[rush-daemon][WS2.9][8/9] Merge shared builds

[mojaza]

Summary

Merge compatible concurrent phased SHARED-BUILD requests into one warm operation-graph iteration while preserving client-scoped output and results.

Depends on #7. This stack follows merged microsoft#5949 and advances microsoft#5897. Azure PBI: https://onedrive.visualstudio.com/EFun/_workitems/edit/3216025

Details

• Adds a graph-owned coordinator with deterministic event-loop-turn batch boundaries for admitted shared builds.
• Reconciles retained workspace invalidations once per batch, unions each client's dependency closure, and performs one setEnabledStates/schedule/execute cycle.
• Fans raw streams and structured events out only to each client's retained operation subset with independent ordered backpressure.
• Derives exit semantics from each client's subset, so disjoint failures remain isolated and shared failures propagate only to clients containing the failed operation.
• Removes pre-start cancellations, unsubscribes mid-run cancellations without stopping work needed by live clients, and aborts only when the batch has no live client.
• Preserves FIFO/exclusive admission gating, warm retained results, late-arrival batching, exact-once cleanup, and recovery after failed iterations.

Limitations

This remains opt-in and dead-code-safe. It uses the existing injected real graph/shape seam and does not integrate with apps/rush or construct PhasedScriptAction. Command-independent graph/plugin construction and complete per-iteration runner lifetime remain blocked by open microsoft#5895.

How it was tested

• rushx build in libraries/rush-daemon (TypeScript, ESLint, API Extractor)
• Direct Jest execution of PhasedRequestBatching.test.js, PhasedRequestRouter.test.js, and RequestAdmissionIntegration.test.js: 30 tests passed
• rush change --verify
• Focused read-only correctness review after implementation and after timing/cancellation fixes

[Copilot]

🔵 Needs a closer look

It substantially refactors core phased graph-execution concurrency, batching, and cancellation behavior, which warrants final human validation beyond automated review.

Pull request overview

This PR enhances the @rushstack/rush-daemon phased request routing so that compatible concurrent SHARED-BUILD requests can be coalesced into a single warm operation-graph iteration, while still delivering per-client scoped output/events/results and preserving FIFO/exclusive admission semantics.

Changes:

• Introduces a graph-owned batching coordinator that merges admitted SHARED-BUILD phased requests at deterministic event-loop-turn boundaries and executes a single unioned graph iteration.
• Updates event fan-out to support multiple simultaneous per-client sinks with independent ordered backpressure, plus refined cancellation behavior (unsubscribe a client without aborting shared work unless no live clients remain).
• Adds/updates Jest coverage for batching/coalescing behavior and the exclusive FIFO gating between shared-build batches.

File summaries

File	Description
libraries/rush-daemon/src/test/RequestAdmissionIntegration.test.ts	Adds an integration test asserting exclusive FIFO gating between shared-build batches.
libraries/rush-daemon/src/test/PhasedRequestRouterTestUtilities.ts	Extends the test phased client to allow distinct sessionId values for multi-client scenarios.
libraries/rush-daemon/src/test/PhasedRequestBatching.test.ts	New test suite covering shared-build batching/coalescing, isolation of streams/events/results, and cancellation semantics.
libraries/rush-daemon/src/PhasedRequestRouter.ts	Implements shared-build batching via a graph-owned coordinator; unions selections per batch; refactors result derivation per client subset.
libraries/rush-daemon/src/PhasedRequestEventSink.ts	Threads write-failure errors through the sink writer so the router can deactivate a client with error context.
libraries/rush-daemon/src/PhasedRequestEventMultiplexer.ts	Expands from a single request sink to a set of concurrent request sinks for multi-client fan-out.
libraries/rush-daemon/README.md	Updates documentation to describe shared-build coalescing, batching boundaries, and cancellation semantics.
common/changes/@rushstack/rush-daemon/mojazayeri-shared-build-merging_2026-08-22-00-11.json	Records a minor change note for shared-build request merging behavior.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 0
• Review effort level: Lite

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