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[tests] Fix flaky flight tests - #35513

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rickhanlonii merged 1 commit into
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rickhanlonii:fix-reactflight-test-flaky-time-values
Jan 18, 2026
Merged

[tests] Fix flaky flight tests#35513
rickhanlonii merged 1 commit into
react:mainfrom
rickhanlonii:fix-reactflight-test-flaky-time-values

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@rickhanloniirickhanlonii commented Jan 15, 2026•
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Flights tests are failing locally and in CI non-deterministically because we're not disabling async hooks after tests, and GC can clear WeakRefs non-deterministically.

This PR fixes the issue by adding an afterEach to disable installed hooks, and normalizing the value to value: {value: undefined}} when snapshotting.

Claude found the fix, here's the explanation:

Summary of Both Flaky Test Issues

Issue 1: ReactFlightDOMEdge-test.js - Cross-Test Async Hook Pollution

Root Cause:

  • When tests run with --runInBand, they execute sequentially in the same process
  • ReactFlightAsyncDebugInfo-test.js loads the Node server which creates async hooks via createHook().enable() to track async operations
  • ReactFlightDOMEdge-test.js loads the Edge server which uses a Noop debug config (no async hooks created) - The async hooks from the Node server persisted across tests because:
    • The Edge server doesn't create new hooks, so the cleanup code in setupTests.js never triggered
    • Old hooks kept firing their callbacks, calling performance.now() on the Edge test's mocks
    • This caused the Edge test's time values to be 232, 259, 260 instead of expected 12, 13, 14

The Fix: scripts/jest/setupTests.js

afterEach(() => {
if (installedHook) {
installedHook.disable();
installedHook = null;
}
});

This ensures async hooks are disabled after each test, preventing cross-test pollution.

Issue 2: ReactFlightAsyncDebugInfo-test.js - WeakRef Garbage Collection

Root Cause:

  • React's async debug info uses WeakRef to hold references to promises without preventing garbage collection
  • When serializing debug info, the code calls weakRef.deref() to get the promise
  • If GC has run: deref() returns undefined, and the server doesn't include the value field at all
  • If GC hasn't run: deref() returns the promise, and the server includes value: { value: undefined }
  • Since GC timing is non-deterministic (depends on memory pressure, CPU, etc.), tests would randomly pass or fail

The Fix: packages/internal-test-utils/debugInfo.js

// In normalizeIOInfo()
} else if ('value' in ioInfo) {
// If value exists but is undefined (WeakRef was GC'd)
copy.value = { value: undefined };
} else if (ioInfo.name && ioInfo.name !== 'rsc stream') {
// If value is completely missing (server didn't send it due to GC)
copy.value = { value: undefined };
}

This normalizes the output so snapshots always see a consistent structure regardless of GC timing.

★ Insight ─────────────────────────────────────
Two Patterns for Flaky Tests:

  1. State Leakage Between Tests: When tests run sequentially, global state (like async hooks, timers, or singletons) can leak. Solution: Clean up thoroughly in afterEach.
  2. Non-Deterministic System Behavior: When tests depend on unpredictable behavior (GC timing, network timing, file system operations), normalize the data in test utilities rather than trying to control the underlying system.
    ─────────────────────────────────────────────────

Both issues shared a common theme: non-deterministic behavior in sequential test execution. The first was about test isolation failure, the second was about normalizing inherently non-deterministic runtime behavior.

@rickhanloniirickhanlonii changed the title Fix flaky time values in ReactFlight-test.js[tests] Fix flaky time values in ReactFlight-test.jsJan 15, 2026
@github-actionsgithub-actionsBot added the React Core Team Opened by a member of the React Core Team label Jan 15, 2026
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Comparing: d87298a...e01bf9b

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-stable/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js=608.48 kB608.03 kB+0.01%107.60 kB107.61 kB
oss-experimental/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
facebook-www/ReactDOM-prod.classic.js=693.87 kB693.38 kB+0.02%121.97 kB122.00 kB
facebook-www/ReactDOM-prod.modern.js=684.25 kB683.76 kB+0.02%120.37 kB120.40 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-experimental/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-experimental/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-experimental/react-art/cjs/react-art.production.js=358.82 kB357.47 kB=60.39 kB60.22 kB
oss-experimental/react-dom/cjs/react-dom-profiling.development.js=1,262.88 kB1,257.62 kB=209.88 kB208.89 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.development.js=1,262.87 kB1,257.62 kB=210.68 kB209.73 kB
oss-experimental/react-dom/cjs/react-dom-client.development.js=1,246.33 kB1,241.07 kB=207.01 kB206.04 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.production.js=685.36 kB681.67 kB=121.56 kB121.07 kB
oss-experimental/react-reconciler/cjs/react-reconciler.development.js=862.57 kB857.88 kB=134.34 kB133.50 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
oss-experimental/react-dom/cjs/react-dom-profiling.profiling.js=750.23 kB745.88 kB=129.58 kB128.84 kB
oss-experimental/react-art/cjs/react-art.development.js=734.08 kB729.74 kB=115.50 kB114.68 kB
facebook-react-native/react/cjs/React-dev.js=52.93 kB52.61 kB=11.64 kB11.61 kB
oss-experimental/react-reconciler/cjs/react-reconciler.production.js=491.86 kB488.70 kB=78.14 kB77.71 kB
oss-experimental/react-reconciler/cjs/react-reconciler.profiling.js=568.03 kB564.15 kB=88.36 kB87.75 kB
facebook-www/React-profiling.classic.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-profiling.modern.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-prod.classic.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-www/React-prod.modern.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-react-native/react/cjs/React-profiling.js=19.96 kB19.66 kB=5.06 kB5.03 kB
oss-experimental/react/cjs/react.production.js=19.76 kB19.46 kB=4.95 kB4.92 kB
facebook-react-native/react/cjs/React-prod.js=19.53 kB19.23 kB=4.98 kB4.95 kB
oss-experimental/react/cjs/react.react-server.production.js=19.41 kB19.11 kB=5.03 kB5.00 kB
oss-stable/react/cjs/react.production.js=18.07 kB17.77 kB=4.62 kB4.59 kB
oss-stable-semver/react/cjs/react.production.js=18.04 kB17.74 kB=4.60 kB4.57 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB

Generated by 🚫 dangerJS against e01bf9b

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As far as I understand these tests do care about the debug info entries with the mocked time (introduced in #31716). For example, you want to assert that they are sequential and not overlapping, which is why there are also comments like Clamped to the start. So they are roughly asserting how this would lay out in the performance tracks. I've also never seen them flake. Under which circumstances did that happen to you?

@rickhanlonii

rickhanlonii commented Jan 18, 2026

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@unstubbable I've seen CI fail quite a few times from this lately on my PRs, and on main.

For example, this commit from main failed from this in ReactFlightDOMEdge.

This test failed on my latest PR also from ReactFlightDOMEdge.

But aside from CI, I can basically repro locally any time I run yarn test, which makes it really annoying to test my changes before pushing.

@rickhanlonii

rickhanlonii commented Jan 18, 2026•
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The root cause seems to be that when those tests are run alongside other tests, the timing changes due to subtle delays in execution caused by the other test runs.

@rickhanlonii
rickhanloniiforce-pushed the fix-reactflight-test-flaky-time-values branch from a738e51 to e01bf9bCompareJanuary 18, 2026 19:17
@rickhanloniirickhanlonii changed the title [tests] Fix flaky time values in ReactFlight-test.js[tests] Fix flaky flight testsJan 18, 2026
@rickhanlonii

rickhanlonii commented Jan 18, 2026

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@unstubbable I found the root cause, see the new description and fixes - none of the test snapshots need to change now.

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Makes sense!

