singleflight deduplication followers.

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✨ Finishing Touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch jensneuse/fix-abstract-check

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[StarpTech]

LGTM