fix(ssr): the end of a streamed render waits for a suspended shell

[ryansolid]

Stacked on #3770 (base fix/ssr-shell-suspend-retry). The fix here stands alone; its <Errored> awaited-mode tests also need #3770's fixes to pass. Retarget to next once #3770 merges.

Summary

flushEnd flushed the serializer whenever the fragment registry was empty. A <Loading> fragment settling while the shell was still suspended — a lazy memo throwing NotReadyError above it, the router's flash-decode shape — emptied the registry before the shell's root holes re-pulled, so the flush ran first. Seroval drops every write() after flush() and fires onDone once, so:

• Hydration data went missing in both forms. Whatever the root-hole re-pull serialized (an async value in the re-rendered shell, say) never reached the document. The piped form still shipped the markup, so this only shows on hydration.
• The awaited form returned an empty document. renderToStream(...).then resolved "": onDone's doShell() bailed on the pending holes and completion ran anyway.
• With an <Errored> holding the suspended subtree, the awaited render never completed — a synchronous loop that blocked even a test's timeout.

Fix: pending root holes gate flushEnd. Both consumers' flush loops already call it again once the holes resolve. The existing serializeRootAssets guard ("A Loading boundary's resolve path can queue flushEnd while the shell is still pending…") was an earlier patch for one symptom of this race; it stays, now redundant on this path.

These are the awaited-mode bugs listed under "Not covered here" in #3770. They matter for the mode an app picks when it needs no-JS correctness (renderMode: 'async' for crawlers or no-JS pages).

Public API changes

None.

Tests

test/server/then-shell-suspend.spec.tsx:

• awaited render, shell suspension above <Loading>, with/without <Errored> and <Document>, in both settle orders: shell present, content present, fallback gone. On next the content-first, no-<Errored> cases resolve "" and the <Errored> cases hang.
• a shell component re-run after the suspension serializes an async value: it must appear in a hydration script, piped and awaited. Without the gate the piped form ships the markup without the data and the awaited form is empty.

web (client, server, hydrate), solid suites and web type tests pass. Size: every scenario within its cap (renderToString doesn't bundle this path).

[changeset-bot]

🦋 Changeset detected

Latest commit: 2d381ca

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 12 packages

Name	Type
@solidjs/web	Patch
@solidjs/babel-plugin	Patch
@solidjs/diagnostics	Patch
@solidjs/element	Patch
@solidjs/h	Patch
@solidjs/html	Patch
test-integration	Patch
todos-server-example	Patch
@solidjs/compiler	Patch
@solidjs/signals	Patch
solid-js	Patch
@solidjs/universal	Patch

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[github-actions]

Size (brotli, eager entry chunk)

scenario	head	vs base	cap		lazy chunks (not counted)
signals: core floor (createSignal/Memo/Effect/Root/flush)	7.32 KB	0 B	7.33 KB	✅
signals: + createStore	14.51 KB	0 B	14.53 KB	✅
signals: + isPending/latest	9.45 KB	0 B	9.45 KB	✅
app: render + one signal (the simple-app floor)	9.81 KB	0 B	9.83 KB	✅
app: hydrating (no stores) with Show/For/Loading/Errored/lazy	17.65 KB	0 B	17.66 KB	✅	lazy-page.js 0.04 KB
app: hydrating + every store primitive family	28.79 KB	0 B	28.80 KB	✅	lazy-page.js 0.04 KB
app: CSR with Show/For/Loading/Errored/lazy	12.81 KB	0 B	12.82 KB	✅	lazy-page.js 0.04 KB
app: CSR, observe tier (same app on the observe artifacts)	14.39 KB	0 B	14.39 KB	✅	lazy-page.js 0.04 KB
app: CSR, observe tier + attribution engine enabled	28.59 KB	0 B	28.61 KB	✅	lazy-page.js 0.04 KB
frames: eager client consumer (frames client + transport, lazy codec)	13.77 KB	0 B	13.78 KB	✅
page: base server components (hydrating + dynamic + frames + sf reference)	44.76 KB	0 B	44.78 KB	✅	decode.js 6.07 KB, lazy-page.js 0.04 KB
page: live server components (base + live/GET + action + isPending/latest)	48.44 KB	0 B	48.45 KB	✅	decode.js 6.07 KB, lazy-page.js 0.04 KB
server: floor (getRequestEvent + isServer)	1.33 KB	0 B	1.34 KB	✅
server: renderToString (the server-render floor)	20.41 KB	0 B	20.42 KB	✅

_{Bundled with Rolldown (what Vite ships), brotli q11, decimal KB. Caps in scripts/size/scenarios.js; the floor and page caps in floor-caps.json are frozen (lower only, or Size-Exception: in the PR body).}

[coveralls]

Coverage Report for CI Build 37286304114

Warning

Build has drifted: This PR's base is out of sync with its target branch, so coverage data may include unrelated changes.
Quick fix: rebase this PR. Learn more →

Coverage remained the same at 75.991%

Details

• Coverage remained the same as the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	1195
Covered Lines:	962
Line Coverage:	80.5%
Relevant Branches:	925
Covered Branches:	649
Branch Coverage:	70.16%
Branches in Coverage %:	Yes
Coverage Strength:	27.83 hits per line

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💛 - Coveralls

[codspeed]

Merging this PR will improve performance by 23.99%

⚠️ 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

⚡ 2 improved benchmarks
✅ 186 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	projection derive: write one NESTED field (reference)	2.6 ms	2 ms	+28.72%
⚡	memo + sync render effect only (reference)	32.1 ms	26.8 ms	+19.44%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing fix/ssr-shell-suspend-serializer-flush (2d381ca) with next (7addcc6)}