perf(router-core): avoid byte round-trip in SSR stream transform

[Sheraff]

Summary

• transformStreamWithRouter decoded every app chunk Uint8Array→string, buffered strings in pendingWrites, and re-encoded at enqueue — two full transcodes per byte even when no router injection happens near the chunk.
• App chunks now stay raw Uint8Array end-to-end: byte-level closing-tag/barrier-marker scanner (safe by construction: closing tags are ASCII; UTF-8 continuation bytes are ≥ 0x80 so ASCII matching/slicing can never split a multi-byte char), growable/compacting byte buffer replacing string leftover/pendingTail, polymorphic output queue (string | Uint8Array). Markers split across written ranges now count too, with overlap re-scan.

Audit catch

Found + fixed a real bug during review: state = HoldingTail was set before the body-prefix emit whose marker scan is gated on state < HoldingTail — if the barrier script and </body> arrived in one chunk, the barrier never lifted and router scripts were dropped. Reordered emit-before-transition + regression test added.

Performance

workload	before	after	delta
full transform, large body passthrough	385 ops/s	423 ops/s	+10%
full transform, small body w/ frequent injections	12.4k ops/s	18.9k ops/s	+52%
scanner micro: medium chunk boundary scan	—	—	+60%
~13KB no-</body> scan	—	—	+57%

Verification

• test:unit ✅ (1610 pass, 4 new incl. tag-straddling-boundary & multibyte cases) · eslint ✅ · types ✅ · prettier ✅
• Correctness argument for multibyte/boundary handling detailed in RESULT-perf-task5.md on the branch.

[nx-cloud]

View your CI Pipeline Execution ↗ for commit 36e17cc

Command	Status	Duration	Result
nx affected --targets=test:eslint,test:unit,tes...	✅ Succeeded	2m 51s	View ↗
nx run-many --target=build --exclude=examples/*...	✅ Succeeded	28s	View ↗

---

☁️ Nx Cloud last updated this comment at 2026-08-22 06:00:51 UTC

[github-actions]

🚀 Changeset Version Preview

4 package(s) bumped directly, 19 bumped as dependents.

🟩 Patch bumps

Package	Version	Reason
@tanstack/react-router	1.170.31 → 1.170.32	Changeset
@tanstack/router-core	1.171.26 → 1.171.27	Changeset
@tanstack/solid-router	1.170.29 → 1.170.30	Changeset
@tanstack/vue-router	1.170.28 → 1.170.29	Changeset
@tanstack/react-start	1.168.48 → 1.168.49	Dependent
@tanstack/react-start-client	1.168.29 → 1.168.30	Dependent
@tanstack/react-start-rsc	0.1.47 → 0.1.48	Dependent
@tanstack/react-start-server	1.167.36 → 1.167.37	Dependent
@tanstack/router-cli	1.167.32 → 1.167.33	Dependent
@tanstack/router-generator	1.167.32 → 1.167.33	Dependent
@tanstack/router-plugin	1.168.34 → 1.168.35	Dependent
@tanstack/router-vite-plugin	1.167.34 → 1.167.35	Dependent
@tanstack/solid-start	1.168.46 → 1.168.47	Dependent
@tanstack/solid-start-client	1.168.28 → 1.168.29	Dependent
@tanstack/solid-start-server	1.167.35 → 1.167.36	Dependent
@tanstack/start-client-core	1.170.26 → 1.170.27	Dependent
@tanstack/start-plugin-core	1.171.38 → 1.171.39	Dependent
@tanstack/start-server-core	1.169.30 → 1.169.31	Dependent
@tanstack/start-static-server-functions	1.167.31 → 1.167.32	Dependent
@tanstack/start-storage-context	1.167.28 → 1.167.29	Dependent
@tanstack/vue-start	1.168.45 → 1.168.46	Dependent
@tanstack/vue-start-client	1.167.31 → 1.167.32	Dependent
@tanstack/vue-start-server	1.167.35 → 1.167.36	Dependent

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[github-actions]

Bundle Size Benchmarks

This pull request does not affect bundle size in any measured scenario.

[nx-cloud]

Important

At least one additional CI pipeline execution has run since the conclusion below was written and it may no longer be applicable.

Nx Cloud has identified a possible root cause for your failed CI:

We determined this failure is an environment issue rather than a regression from the PR changes. The serve package was not pre-installed and required an on-the-fly npm download, causing the test server to not be ready in time — resulting in a missing element on the prerendered page and a subsequent connection refused error. Our PR's changes are scoped to SSR streaming only and do not affect the SPA static-serving path exercised by these tests.

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commit: 36e17cc

[codspeed-hq]

Merging this PR will regress 5 benchmarks

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 13 improved benchmarks
❌ 5 regressed benchmarks
✅ 162 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Memory	mem server error-paths error (solid)	978.9 KB	1,207.1 KB	-18.91%
❌	Memory	mem server request-churn (solid)	712.7 KB	804.6 KB	-11.42%
❌	Memory	mem server request-churn (react)	662.6 KB	701.3 KB	-5.53%
❌	Memory	mem server server-fn-churn (react)	378.9 KB	392.5 KB	-3.47%
❌	Memory	mem client navigation-churn (solid)	592.5 KB	612.3 KB	-3.23%
⚡	Memory	mem server streaming-peak chunked (solid)	30.5 MB	2.2 MB	×14
⚡	Memory	mem server streaming-peak chunked (vue)	11.2 MB	1.1 MB	×10
⚡	Memory	mem server streaming-peak chunked (react)	11.1 MB	1.7 MB	×6.6
⚡	Memory	mem server error-paths not-found (vue)	2,304.3 KB	478.2 KB	×4.8
⚡	Memory	mem server aborted-requests (solid)	1,277.9 KB	787.2 KB	+62.33%
⚡	Memory	mem server error-paths unmatched (react)	544.1 KB	444.3 KB	+22.46%
⚡	Memory	mem server aborted-requests (react)	855.7 KB	700.3 KB	+22.19%
⚡	Memory	mem server aborted-requests (vue)	1.1 MB	1 MB	+6.39%
⚡	Simulation	ssr server-fn during document ssr (vue)	674.5 ms	635.5 ms	+6.13%
⚡	Memory	mem server error-paths redirect (solid)	382.2 KB	363.7 KB	+5.08%
⚡	Memory	mem server error-paths redirect (vue)	427.7 KB	411.2 KB	+4.03%
⚡	Memory	mem server serialization-payload (vue)	4.5 MB	4.3 MB	+3.44%
⚡	Memory	mem server peak-large-page (react)	1.2 MB	1.2 MB	+3.01%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing perf/task5-stream-byte-passthrough (36e17cc) with main (d83a896)}