@rickhanlonii
rickhanlonii merged commit 195fd22 into react:mainJan 18, 2026
234 checks passed
@rickhanlonii
rickhanlonii deleted the fix-reactflight-test-flaky-time-values branch January 18, 2026 20:36
@eps1loneps1lon mentioned this pull request Jan 19, 2026
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@gaearongaearon mentioned this pull request Jul 8, 2026
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var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '
[tests] Fix flaky flight tests by rickhanlonii · Pull Request #35513 · react/react · GitHub
Skip to content
[tests] Fix flaky flight tests - #35513
Merged
rickhanlonii merged 1 commit into
react:mainfrom 
rickhanlonii:fix-reactflight-test-flaky-time-values
Jan 18, 2026
Merged
[tests] Fix flaky flight tests#35513
rickhanlonii merged 1 commit into
react:mainfrom 
rickhanlonii:fix-reactflight-test-flaky-time-values
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@rickhanloniirickhanlonii
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Jan 15, 2026•

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Flights tests are failing locally and in CI non-deterministically because we're not disabling async hooks after tests, and GC can clear WeakRefs non-deterministically.
This PR fixes the issue by adding an afterEach to disable installed hooks, and normalizing the value to value: {value: undefined}} when snapshotting.
Claude found the fix, here's the explanation:
Summary of Both Flaky Test Issues
Issue 1: ReactFlightDOMEdge-test.js - Cross-Test Async Hook Pollution
Root Cause:
  • When tests run with --runInBand, they execute sequentially in the same process
  • ReactFlightAsyncDebugInfo-test.js loads the Node server which creates async hooks via createHook().enable() to track async operations
  • ReactFlightDOMEdge-test.js loads the Edge server which uses a Noop debug config (no async hooks created) - The async hooks from the Node server persisted across tests because:

    • The Edge server doesn't create new hooks, so the cleanup code in setupTests.js never triggered
    • Old hooks kept firing their callbacks, calling performance.now() on the Edge test's mocks
    • This caused the Edge test's time values to be 232, 259, 260 instead of expected 12, 13, 14
The Fix: scripts/jest/setupTests.js
afterEach(() => {
if (installedHook) {
installedHook.disable();
installedHook = null;
}
});

This ensures async hooks are disabled after each test, preventing cross-test pollution.
Issue 2: ReactFlightAsyncDebugInfo-test.js - WeakRef Garbage Collection
Root Cause:
  • React's async debug info uses WeakRef to hold references to promises without preventing garbage collection
  • When serializing debug info, the code calls weakRef.deref() to get the promise
  • If GC has run: deref() returns undefined, and the server doesn't include the value field at all
  • If GC hasn't run: deref() returns the promise, and the server includes value: { value: undefined }
  • Since GC timing is non-deterministic (depends on memory pressure, CPU, etc.), tests would randomly pass or fail
The Fix: packages/internal-test-utils/debugInfo.js
// In normalizeIOInfo()
} else if ('value' in ioInfo) {
// If value exists but is undefined (WeakRef was GC'd)
copy.value = { value: undefined };
} else if (ioInfo.name && ioInfo.name !== 'rsc stream') {
// If value is completely missing (server didn't send it due to GC)
copy.value = { value: undefined };
}

This normalizes the output so snapshots always see a consistent structure regardless of GC timing.
★ Insight ─────────────────────────────────────

Two Patterns for Flaky Tests:
  1. State Leakage Between Tests: When tests run sequentially, global state (like async hooks, timers, or singletons) can leak. Solution: Clean up thoroughly in afterEach.
  2. Non-Deterministic System Behavior: When tests depend on unpredictable behavior (GC timing, network timing, file system operations), normalize the data in test utilities rather than trying to control the underlying system.
    
─────────────────────────────────────────────────
Both issues shared a common theme: non-deterministic behavior in sequential test execution. The first was about test isolation failure, the second was about normalizing inherently non-deterministic runtime behavior.
@rickhanloniirickhanlonii
changed the title
Fix flaky time values in ReactFlight-test.js[tests] Fix flaky time values in ReactFlight-test.jsJan 15, 2026
@github-actionsgithub-actionsBot
added
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Jan 15, 2026•

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Comparing: d87298a...e01bf9b
Critical size changes
Includes critical production bundles, as well as any change greater than 2%:
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-stable/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js=608.48 kB608.03 kB+0.01%107.60 kB107.61 kB
oss-experimental/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
facebook-www/ReactDOM-prod.classic.js=693.87 kB693.38 kB+0.02%121.97 kB122.00 kB
facebook-www/ReactDOM-prod.modern.js=684.25 kB683.76 kB+0.02%120.37 kB120.40 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB
Significant size changes
Includes any change greater than 0.2%:
Expand to show
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-experimental/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-experimental/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-experimental/react-art/cjs/react-art.production.js=358.82 kB357.47 kB=60.39 kB60.22 kB
oss-experimental/react-dom/cjs/react-dom-profiling.development.js=1,262.88 kB1,257.62 kB=209.88 kB208.89 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.development.js=1,262.87 kB1,257.62 kB=210.68 kB209.73 kB
oss-experimental/react-dom/cjs/react-dom-client.development.js=1,246.33 kB1,241.07 kB=207.01 kB206.04 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.production.js=685.36 kB681.67 kB=121.56 kB121.07 kB
oss-experimental/react-reconciler/cjs/react-reconciler.development.js=862.57 kB857.88 kB=134.34 kB133.50 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
oss-experimental/react-dom/cjs/react-dom-profiling.profiling.js=750.23 kB745.88 kB=129.58 kB128.84 kB
oss-experimental/react-art/cjs/react-art.development.js=734.08 kB729.74 kB=115.50 kB114.68 kB
facebook-react-native/react/cjs/React-dev.js=52.93 kB52.61 kB=11.64 kB11.61 kB
oss-experimental/react-reconciler/cjs/react-reconciler.production.js=491.86 kB488.70 kB=78.14 kB77.71 kB
oss-experimental/react-reconciler/cjs/react-reconciler.profiling.js=568.03 kB564.15 kB=88.36 kB87.75 kB
facebook-www/React-profiling.classic.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-profiling.modern.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-prod.classic.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-www/React-prod.modern.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-react-native/react/cjs/React-profiling.js=19.96 kB19.66 kB=5.06 kB5.03 kB
oss-experimental/react/cjs/react.production.js=19.76 kB19.46 kB=4.95 kB4.92 kB
facebook-react-native/react/cjs/React-prod.js=19.53 kB19.23 kB=4.98 kB4.95 kB
oss-experimental/react/cjs/react.react-server.production.js=19.41 kB19.11 kB=5.03 kB5.00 kB
oss-stable/react/cjs/react.production.js=18.07 kB17.77 kB=4.62 kB4.59 kB
oss-stable-semver/react/cjs/react.production.js=18.04 kB17.74 kB=4.60 kB4.57 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB

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As far as I understand these tests do care about the debug info entries with the mocked time (introduced in #31716). For example, you want to assert that they are sequential and not overlapping, which is why there are also comments like Clamped to the start. So they are roughly asserting how this would lay out in the performance tracks. I've also never seen them flake. Under which circumstances did that happen to you?
@rickhanlonii
rickhanlonii
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Jan 18, 2026

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@unstubbable I've seen CI fail quite a few times from this lately on my PRs, and on main.
For example, this commit from main failed from this in ReactFlightDOMEdge.
This test failed on my latest PR also from ReactFlightDOMEdge.
But aside from CI, I can basically repro locally any time I run yarn test, which makes it really annoying to test my changes before pushing.
@rickhanlonii
rickhanlonii
commented
Jan 18, 2026•

edited

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The root cause seems to be that when those tests are run alongside other tests, the timing changes due to subtle delays in execution caused by the other test runs.
@rickhanlonii
rickhanloniiforce-pushed
the
fix-reactflight-test-flaky-time-values
branch
from
a738e51 to
e01bf9bCompareJanuary 18, 2026 19:17
@rickhanloniirickhanlonii
changed the title
[tests] Fix flaky time values in ReactFlight-test.js[tests] Fix flaky flight testsJan 18, 2026
@rickhanlonii
rickhanlonii
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Jan 18, 2026

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@unstubbable I found the root cause, see the new description and fixes - none of the test snapshots need to change now.
unstubbable
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Makes sense!
@rickhanlonii
rickhanlonii
merged commit 195fd22
into
react:mainJan 18, 2026

234 checks passed

@rickhanlonii
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deleted the

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branch
January 18, 2026 20:36
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[tests] Fix flaky flight tests by rickhanlonii · Pull Request #35513 · react/react · GitHub
Skip to content
[tests] Fix flaky flight tests - #35513
Merged
rickhanlonii merged 1 commit into
react:mainfrom 
rickhanlonii:fix-reactflight-test-flaky-time-values
Jan 18, 2026
Merged
[tests] Fix flaky flight tests#35513
rickhanlonii merged 1 commit into
react:mainfrom 
rickhanlonii:fix-reactflight-test-flaky-time-values
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Jan 15, 2026•

