Fix memory leak in createPrefetchResponseStream by releasing reader lock by timneutkens · Pull Request #89485 · vercel/next.js

timneutkens · 2026-02-04T12:31:01Z

What?

Fix memory leak in createPrefetchResponseStream where the ReadableStreamDefaultReader is never released after consuming the original stream.

Why?

The unreleased reader maintains a strong reference chain: originalFlightStream.[[reader]] → reader → closures, preventing garbage collection and causing ~32MB memory growth per 10 navigation cycles on the test app.

How?

Add reader.releaseLock() when the stream completes. This breaks the reference chain while preserving the intentional behavior of keeping the target stream open (for Flight's hanging promise handling in PPR).

Test Results

Scenario	Cycle 10	Cycle 20	Growth (10→20)
No fix	59.9 MB	91.8 MB	31.91 MB
controller.close()	5.5 MB	5.5 MB	0.03 MB
reader.releaseLock()	5.4 MB	5.5 MB	0.04 MB

timneutkens · 2026-02-04T12:31:20Z

Fix memory leak in createPrefetchResponseStream by releasing reader lock #89485 👈 (View in Graphite)
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nextjs-bot · 2026-02-04T12:37:17Z

Tests Passed

nextjs-bot · 2026-02-04T12:49:50Z

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⚡ Dev Server

Metric	Canary	PR	Change	Trend
Cold (Listen)	455ms	455ms	✓	▁▁▁▁█
Cold (Ready in log)	438ms	438ms	✓	▇▇▁█▃
Cold (First Request)	1.165s	1.174s	✓	██▁█▁
Warm (Listen)	457ms	456ms	✓	▁▁▁▁▁
Warm (Ready in log)	446ms	443ms	✓	▁▁▁▁▆
Warm (First Request)	344ms	342ms	✓	▂▁▆▁█

📦 Dev Server (Webpack) (Legacy)

📦 Dev Server (Webpack)

Metric	Canary	PR	Change	Trend
Cold (Listen)	455ms	455ms	✓	▁▁▁▁▁
Cold (Ready in log)	438ms	439ms	✓	▆▅▁▅▃
Cold (First Request)	1.869s	1.856s	✓	▄▄▁▃▂
Warm (Listen)	456ms	457ms	✓	▁▁▁▁▁
Warm (Ready in log)	437ms	438ms	✓	▅▃▁▄▃
Warm (First Request)	1.881s	1.865s	✓	▃▃▁▃▂

⚡ Production Builds

Metric	Canary	PR	Change	Trend
Fresh Build	3.990s	3.901s	✓	▁▁▃▁▄
Cached Build	3.959s	4.018s	✓	▁▁▃▁▄

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Metric	Canary	PR	Change	Trend
Fresh Build	13.832s	13.827s	✓	▁▁▁▁▁
Cached Build	13.972s	13.938s	✓	▁▁▁▁▁
node_modules Size	465 MB	465 MB	✓	▁▁▁▁▁

📦 Bundle Sizes

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Main Bundles: **437 kB** → **437 kB** ⚠️ +3 B

81 files with content-based hashes (individual files not comparable between builds)

Server

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	766 B	763 B	✓
Total	766 B	763 B	✅ -3 B

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_buildManifest.js gzip	449 B	451 B	✓
Total	449 B	451 B	⚠️ +2 B

📦 Webpack

Client

Main Bundles

	Canary	PR	Change
5528-HASH.js gzip	5.47 kB	N/A	-
6280-HASH.js gzip	56.8 kB	N/A	-
6335.HASH.js gzip	169 B	N/A	-
912-HASH.js gzip	4.53 kB	N/A	-
e8aec2e4-HASH.js gzip	62.5 kB	N/A	-
framework-HASH.js gzip	59.7 kB	59.7 kB	✓
main-app-HASH.js gzip	256 B	254 B	✓
main-HASH.js gzip	39 kB	39 kB	✓
webpack-HASH.js gzip	1.68 kB	1.68 kB	✓
262-HASH.js gzip	N/A	4.52 kB	-
2889.HASH.js gzip	N/A	169 B	-
5602-HASH.js gzip	N/A	5.48 kB	-
6948ada0-HASH.js gzip	N/A	62.5 kB	-
9544-HASH.js gzip	N/A	57.5 kB	-
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Polyfills

	Canary	PR	Change
polyfills-HASH.js gzip	39.4 kB	39.4 kB	✓
Total	39.4 kB	39.4 kB	✓

Pages

	Canary	PR	Change
_app-HASH.js gzip	194 B	194 B	✓
_error-HASH.js gzip	183 B	180 B	🟢 3 B (-2%)
css-HASH.js gzip	331 B	330 B	✓
dynamic-HASH.js gzip	1.81 kB	1.81 kB	✓
edge-ssr-HASH.js gzip	256 B	256 B	✓
head-HASH.js gzip	351 B	352 B	✓
hooks-HASH.js gzip	384 B	383 B	✓
image-HASH.js gzip	580 B	581 B	✓
index-HASH.js gzip	260 B	260 B	✓
link-HASH.js gzip	2.49 kB	2.49 kB	✓
routerDirect..HASH.js gzip	320 B	319 B	✓
script-HASH.js gzip	386 B	386 B	✓
withRouter-HASH.js gzip	315 B	315 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
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Server