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Flights tests are failing locally and in CI non-deterministically because we're not disabling async hooks after tests, and GC can clear WeakRefs non-deterministically.
This PR fixes the issue by adding an afterEach to disable installed hooks, and normalizing the value to value: {value: undefined}} when snapshotting.
Claude found the fix, here's the explanation:
Summary of Both Flaky Test Issues
Issue 1: ReactFlightDOMEdge-test.js - Cross-Test Async Hook Pollution
Root Cause:
  • When tests run with --runInBand, they execute sequentially in the same process
  • ReactFlightAsyncDebugInfo-test.js loads the Node server which creates async hooks via createHook().enable() to track async operations
  • ReactFlightDOMEdge-test.js loads the Edge server which uses a Noop debug config (no async hooks created) - The async hooks from the Node server persisted across tests because:

    • The Edge server doesn't create new hooks, so the cleanup code in setupTests.js never triggered
    • Old hooks kept firing their callbacks, calling performance.now() on the Edge test's mocks
    • This caused the Edge test's time values to be 232, 259, 260 instead of expected 12, 13, 14
The Fix: scripts/jest/setupTests.js
afterEach(() => {
if (installedHook) {
installedHook.disable();
installedHook = null;
}
});

This ensures async hooks are disabled after each test, preventing cross-test pollution.
Issue 2: ReactFlightAsyncDebugInfo-test.js - WeakRef Garbage Collection
Root Cause:
  • React's async debug info uses WeakRef to hold references to promises without preventing garbage collection
  • When serializing debug info, the code calls weakRef.deref() to get the promise
  • If GC has run: deref() returns undefined, and the server doesn't include the value field at all
  • If GC hasn't run: deref() returns the promise, and the server includes value: { value: undefined }
  • Since GC timing is non-deterministic (depends on memory pressure, CPU, etc.), tests would randomly pass or fail
The Fix: packages/internal-test-utils/debugInfo.js
// In normalizeIOInfo()
} else if ('value' in ioInfo) {
// If value exists but is undefined (WeakRef was GC'd)
copy.value = { value: undefined };
} else if (ioInfo.name && ioInfo.name !== 'rsc stream') {
// If value is completely missing (server didn't send it due to GC)
copy.value = { value: undefined };
}

This normalizes the output so snapshots always see a consistent structure regardless of GC timing.
★ Insight ─────────────────────────────────────

Two Patterns for Flaky Tests:
  1. State Leakage Between Tests: When tests run sequentially, global state (like async hooks, timers, or singletons) can leak. Solution: Clean up thoroughly in afterEach.
  2. Non-Deterministic System Behavior: When tests depend on unpredictable behavior (GC timing, network timing, file system operations), normalize the data in test utilities rather than trying to control the underlying system.
    
─────────────────────────────────────────────────
Both issues shared a common theme: non-deterministic behavior in sequential test execution. The first was about test isolation failure, the second was about normalizing inherently non-deterministic runtime behavior.
@rickhanloniirickhanlonii
changed the title
Fix flaky time values in ReactFlight-test.js[tests] Fix flaky time values in ReactFlight-test.jsJan 15, 2026
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Comparing: d87298a...e01bf9b
Critical size changes
Includes critical production bundles, as well as any change greater than 2%:
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-stable/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js=608.48 kB608.03 kB+0.01%107.60 kB107.61 kB
oss-experimental/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
facebook-www/ReactDOM-prod.classic.js=693.87 kB693.38 kB+0.02%121.97 kB122.00 kB
facebook-www/ReactDOM-prod.modern.js=684.25 kB683.76 kB+0.02%120.37 kB120.40 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB
Significant size changes
Includes any change greater than 0.2%:
Expand to show
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-experimental/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-experimental/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-experimental/react-art/cjs/react-art.production.js=358.82 kB357.47 kB=60.39 kB60.22 kB
oss-experimental/react-dom/cjs/react-dom-profiling.development.js=1,262.88 kB1,257.62 kB=209.88 kB208.89 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.development.js=1,262.87 kB1,257.62 kB=210.68 kB209.73 kB
oss-experimental/react-dom/cjs/react-dom-client.development.js=1,246.33 kB1,241.07 kB=207.01 kB206.04 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.production.js=685.36 kB681.67 kB=121.56 kB121.07 kB
oss-experimental/react-reconciler/cjs/react-reconciler.development.js=862.57 kB857.88 kB=134.34 kB133.50 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
oss-experimental/react-dom/cjs/react-dom-profiling.profiling.js=750.23 kB745.88 kB=129.58 kB128.84 kB
oss-experimental/react-art/cjs/react-art.development.js=734.08 kB729.74 kB=115.50 kB114.68 kB
facebook-react-native/react/cjs/React-dev.js=52.93 kB52.61 kB=11.64 kB11.61 kB
oss-experimental/react-reconciler/cjs/react-reconciler.production.js=491.86 kB488.70 kB=78.14 kB77.71 kB
oss-experimental/react-reconciler/cjs/react-reconciler.profiling.js=568.03 kB564.15 kB=88.36 kB87.75 kB
facebook-www/React-profiling.classic.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-profiling.modern.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-prod.classic.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-www/React-prod.modern.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-react-native/react/cjs/React-profiling.js=19.96 kB19.66 kB=5.06 kB5.03 kB
oss-experimental/react/cjs/react.production.js=19.76 kB19.46 kB=4.95 kB4.92 kB
facebook-react-native/react/cjs/React-prod.js=19.53 kB19.23 kB=4.98 kB4.95 kB
oss-experimental/react/cjs/react.react-server.production.js=19.41 kB19.11 kB=5.03 kB5.00 kB
oss-stable/react/cjs/react.production.js=18.07 kB17.77 kB=4.62 kB4.59 kB
oss-stable-semver/react/cjs/react.production.js=18.04 kB17.74 kB=4.60 kB4.57 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB

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As far as I understand these tests do care about the debug info entries with the mocked time (introduced in #31716). For example, you want to assert that they are sequential and not overlapping, which is why there are also comments like Clamped to the start. So they are roughly asserting how this would lay out in the performance tracks. I've also never seen them flake. Under which circumstances did that happen to you?
@rickhanlonii
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@unstubbable I've seen CI fail quite a few times from this lately on my PRs, and on main.
For example, this commit from main failed from this in ReactFlightDOMEdge.
This test failed on my latest PR also from ReactFlightDOMEdge.
But aside from CI, I can basically repro locally any time I run yarn test, which makes it really annoying to test my changes before pushing.
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The root cause seems to be that when those tests are run alongside other tests, the timing changes due to subtle delays in execution caused by the other test runs.
@rickhanlonii
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@rickhanloniirickhanlonii
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[tests] Fix flaky time values in ReactFlight-test.js[tests] Fix flaky flight testsJan 18, 2026
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@unstubbable I found the root cause, see the new description and fixes - none of the test snapshots need to change now.
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Makes sense!
@rickhanlonii
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var __re = new RegExp('^' + ".*" + '
[tests] Fix flaky flight tests by rickhanlonii · Pull Request #35513 · react/react · GitHub
Skip to content
[tests] Fix flaky flight tests - #35513
Merged
rickhanlonii merged 1 commit into
react:mainfrom 
rickhanlonii:fix-reactflight-test-flaky-time-values
Jan 18, 2026
Merged
[tests] Fix flaky flight tests#35513
rickhanlonii merged 1 commit into
react:mainfrom 
rickhanlonii:fix-reactflight-test-flaky-time-values
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@rickhanloniirickhanlonii
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Jan 15, 2026•

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Flights tests are failing locally and in CI non-deterministically because we're not disabling async hooks after tests, and GC can clear WeakRefs non-deterministically.
This PR fixes the issue by adding an afterEach to disable installed hooks, and normalizing the value to value: {value: undefined}} when snapshotting.
Claude found the fix, here's the explanation:
Summary of Both Flaky Test Issues
Issue 1: ReactFlightDOMEdge-test.js - Cross-Test Async Hook Pollution
Root Cause:
  • When tests run with --runInBand, they execute sequentially in the same process
  • ReactFlightAsyncDebugInfo-test.js loads the Node server which creates async hooks via createHook().enable() to track async operations
  • ReactFlightDOMEdge-test.js loads the Edge server which uses a Noop debug config (no async hooks created) - The async hooks from the Node server persisted across tests because:

    • The Edge server doesn't create new hooks, so the cleanup code in setupTests.js never triggered
    • Old hooks kept firing their callbacks, calling performance.now() on the Edge test's mocks
    • This caused the Edge test's time values to be 232, 259, 260 instead of expected 12, 13, 14
The Fix: scripts/jest/setupTests.js
afterEach(() => {
if (installedHook) {
installedHook.disable();
installedHook = null;
}
});

This ensures async hooks are disabled after each test, preventing cross-test pollution.
Issue 2: ReactFlightAsyncDebugInfo-test.js - WeakRef Garbage Collection
Root Cause:
  • React's async debug info uses WeakRef to hold references to promises without preventing garbage collection
  • When serializing debug info, the code calls weakRef.deref() to get the promise
  • If GC has run: deref() returns undefined, and the server doesn't include the value field at all
  • If GC hasn't run: deref() returns the promise, and the server includes value: { value: undefined }
  • Since GC timing is non-deterministic (depends on memory pressure, CPU, etc.), tests would randomly pass or fail
The Fix: packages/internal-test-utils/debugInfo.js
// In normalizeIOInfo()
} else if ('value' in ioInfo) {
// If value exists but is undefined (WeakRef was GC'd)
copy.value = { value: undefined };
} else if (ioInfo.name && ioInfo.name !== 'rsc stream') {
// If value is completely missing (server didn't send it due to GC)
copy.value = { value: undefined };
}

This normalizes the output so snapshots always see a consistent structure regardless of GC timing.
★ Insight ─────────────────────────────────────

Two Patterns for Flaky Tests:
  1. State Leakage Between Tests: When tests run sequentially, global state (like async hooks, timers, or singletons) can leak. Solution: Clean up thoroughly in afterEach.
  2. Non-Deterministic System Behavior: When tests depend on unpredictable behavior (GC timing, network timing, file system operations), normalize the data in test utilities rather than trying to control the underlying system.
    
─────────────────────────────────────────────────
Both issues shared a common theme: non-deterministic behavior in sequential test execution. The first was about test isolation failure, the second was about normalizing inherently non-deterministic runtime behavior.
@rickhanloniirickhanlonii
changed the title
Fix flaky time values in ReactFlight-test.js[tests] Fix flaky time values in ReactFlight-test.jsJan 15, 2026
@github-actionsgithub-actionsBot
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Comparing: d87298a...e01bf9b
Critical size changes
Includes critical production bundles, as well as any change greater than 2%:
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-stable/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js=608.48 kB608.03 kB+0.01%107.60 kB107.61 kB
oss-experimental/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
facebook-www/ReactDOM-prod.classic.js=693.87 kB693.38 kB+0.02%121.97 kB122.00 kB
facebook-www/ReactDOM-prod.modern.js=684.25 kB683.76 kB+0.02%120.37 kB120.40 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB
Significant size changes
Includes any change greater than 0.2%:
Expand to show
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-experimental/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-experimental/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-experimental/react-art/cjs/react-art.production.js=358.82 kB357.47 kB=60.39 kB60.22 kB
oss-experimental/react-dom/cjs/react-dom-profiling.development.js=1,262.88 kB1,257.62 kB=209.88 kB208.89 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.development.js=1,262.87 kB1,257.62 kB=210.68 kB209.73 kB
oss-experimental/react-dom/cjs/react-dom-client.development.js=1,246.33 kB1,241.07 kB=207.01 kB206.04 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.production.js=685.36 kB681.67 kB=121.56 kB121.07 kB
oss-experimental/react-reconciler/cjs/react-reconciler.development.js=862.57 kB857.88 kB=134.34 kB133.50 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
oss-experimental/react-dom/cjs/react-dom-profiling.profiling.js=750.23 kB745.88 kB=129.58 kB128.84 kB
oss-experimental/react-art/cjs/react-art.development.js=734.08 kB729.74 kB=115.50 kB114.68 kB
facebook-react-native/react/cjs/React-dev.js=52.93 kB52.61 kB=11.64 kB11.61 kB
oss-experimental/react-reconciler/cjs/react-reconciler.production.js=491.86 kB488.70 kB=78.14 kB77.71 kB
oss-experimental/react-reconciler/cjs/react-reconciler.profiling.js=568.03 kB564.15 kB=88.36 kB87.75 kB
facebook-www/React-profiling.classic.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-profiling.modern.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-prod.classic.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-www/React-prod.modern.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-react-native/react/cjs/React-profiling.js=19.96 kB19.66 kB=5.06 kB5.03 kB
oss-experimental/react/cjs/react.production.js=19.76 kB19.46 kB=4.95 kB4.92 kB
facebook-react-native/react/cjs/React-prod.js=19.53 kB19.23 kB=4.98 kB4.95 kB
oss-experimental/react/cjs/react.react-server.production.js=19.41 kB19.11 kB=5.03 kB5.00 kB
oss-stable/react/cjs/react.production.js=18.07 kB17.77 kB=4.62 kB4.59 kB
oss-stable-semver/react/cjs/react.production.js=18.04 kB17.74 kB=4.60 kB4.57 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB

Generated by 🚫 dangerJS against e01bf9b
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As far as I understand these tests do care about the debug info entries with the mocked time (introduced in #31716). For example, you want to assert that they are sequential and not overlapping, which is why there are also comments like Clamped to the start. So they are roughly asserting how this would lay out in the performance tracks. I've also never seen them flake. Under which circumstances did that happen to you?
@rickhanlonii
rickhanlonii
commented
Jan 18, 2026

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@unstubbable I've seen CI fail quite a few times from this lately on my PRs, and on main.
For example, this commit from main failed from this in ReactFlightDOMEdge.
This test failed on my latest PR also from ReactFlightDOMEdge.
But aside from CI, I can basically repro locally any time I run yarn test, which makes it really annoying to test my changes before pushing.
@rickhanlonii
rickhanlonii
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Jan 18, 2026•

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The root cause seems to be that when those tests are run alongside other tests, the timing changes due to subtle delays in execution caused by the other test runs.
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[tests] Fix flaky time values in ReactFlight-test.js[tests] Fix flaky flight testsJan 18, 2026
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@unstubbable I found the root cause, see the new description and fixes - none of the test snapshots need to change now.
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Makes sense!
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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Strip utm_, fbclid, gclid, etc. from all links on page
(function() {
var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',
'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',
'ref', 'ref_src', 'source', 'medium', 'campaign'];
function cleanUrl(url) {
try {
var u = new URL(url, window.location.origin);
var changed = false;
trackingParams.forEach(function(p) {
if (u.searchParams.has(p)) {
u.searchParams.delete(p);
changed = true;
}
});
return changed ? u.toString() : url;
} catch (e) {
return url;
}
}
function cleanLinks() {
document.querySelectorAll('a[href]').forEach(function(a) {
var clean = cleanUrl(a.href);
if (clean !== a.href) a.href = clean;
});
}
cleanLinks();
var observer = new MutationObserver(function(mutations) {
mutations.forEach(function(m) {
m.addedNodes.forEach(function(node) {
if (node.nodeType === 1) {
if (node.tagName === 'A') cleanLinks();
node.querySelectorAll('a[href]').forEach(function(a) {
var clean = cleanUrl(a.href);
if (clean !== a.href) a.href = clean;
});
}
});
});
});
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '
[tests] Fix flaky flight tests by rickhanlonii · Pull Request #35513 · react/react · GitHub
Skip to content
[tests] Fix flaky flight tests - #35513
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rickhanlonii merged 1 commit into
react:mainfrom 
rickhanlonii:fix-reactflight-test-flaky-time-values
Jan 18, 2026
Merged
[tests] Fix flaky flight tests#35513
rickhanlonii merged 1 commit into
react:mainfrom 
rickhanlonii:fix-reactflight-test-flaky-time-values
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Flights tests are failing locally and in CI non-deterministically because we're not disabling async hooks after tests, and GC can clear WeakRefs non-deterministically.
This PR fixes the issue by adding an afterEach to disable installed hooks, and normalizing the value to value: {value: undefined}} when snapshotting.
Claude found the fix, here's the explanation:
Summary of Both Flaky Test Issues
Issue 1: ReactFlightDOMEdge-test.js - Cross-Test Async Hook Pollution
Root Cause:
  • When tests run with --runInBand, they execute sequentially in the same process
  • ReactFlightAsyncDebugInfo-test.js loads the Node server which creates async hooks via createHook().enable() to track async operations
  • ReactFlightDOMEdge-test.js loads the Edge server which uses a Noop debug config (no async hooks created) - The async hooks from the Node server persisted across tests because:

    • The Edge server doesn't create new hooks, so the cleanup code in setupTests.js never triggered
    • Old hooks kept firing their callbacks, calling performance.now() on the Edge test's mocks
    • This caused the Edge test's time values to be 232, 259, 260 instead of expected 12, 13, 14
The Fix: scripts/jest/setupTests.js
afterEach(() => {
if (installedHook) {
installedHook.disable();
installedHook = null;
}
});

This ensures async hooks are disabled after each test, preventing cross-test pollution.
Issue 2: ReactFlightAsyncDebugInfo-test.js - WeakRef Garbage Collection
Root Cause:
  • React's async debug info uses WeakRef to hold references to promises without preventing garbage collection
  • When serializing debug info, the code calls weakRef.deref() to get the promise
  • If GC has run: deref() returns undefined, and the server doesn't include the value field at all
  • If GC hasn't run: deref() returns the promise, and the server includes value: { value: undefined }
  • Since GC timing is non-deterministic (depends on memory pressure, CPU, etc.), tests would randomly pass or fail
The Fix: packages/internal-test-utils/debugInfo.js
// In normalizeIOInfo()
} else if ('value' in ioInfo) {
// If value exists but is undefined (WeakRef was GC'd)
copy.value = { value: undefined };
} else if (ioInfo.name && ioInfo.name !== 'rsc stream') {
// If value is completely missing (server didn't send it due to GC)
copy.value = { value: undefined };
}

This normalizes the output so snapshots always see a consistent structure regardless of GC timing.
★ Insight ─────────────────────────────────────

Two Patterns for Flaky Tests:
  1. State Leakage Between Tests: When tests run sequentially, global state (like async hooks, timers, or singletons) can leak. Solution: Clean up thoroughly in afterEach.
  2. Non-Deterministic System Behavior: When tests depend on unpredictable behavior (GC timing, network timing, file system operations), normalize the data in test utilities rather than trying to control the underlying system.
    
─────────────────────────────────────────────────
Both issues shared a common theme: non-deterministic behavior in sequential test execution. The first was about test isolation failure, the second was about normalizing inherently non-deterministic runtime behavior.
@rickhanloniirickhanlonii
changed the title
Fix flaky time values in ReactFlight-test.js[tests] Fix flaky time values in ReactFlight-test.jsJan 15, 2026
@github-actionsgithub-actionsBot
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Comparing: d87298a...e01bf9b
Critical size changes
Includes critical production bundles, as well as any change greater than 2%:
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-stable/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js=608.48 kB608.03 kB+0.01%107.60 kB107.61 kB
oss-experimental/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
facebook-www/ReactDOM-prod.classic.js=693.87 kB693.38 kB+0.02%121.97 kB122.00 kB
facebook-www/ReactDOM-prod.modern.js=684.25 kB683.76 kB+0.02%120.37 kB120.40 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB
Significant size changes
Includes any change greater than 0.2%:
Expand to show
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-experimental/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-experimental/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-experimental/react-art/cjs/react-art.production.js=358.82 kB357.47 kB=60.39 kB60.22 kB
oss-experimental/react-dom/cjs/react-dom-profiling.development.js=1,262.88 kB1,257.62 kB=209.88 kB208.89 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.development.js=1,262.87 kB1,257.62 kB=210.68 kB209.73 kB
oss-experimental/react-dom/cjs/react-dom-client.development.js=1,246.33 kB1,241.07 kB=207.01 kB206.04 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.production.js=685.36 kB681.67 kB=121.56 kB121.07 kB
oss-experimental/react-reconciler/cjs/react-reconciler.development.js=862.57 kB857.88 kB=134.34 kB133.50 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
oss-experimental/react-dom/cjs/react-dom-profiling.profiling.js=750.23 kB745.88 kB=129.58 kB128.84 kB
oss-experimental/react-art/cjs/react-art.development.js=734.08 kB729.74 kB=115.50 kB114.68 kB
facebook-react-native/react/cjs/React-dev.js=52.93 kB52.61 kB=11.64 kB11.61 kB
oss-experimental/react-reconciler/cjs/react-reconciler.production.js=491.86 kB488.70 kB=78.14 kB77.71 kB
oss-experimental/react-reconciler/cjs/react-reconciler.profiling.js=568.03 kB564.15 kB=88.36 kB87.75 kB
facebook-www/React-profiling.classic.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-profiling.modern.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-prod.classic.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-www/React-prod.modern.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-react-native/react/cjs/React-profiling.js=19.96 kB19.66 kB=5.06 kB5.03 kB
oss-experimental/react/cjs/react.production.js=19.76 kB19.46 kB=4.95 kB4.92 kB
facebook-react-native/react/cjs/React-prod.js=19.53 kB19.23 kB=4.98 kB4.95 kB
oss-experimental/react/cjs/react.react-server.production.js=19.41 kB19.11 kB=5.03 kB5.00 kB
oss-stable/react/cjs/react.production.js=18.07 kB17.77 kB=4.62 kB4.59 kB
oss-stable-semver/react/cjs/react.production.js=18.04 kB17.74 kB=4.60 kB4.57 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB

Generated by 🚫 dangerJS against e01bf9b
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As far as I understand these tests do care about the debug info entries with the mocked time (introduced in #31716). For example, you want to assert that they are sequential and not overlapping, which is why there are also comments like Clamped to the start. So they are roughly asserting how this would lay out in the performance tracks. I've also never seen them flake. Under which circumstances did that happen to you?
@rickhanlonii
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@unstubbable I've seen CI fail quite a few times from this lately on my PRs, and on main.
For example, this commit from main failed from this in ReactFlightDOMEdge.
This test failed on my latest PR also from ReactFlightDOMEdge.
But aside from CI, I can basically repro locally any time I run yarn test, which makes it really annoying to test my changes before pushing.
@rickhanlonii
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Jan 18, 2026•

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The root cause seems to be that when those tests are run alongside other tests, the timing changes due to subtle delays in execution caused by the other test runs.
@rickhanlonii
rickhanloniiforce-pushed
the
fix-reactflight-test-flaky-time-values
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from
a738e51 to
e01bf9bCompareJanuary 18, 2026 19:17
@rickhanloniirickhanlonii
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[tests] Fix flaky time values in ReactFlight-test.js[tests] Fix flaky flight testsJan 18, 2026
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@unstubbable I found the root cause, see the new description and fixes - none of the test snapshots need to change now.
unstubbable
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Makes sense!
@rickhanlonii
rickhanlonii
merged commit 195fd22
into
react:mainJan 18, 2026

234 checks passed

@rickhanlonii
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deleted the

fix-reactflight-test-flaky-time-values

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January 18, 2026 20:36
@eps1loneps1lon
mentioned this pull request
Jan 19, 2026
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mentioned this pull request
Jun 5, 2026
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@gaearongaearon
mentioned this pull request
Jul 8, 2026
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@rickhanlonii@react-sizebot@unstubbable