Edge SSR

	Canary	PR	Change
edge-ssr.js gzip	126 kB	126 kB	✓
page.js gzip	249 kB	249 kB	✓
Total	375 kB	375 kB	⚠️ +140 B

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	615 B	616 B	✓
middleware-r..fest.js gzip	156 B	155 B	✓
middleware.js gzip	33.1 kB	33.2 kB	✓
edge-runtime..pack.js gzip	842 B	842 B	✓
Total	34.7 kB	34.8 kB	⚠️ +98 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	732 B	736 B	✓
Total	732 B	736 B	⚠️ +4 B

Build Cache

	Canary	PR	Change
0.pack gzip	3.82 MB	3.82 MB	🔴 +7.28 kB (+0%)
index.pack gzip	103 kB	103 kB	✓
index.pack.old gzip	103 kB	103 kB	✓
Total	4.02 MB	4.03 MB	⚠️ +8.11 kB

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	Canary	PR	Change
app-page-exp...dev.js gzip	312 kB	312 kB	✓
app-page-exp..prod.js gzip	166 kB	166 kB	✓
app-page-tur...dev.js gzip	312 kB	312 kB	✓
app-page-tur..prod.js gzip	166 kB	166 kB	✓
app-page-tur...dev.js gzip	308 kB	308 kB	✓
app-page-tur..prod.js gzip	164 kB	164 kB	✓
app-page.run...dev.js gzip	309 kB	309 kB	✓
app-page.run..prod.js gzip	164 kB	164 kB	✓
app-route-ex...dev.js gzip	70.4 kB	70.4 kB	✓
app-route-ex..prod.js gzip	48.9 kB	48.9 kB	✓
app-route-tu...dev.js gzip	70.5 kB	70.5 kB	✓
app-route-tu..prod.js gzip	49 kB	49 kB	✓
app-route-tu...dev.js gzip	70.1 kB	70.1 kB	✓
app-route-tu..prod.js gzip	48.7 kB	48.7 kB	✓
app-route.ru...dev.js gzip	70 kB	70 kB	✓
app-route.ru..prod.js gzip	48.7 kB	48.7 kB	✓
dist_client_...dev.js gzip	324 B	324 B	✓
dist_client_...dev.js gzip	326 B	326 B	✓
dist_client_...dev.js gzip	318 B	318 B	✓
dist_client_...dev.js gzip	317 B	317 B	✓
pages-api-tu...dev.js gzip	43.2 kB	43.2 kB	✓
pages-api-tu..prod.js gzip	32.9 kB	32.9 kB	✓
pages-api.ru...dev.js gzip	43.1 kB	43.1 kB	✓
pages-api.ru..prod.js gzip	32.8 kB	32.8 kB	✓
pages-turbo....dev.js gzip	52.5 kB	52.5 kB	✓
pages-turbo...prod.js gzip	39.4 kB	39.4 kB	✓
pages.runtim...dev.js gzip	52.4 kB	52.4 kB	✓
pages.runtim..prod.js gzip	39.3 kB	39.3 kB	✓
server.runti..prod.js gzip	62.6 kB	62.6 kB	✓
Total	2.78 MB	2.78 MB	⚠️ +187 B

📝 Changed Files (8 files)

Files with changes:

app-page-exp..ntime.dev.js
app-page-exp..time.prod.js
app-page-tur..ntime.dev.js
app-page-tur..time.prod.js
app-page-tur..ntime.dev.js
app-page-tur..time.prod.js
app-page.runtime.dev.js
app-page.runtime.prod.js

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Previously, `createPrefetchResponseStream` intentionally never called `controller.close()` on the wrapper stream, to prevent React Flight from erroring on unresolved references (dynamic holes). However, Chrome and Firefox keep unclosed ReadableStreams with pending reads as native GC roots, preventing the stream — and the entire FlightResponse captured in the `reader.read().then(progress)` closure chain — from being garbage-collected. Now that React Flight supports `unstable_allowPartialStream` as an option for `createFromReadableStream` (facebook/react#35731), we can close the stream normally. Flight will mark unresolved chunks as "halted" instead of erroring, which is the correct behavior for prefetch responses. Also removes the now-unnecessary `createUnclosingPrefetchStream` wrapper from the legacy prefetch path in `fetch-server-response.ts`. closes #89485

Fix memory leak in createPrefetchResponseStream by releasing reader lock

c93eaa5

nextjs-bot added created-by: Turbopack team PRs by the Turbopack team. type: next labels Feb 4, 2026

timneutkens marked this pull request as ready for review February 4, 2026 12:32

unstubbable reviewed Feb 4, 2026

View reviewed changes

Comment thread packages/next/src/client/components/segment-cache/cache.ts

Update cache.ts

b8988d7

unstubbable marked this pull request as draft February 5, 2026 11:52

unstubbable mentioned this pull request Feb 9, 2026

Avoid using unclosing prefetch streams in the browser #89610

Merged

unstubbable closed this in #89610 Feb 9, 2026

github-actions Bot added the locked label Feb 24, 2026

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Fix memory leak in createPrefetchResponseStream by releasing reader lock#89485

Fix memory leak in createPrefetchResponseStream by releasing reader lock#89485
timneutkens wants to merge 2 commits intocanaryfrom
02-04-fix_memory_leak_in_createprefetchresponsestream_by_releasing_reader_lock

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