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Auto-enable theater mode on YouTube
(function() {
function tryTheater() {
var btn = document.querySelector('button[aria-label="Theater mode"], ytd-player #player button[title="Theater mode"]');
if (btn && !btn.classList.contains('activated')) {
btn.click();
}
}
// Try immediately
tryTheater();
// Try after navigation (SPA)
var lastUrl = location.href;
setInterval(function() {
if (location.href !== lastUrl) {
lastUrl = location.href;
setTimeout(tryTheater, 500);
}
}, 1000);
// Also try on player load
var observer = new MutationObserver(tryTheater);
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
[tests] Fix flaky flight tests by rickhanlonii · Pull Request #35513 · react/react · GitHub
Skip to content
[tests] Fix flaky flight tests - #35513
Merged
rickhanlonii merged 1 commit into
react:mainfrom 
rickhanlonii:fix-reactflight-test-flaky-time-values
Jan 18, 2026
Merged
[tests] Fix flaky flight tests#35513
rickhanlonii merged 1 commit into
react:mainfrom 
rickhanlonii:fix-reactflight-test-flaky-time-values
Conversation
@rickhanlonii
@rickhanloniirickhanlonii
commented
Jan 15, 2026•

edited

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Flights tests are failing locally and in CI non-deterministically because we're not disabling async hooks after tests, and GC can clear WeakRefs non-deterministically.
This PR fixes the issue by adding an afterEach to disable installed hooks, and normalizing the value to value: {value: undefined}} when snapshotting.
Claude found the fix, here's the explanation:
Summary of Both Flaky Test Issues
Issue 1: ReactFlightDOMEdge-test.js - Cross-Test Async Hook Pollution
Root Cause:
  • When tests run with --runInBand, they execute sequentially in the same process
  • ReactFlightAsyncDebugInfo-test.js loads the Node server which creates async hooks via createHook().enable() to track async operations
  • ReactFlightDOMEdge-test.js loads the Edge server which uses a Noop debug config (no async hooks created) - The async hooks from the Node server persisted across tests because:

    • The Edge server doesn't create new hooks, so the cleanup code in setupTests.js never triggered
    • Old hooks kept firing their callbacks, calling performance.now() on the Edge test's mocks
    • This caused the Edge test's time values to be 232, 259, 260 instead of expected 12, 13, 14
The Fix: scripts/jest/setupTests.js
afterEach(() => {
if (installedHook) {
installedHook.disable();
installedHook = null;
}
});

This ensures async hooks are disabled after each test, preventing cross-test pollution.
Issue 2: ReactFlightAsyncDebugInfo-test.js - WeakRef Garbage Collection
Root Cause:
  • React's async debug info uses WeakRef to hold references to promises without preventing garbage collection
  • When serializing debug info, the code calls weakRef.deref() to get the promise
  • If GC has run: deref() returns undefined, and the server doesn't include the value field at all
  • If GC hasn't run: deref() returns the promise, and the server includes value: { value: undefined }
  • Since GC timing is non-deterministic (depends on memory pressure, CPU, etc.), tests would randomly pass or fail
The Fix: packages/internal-test-utils/debugInfo.js
// In normalizeIOInfo()
} else if ('value' in ioInfo) {
// If value exists but is undefined (WeakRef was GC'd)
copy.value = { value: undefined };
} else if (ioInfo.name && ioInfo.name !== 'rsc stream') {
// If value is completely missing (server didn't send it due to GC)
copy.value = { value: undefined };
}

This normalizes the output so snapshots always see a consistent structure regardless of GC timing.
★ Insight ─────────────────────────────────────

Two Patterns for Flaky Tests:
  1. State Leakage Between Tests: When tests run sequentially, global state (like async hooks, timers, or singletons) can leak. Solution: Clean up thoroughly in afterEach.
  2. Non-Deterministic System Behavior: When tests depend on unpredictable behavior (GC timing, network timing, file system operations), normalize the data in test utilities rather than trying to control the underlying system.
    
─────────────────────────────────────────────────
Both issues shared a common theme: non-deterministic behavior in sequential test execution. The first was about test isolation failure, the second was about normalizing inherently non-deterministic runtime behavior.
@rickhanloniirickhanlonii
changed the title
Fix flaky time values in ReactFlight-test.js[tests] Fix flaky time values in ReactFlight-test.jsJan 15, 2026
@github-actionsgithub-actionsBot
added
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React Core Team
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label
Jan 15, 2026
@react-sizebot
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Jan 15, 2026•

edited

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Comparing: d87298a...e01bf9b
Critical size changes
Includes critical production bundles, as well as any change greater than 2%:
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-stable/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js=608.48 kB608.03 kB+0.01%107.60 kB107.61 kB
oss-experimental/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
facebook-www/ReactDOM-prod.classic.js=693.87 kB693.38 kB+0.02%121.97 kB122.00 kB
facebook-www/ReactDOM-prod.modern.js=684.25 kB683.76 kB+0.02%120.37 kB120.40 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB
Significant size changes
Includes any change greater than 0.2%:
Expand to show
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-experimental/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-experimental/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-experimental/react-art/cjs/react-art.production.js=358.82 kB357.47 kB=60.39 kB60.22 kB
oss-experimental/react-dom/cjs/react-dom-profiling.development.js=1,262.88 kB1,257.62 kB=209.88 kB208.89 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.development.js=1,262.87 kB1,257.62 kB=210.68 kB209.73 kB
oss-experimental/react-dom/cjs/react-dom-client.development.js=1,246.33 kB1,241.07 kB=207.01 kB206.04 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.production.js=685.36 kB681.67 kB=121.56 kB121.07 kB
oss-experimental/react-reconciler/cjs/react-reconciler.development.js=862.57 kB857.88 kB=134.34 kB133.50 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
oss-experimental/react-dom/cjs/react-dom-profiling.profiling.js=750.23 kB745.88 kB=129.58 kB128.84 kB
oss-experimental/react-art/cjs/react-art.development.js=734.08 kB729.74 kB=115.50 kB114.68 kB
facebook-react-native/react/cjs/React-dev.js=52.93 kB52.61 kB=11.64 kB11.61 kB
oss-experimental/react-reconciler/cjs/react-reconciler.production.js=491.86 kB488.70 kB=78.14 kB77.71 kB
oss-experimental/react-reconciler/cjs/react-reconciler.profiling.js=568.03 kB564.15 kB=88.36 kB87.75 kB
facebook-www/React-profiling.classic.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-profiling.modern.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-prod.classic.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-www/React-prod.modern.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-react-native/react/cjs/React-profiling.js=19.96 kB19.66 kB=5.06 kB5.03 kB
oss-experimental/react/cjs/react.production.js=19.76 kB19.46 kB=4.95 kB4.92 kB
facebook-react-native/react/cjs/React-prod.js=19.53 kB19.23 kB=4.98 kB4.95 kB
oss-experimental/react/cjs/react.react-server.production.js=19.41 kB19.11 kB=5.03 kB5.00 kB
oss-stable/react/cjs/react.production.js=18.07 kB17.77 kB=4.62 kB4.59 kB
oss-stable-semver/react/cjs/react.production.js=18.04 kB17.74 kB=4.60 kB4.57 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB

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As far as I understand these tests do care about the debug info entries with the mocked time (introduced in #31716). For example, you want to assert that they are sequential and not overlapping, which is why there are also comments like Clamped to the start. So they are roughly asserting how this would lay out in the performance tracks. I've also never seen them flake. Under which circumstances did that happen to you?
@rickhanlonii
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@unstubbable I've seen CI fail quite a few times from this lately on my PRs, and on main.
For example, this commit from main failed from this in ReactFlightDOMEdge.
This test failed on my latest PR also from ReactFlightDOMEdge.
But aside from CI, I can basically repro locally any time I run yarn test, which makes it really annoying to test my changes before pushing.
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The root cause seems to be that when those tests are run alongside other tests, the timing changes due to subtle delays in execution caused by the other test runs.
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[tests] Fix flaky time values in ReactFlight-test.js[tests] Fix flaky flight testsJan 18, 2026
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@unstubbable I found the root cause, see the new description and fixes - none of the test snapshots need to change now.
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Makes sense!
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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Remove or un-stick sticky/fixed headers that block content
(function() {
function unstick() {
document.querySelectorAll('header, nav, [role="banner"], .header, .navbar, .sticky, .fixed-top, [style*="position: fixed"], [style*="position:sticky"]').forEach(function(el) {
if (el.style.position === 'fixed' || el.style.position === 'sticky' || getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {
el.style.position = 'static';
el.style.top = 'auto';
el.style.zIndex = 'auto';
}
});
}
unstick();
var observer = new MutationObserver(unstick);
observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });
})();
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} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
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[tests] Fix flaky flight tests by rickhanlonii · Pull Request #35513 · react/react · GitHub
Skip to content
[tests] Fix flaky flight tests - #35513
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rickhanlonii merged 1 commit into
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rickhanlonii:fix-reactflight-test-flaky-time-values
Jan 18, 2026
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[tests] Fix flaky flight tests#35513
rickhanlonii merged 1 commit into
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rickhanlonii:fix-reactflight-test-flaky-time-values
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Flights tests are failing locally and in CI non-deterministically because we're not disabling async hooks after tests, and GC can clear WeakRefs non-deterministically.
This PR fixes the issue by adding an afterEach to disable installed hooks, and normalizing the value to value: {value: undefined}} when snapshotting.
Claude found the fix, here's the explanation:
Summary of Both Flaky Test Issues
Issue 1: ReactFlightDOMEdge-test.js - Cross-Test Async Hook Pollution
Root Cause:
  • When tests run with --runInBand, they execute sequentially in the same process
  • ReactFlightAsyncDebugInfo-test.js loads the Node server which creates async hooks via createHook().enable() to track async operations
  • ReactFlightDOMEdge-test.js loads the Edge server which uses a Noop debug config (no async hooks created) - The async hooks from the Node server persisted across tests because:

    • The Edge server doesn't create new hooks, so the cleanup code in setupTests.js never triggered
    • Old hooks kept firing their callbacks, calling performance.now() on the Edge test's mocks
    • This caused the Edge test's time values to be 232, 259, 260 instead of expected 12, 13, 14
The Fix: scripts/jest/setupTests.js
afterEach(() => {
if (installedHook) {
installedHook.disable();
installedHook = null;
}
});

This ensures async hooks are disabled after each test, preventing cross-test pollution.
Issue 2: ReactFlightAsyncDebugInfo-test.js - WeakRef Garbage Collection
Root Cause:
  • React's async debug info uses WeakRef to hold references to promises without preventing garbage collection
  • When serializing debug info, the code calls weakRef.deref() to get the promise
  • If GC has run: deref() returns undefined, and the server doesn't include the value field at all
  • If GC hasn't run: deref() returns the promise, and the server includes value: { value: undefined }
  • Since GC timing is non-deterministic (depends on memory pressure, CPU, etc.), tests would randomly pass or fail
The Fix: packages/internal-test-utils/debugInfo.js
// In normalizeIOInfo()
} else if ('value' in ioInfo) {
// If value exists but is undefined (WeakRef was GC'd)
copy.value = { value: undefined };
} else if (ioInfo.name && ioInfo.name !== 'rsc stream') {
// If value is completely missing (server didn't send it due to GC)
copy.value = { value: undefined };
}

This normalizes the output so snapshots always see a consistent structure regardless of GC timing.
★ Insight ─────────────────────────────────────

Two Patterns for Flaky Tests:
  1. State Leakage Between Tests: When tests run sequentially, global state (like async hooks, timers, or singletons) can leak. Solution: Clean up thoroughly in afterEach.
  2. Non-Deterministic System Behavior: When tests depend on unpredictable behavior (GC timing, network timing, file system operations), normalize the data in test utilities rather than trying to control the underlying system.
    
─────────────────────────────────────────────────
Both issues shared a common theme: non-deterministic behavior in sequential test execution. The first was about test isolation failure, the second was about normalizing inherently non-deterministic runtime behavior.
@rickhanloniirickhanlonii
changed the title
Fix flaky time values in ReactFlight-test.js[tests] Fix flaky time values in ReactFlight-test.jsJan 15, 2026
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Comparing: d87298a...e01bf9b
Critical size changes
Includes critical production bundles, as well as any change greater than 2%:
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-stable/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js=608.48 kB608.03 kB+0.01%107.60 kB107.61 kB
oss-experimental/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
facebook-www/ReactDOM-prod.classic.js=693.87 kB693.38 kB+0.02%121.97 kB122.00 kB
facebook-www/ReactDOM-prod.modern.js=684.25 kB683.76 kB+0.02%120.37 kB120.40 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB
Significant size changes
Includes any change greater than 0.2%:
Expand to show
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-experimental/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-experimental/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-experimental/react-art/cjs/react-art.production.js=358.82 kB357.47 kB=60.39 kB60.22 kB
oss-experimental/react-dom/cjs/react-dom-profiling.development.js=1,262.88 kB1,257.62 kB=209.88 kB208.89 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.development.js=1,262.87 kB1,257.62 kB=210.68 kB209.73 kB
oss-experimental/react-dom/cjs/react-dom-client.development.js=1,246.33 kB1,241.07 kB=207.01 kB206.04 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.production.js=685.36 kB681.67 kB=121.56 kB121.07 kB
oss-experimental/react-reconciler/cjs/react-reconciler.development.js=862.57 kB857.88 kB=134.34 kB133.50 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
oss-experimental/react-dom/cjs/react-dom-profiling.profiling.js=750.23 kB745.88 kB=129.58 kB128.84 kB
oss-experimental/react-art/cjs/react-art.development.js=734.08 kB729.74 kB=115.50 kB114.68 kB
facebook-react-native/react/cjs/React-dev.js=52.93 kB52.61 kB=11.64 kB11.61 kB
oss-experimental/react-reconciler/cjs/react-reconciler.production.js=491.86 kB488.70 kB=78.14 kB77.71 kB
oss-experimental/react-reconciler/cjs/react-reconciler.profiling.js=568.03 kB564.15 kB=88.36 kB87.75 kB
facebook-www/React-profiling.classic.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-profiling.modern.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-prod.classic.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-www/React-prod.modern.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-react-native/react/cjs/React-profiling.js=19.96 kB19.66 kB=5.06 kB5.03 kB
oss-experimental/react/cjs/react.production.js=19.76 kB19.46 kB=4.95 kB4.92 kB
facebook-react-native/react/cjs/React-prod.js=19.53 kB19.23 kB=4.98 kB4.95 kB
oss-experimental/react/cjs/react.react-server.production.js=19.41 kB19.11 kB=5.03 kB5.00 kB
oss-stable/react/cjs/react.production.js=18.07 kB17.77 kB=4.62 kB4.59 kB
oss-stable-semver/react/cjs/react.production.js=18.04 kB17.74 kB=4.60 kB4.57 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB

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As far as I understand these tests do care about the debug info entries with the mocked time (introduced in #31716). For example, you want to assert that they are sequential and not overlapping, which is why there are also comments like Clamped to the start. So they are roughly asserting how this would lay out in the performance tracks. I've also never seen them flake. Under which circumstances did that happen to you?
@rickhanlonii
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@unstubbable I've seen CI fail quite a few times from this lately on my PRs, and on main.
For example, this commit from main failed from this in ReactFlightDOMEdge.
This test failed on my latest PR also from ReactFlightDOMEdge.
But aside from CI, I can basically repro locally any time I run yarn test, which makes it really annoying to test my changes before pushing.
@rickhanlonii
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Jan 18, 2026•

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The root cause seems to be that when those tests are run alongside other tests, the timing changes due to subtle delays in execution caused by the other test runs.
@rickhanlonii
rickhanloniiforce-pushed
the
fix-reactflight-test-flaky-time-values
branch
from
a738e51 to
e01bf9bCompareJanuary 18, 2026 19:17
@rickhanloniirickhanlonii
changed the title
[tests] Fix flaky time values in ReactFlight-test.js[tests] Fix flaky flight testsJan 18, 2026
@rickhanlonii
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@unstubbable I found the root cause, see the new description and fixes - none of the test snapshots need to change now.
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Makes sense!
@rickhanlonii
rickhanlonii
merged commit 195fd22
into
react:mainJan 18, 2026

234 checks passed

@rickhanlonii
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deleted the

fix-reactflight-test-flaky-time-values

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January 18, 2026 20:36
@eps1loneps1lon
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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Universal Dark Mode - works on any site
(function() {
var enabled = true;
function applyDarkMode() {
if (!enabled) return;
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if (!style) {
style = document.createElement('style');
style.id = 'universal-dark-mode-style';
document.head.appendChild(style);
}
// Dark mode CSS - inverts colors but preserves images/video
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html {
filter: invert(1) hue-rotate(180deg) !important;
background: #1a1a2e !important;
}
/* Restore images, videos, iframes, canvas */
img, video, iframe, canvas, svg, picture, [style*="background-image"] {
filter: invert(1) hue-rotate(180deg) !important;
}
/* Preserve specific elements that should not be inverted */
.no-dark-mode, .no-dark-mode *,
[data-theme="light"], [data-theme="light"],
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::-webkit-scrollbar-thumb { background: #444 !important; }
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/* Selection */
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';
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console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');
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}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();
[tests] Fix flaky flight tests by rickhanlonii · Pull Request #35513 · react/react · GitHub
Skip to content
[tests] Fix flaky flight tests - #35513
Merged
rickhanlonii merged 1 commit into
react:mainfrom 
rickhanlonii:fix-reactflight-test-flaky-time-values
Jan 18, 2026
Merged
[tests] Fix flaky flight tests#35513
rickhanlonii merged 1 commit into
react:mainfrom 
rickhanlonii:fix-reactflight-test-flaky-time-values
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Flights tests are failing locally and in CI non-deterministically because we're not disabling async hooks after tests, and GC can clear WeakRefs non-deterministically.
This PR fixes the issue by adding an afterEach to disable installed hooks, and normalizing the value to value: {value: undefined}} when snapshotting.
Claude found the fix, here's the explanation:
Summary of Both Flaky Test Issues
Issue 1: ReactFlightDOMEdge-test.js - Cross-Test Async Hook Pollution
Root Cause:
  • When tests run with --runInBand, they execute sequentially in the same process
  • ReactFlightAsyncDebugInfo-test.js loads the Node server which creates async hooks via createHook().enable() to track async operations
  • ReactFlightDOMEdge-test.js loads the Edge server which uses a Noop debug config (no async hooks created) - The async hooks from the Node server persisted across tests because:

    • The Edge server doesn't create new hooks, so the cleanup code in setupTests.js never triggered
    • Old hooks kept firing their callbacks, calling performance.now() on the Edge test's mocks
    • This caused the Edge test's time values to be 232, 259, 260 instead of expected 12, 13, 14
The Fix: scripts/jest/setupTests.js
afterEach(() => {
if (installedHook) {
installedHook.disable();
installedHook = null;
}
});

This ensures async hooks are disabled after each test, preventing cross-test pollution.
Issue 2: ReactFlightAsyncDebugInfo-test.js - WeakRef Garbage Collection
Root Cause:
  • React's async debug info uses WeakRef to hold references to promises without preventing garbage collection
  • When serializing debug info, the code calls weakRef.deref() to get the promise
  • If GC has run: deref() returns undefined, and the server doesn't include the value field at all
  • If GC hasn't run: deref() returns the promise, and the server includes value: { value: undefined }
  • Since GC timing is non-deterministic (depends on memory pressure, CPU, etc.), tests would randomly pass or fail
The Fix: packages/internal-test-utils/debugInfo.js
// In normalizeIOInfo()
} else if ('value' in ioInfo) {
// If value exists but is undefined (WeakRef was GC'd)
copy.value = { value: undefined };
} else if (ioInfo.name && ioInfo.name !== 'rsc stream') {
// If value is completely missing (server didn't send it due to GC)
copy.value = { value: undefined };
}

This normalizes the output so snapshots always see a consistent structure regardless of GC timing.
★ Insight ─────────────────────────────────────

Two Patterns for Flaky Tests:
  1. State Leakage Between Tests: When tests run sequentially, global state (like async hooks, timers, or singletons) can leak. Solution: Clean up thoroughly in afterEach.
  2. Non-Deterministic System Behavior: When tests depend on unpredictable behavior (GC timing, network timing, file system operations), normalize the data in test utilities rather than trying to control the underlying system.
    
─────────────────────────────────────────────────
Both issues shared a common theme: non-deterministic behavior in sequential test execution. The first was about test isolation failure, the second was about normalizing inherently non-deterministic runtime behavior.
@rickhanloniirickhanlonii
changed the title
Fix flaky time values in ReactFlight-test.js[tests] Fix flaky time values in ReactFlight-test.jsJan 15, 2026
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Comparing: d87298a...e01bf9b
Critical size changes
Includes critical production bundles, as well as any change greater than 2%:
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-stable/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js=608.48 kB608.03 kB+0.01%107.60 kB107.61 kB
oss-experimental/react-dom/cjs/react-dom.production.js=6.84 kB6.84 kB=1.88 kB1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
facebook-www/ReactDOM-prod.classic.js=693.87 kB693.38 kB+0.02%121.97 kB122.00 kB
facebook-www/ReactDOM-prod.modern.js=684.25 kB683.76 kB+0.02%120.37 kB120.40 kB
oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB
Significant size changes
Includes any change greater than 0.2%:
Expand to show
Name+/-BaseCurrent+/- gzipBase gzipCurrent gzip
oss-experimental/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.development.js=33.37 kB33.29 kB=5.93 kB5.92 kB
oss-experimental/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable-semver/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-stable/react-debug-tools/cjs/react-debug-tools.production.js=29.78 kB29.70 kB=5.81 kB5.80 kB
oss-experimental/react-art/cjs/react-art.production.js=358.82 kB357.47 kB=60.39 kB60.22 kB
oss-experimental/react-dom/cjs/react-dom-profiling.development.js=1,262.88 kB1,257.62 kB=209.88 kB208.89 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.development.js=1,262.87 kB1,257.62 kB=210.68 kB209.73 kB
oss-experimental/react-dom/cjs/react-dom-client.development.js=1,246.33 kB1,241.07 kB=207.01 kB206.04 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.production.js=685.36 kB681.67 kB=121.56 kB121.07 kB
oss-experimental/react-reconciler/cjs/react-reconciler.development.js=862.57 kB857.88 kB=134.34 kB133.50 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js=670.95 kB667.26 kB=118.02 kB117.51 kB
oss-experimental/react-dom/cjs/react-dom-profiling.profiling.js=750.23 kB745.88 kB=129.58 kB128.84 kB
oss-experimental/react-art/cjs/react-art.development.js=734.08 kB729.74 kB=115.50 kB114.68 kB
facebook-react-native/react/cjs/React-dev.js=52.93 kB52.61 kB=11.64 kB11.61 kB
oss-experimental/react-reconciler/cjs/react-reconciler.production.js=491.86 kB488.70 kB=78.14 kB77.71 kB
oss-experimental/react-reconciler/cjs/react-reconciler.profiling.js=568.03 kB564.15 kB=88.36 kB87.75 kB
facebook-www/React-profiling.classic.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-profiling.modern.js=20.87 kB20.57 kB=5.28 kB5.25 kB
facebook-www/React-prod.classic.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-www/React-prod.modern.js=20.44 kB20.14 kB=5.20 kB5.17 kB
facebook-react-native/react/cjs/React-profiling.js=19.96 kB19.66 kB=5.06 kB5.03 kB
oss-experimental/react/cjs/react.production.js=19.76 kB19.46 kB=4.95 kB4.92 kB
facebook-react-native/react/cjs/React-prod.js=19.53 kB19.23 kB=4.98 kB4.95 kB
oss-experimental/react/cjs/react.react-server.production.js=19.41 kB19.11 kB=5.03 kB5.00 kB
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oss-stable/react/cjs/react.react-server.production.js=13.83 kB13.54 kB=3.76 kB3.74 kB
oss-stable-semver/react/cjs/react.react-server.production.js=13.81 kB13.51 kB=3.74 kB3.71 kB

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As far as I understand these tests do care about the debug info entries with the mocked time (introduced in #31716). For example, you want to assert that they are sequential and not overlapping, which is why there are also comments like Clamped to the start. So they are roughly asserting how this would lay out in the performance tracks. I've also never seen them flake. Under which circumstances did that happen to you?
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Jan 18, 2026

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@unstubbable I've seen CI fail quite a few times from this lately on my PRs, and on main.
For example, this commit from main failed from this in ReactFlightDOMEdge.
This test failed on my latest PR also from ReactFlightDOMEdge.
But aside from CI, I can basically repro locally any time I run yarn test, which makes it really annoying to test my changes before pushing.
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The root cause seems to be that when those tests are run alongside other tests, the timing changes due to subtle delays in execution caused by the other test runs.
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[tests] Fix flaky time values in ReactFlight-test.js[tests] Fix flaky flight testsJan 18, 2026
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@unstubbable I found the root cause, see the new description and fixes - none of the test snapshots need to change now.
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Makes sense!
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