Detect subscriptions wrapped in startTransition

packages/react-reconciler/src/ReactFiberHooks.new.js

@@ -116,6 +116,7 @@ import {
 } from './ReactUpdateQueue.new';
 import {pushInterleavedQueue} from './ReactFiberInterleavedUpdates.new';
 import {getIsStrictModeForDevtools} from './ReactFiberReconciler.new';
+import {warnOnSubscriptionInsideStartTransition} from 'shared/ReactFeatureFlags';
 
 const {ReactCurrentDispatcher, ReactCurrentBatchConfig} = ReactSharedInternals;
 
@@ -1861,6 +1862,23 @@ function startTransition(setPending, callback) {
   } finally {
     setCurrentUpdatePriority(previousPriority);
     ReactCurrentBatchConfig.transition = prevTransition;
+    if (__DEV__) {
+      if (
+        prevTransition !== 1 &&
+        warnOnSubscriptionInsideStartTransition &&
+        ReactCurrentBatchConfig._updatedFibers
+      ) {
+        const updatedFibersCount = ReactCurrentBatchConfig._updatedFibers.size;
+        if (updatedFibersCount > 10) {
+          console.warn(
+            'Detected a large number of updates inside startTransition. ' +
+              'If this is due to a subscription please re-write it to use React provided hooks. ' +
+              'Otherwise concurrent mode guarantees are off the table.',
+          );
+        }
+        ReactCurrentBatchConfig._updatedFibers.clear();
+      }
+    }
   }
 }
 

packages/react-reconciler/src/ReactFiberHooks.old.js

@@ -116,6 +116,7 @@ import {
 } from './ReactUpdateQueue.old';
 import {pushInterleavedQueue} from './ReactFiberInterleavedUpdates.old';
 import {getIsStrictModeForDevtools} from './ReactFiberReconciler.old';
+import {warnOnSubscriptionInsideStartTransition} from 'shared/ReactFeatureFlags';
 
 const {ReactCurrentDispatcher, ReactCurrentBatchConfig} = ReactSharedInternals;
 
@@ -1861,6 +1862,23 @@ function startTransition(setPending, callback) {
   } finally {
     setCurrentUpdatePriority(previousPriority);
     ReactCurrentBatchConfig.transition = prevTransition;
+    if (__DEV__) {
+      if (
+        prevTransition !== 1 &&
+        warnOnSubscriptionInsideStartTransition &&
+        ReactCurrentBatchConfig._updatedFibers
+      ) {
+        const updatedFibersCount = ReactCurrentBatchConfig._updatedFibers.size;
+        if (updatedFibersCount > 10) {
+          console.warn(
+            'Detected a large number of updates inside startTransition. ' +
+              'If this is due to a subscription please re-write it to use React provided hooks. ' +
+              'Otherwise concurrent mode guarantees are off the table.',
+          );
+        }
+        ReactCurrentBatchConfig._updatedFibers.clear();
+      }
+    }
   }
 }
 

packages/react-reconciler/src/ReactFiberWorkLoop.new.js

@@ -30,6 +30,7 @@ import {
   enableStrictEffects,
   skipUnmountedBoundaries,
   enableUpdaterTracking,
+  warnOnSubscriptionInsideStartTransition,
 } from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import invariant from 'shared/invariant';
@@ -385,6 +386,13 @@ export function requestUpdateLane(fiber: Fiber): Lane {
 
   const isTransition = requestCurrentTransition() !== NoTransition;
   if (isTransition) {
+    if (
+      __DEV__ &&
+      warnOnSubscriptionInsideStartTransition &&
+      ReactCurrentBatchConfig._updatedFibers
+    ) {
+      ReactCurrentBatchConfig._updatedFibers.add(fiber);
+    }
     // The algorithm for assigning an update to a lane should be stable for all
     // updates at the same priority within the same event. To do this, the
     // inputs to the algorithm must be the same.

packages/react-reconciler/src/ReactFiberWorkLoop.old.js

@@ -30,6 +30,7 @@ import {
   enableStrictEffects,
   skipUnmountedBoundaries,
   enableUpdaterTracking,
+  warnOnSubscriptionInsideStartTransition,
 } from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import invariant from 'shared/invariant';
@@ -385,6 +386,13 @@ export function requestUpdateLane(fiber: Fiber): Lane {
 
   const isTransition = requestCurrentTransition() !== NoTransition;
   if (isTransition) {
+    if (
+      __DEV__ &&
+      warnOnSubscriptionInsideStartTransition &&
+      ReactCurrentBatchConfig._updatedFibers
+    ) {
+      ReactCurrentBatchConfig._updatedFibers.add(fiber);
+    }
     // The algorithm for assigning an update to a lane should be stable for all
     // updates at the same priority within the same event. To do this, the
     // inputs to the algorithm must be the same.

packages/react/src/ReactCurrentBatchConfig.js

@@ -7,12 +7,22 @@
  * @flow
  */
 
+import type {Fiber} from 'react-reconciler/src/ReactInternalTypes';
+
+type BatchConfig = {
+  transition: number,
+  _updatedFibers?: Set<Fiber>,
+};
 /**
  * Keeps track of the current batch's configuration such as how long an update
  * should suspend for if it needs to.
  */
-const ReactCurrentBatchConfig = {
-  transition: (0: number),
+const ReactCurrentBatchConfig: BatchConfig = {
+  transition: 0,
 };
 
+if (__DEV__) {
+  ReactCurrentBatchConfig._updatedFibers = new Set();
+}
+
 export default ReactCurrentBatchConfig;

packages/react/src/ReactStartTransition.js

@@ -8,6 +8,7 @@
  */
 
 import ReactCurrentBatchConfig from './ReactCurrentBatchConfig';
+import {warnOnSubscriptionInsideStartTransition} from 'shared/ReactFeatureFlags';
 
 export function startTransition(scope: () => void) {
   const prevTransition = ReactCurrentBatchConfig.transition;
@@ -16,5 +17,22 @@ export function startTransition(scope: () => void) {
     scope();
   } finally {
     ReactCurrentBatchConfig.transition = prevTransition;
+    if (__DEV__) {
+      if (
+        prevTransition !== 1 &&
+        warnOnSubscriptionInsideStartTransition &&
+        ReactCurrentBatchConfig._updatedFibers
+      ) {
+        const updatedFibersCount = ReactCurrentBatchConfig._updatedFibers.size;
+        if (updatedFibersCount > 10) {
+          console.warn(
+            'Detected a large number of updates inside startTransition. ' +
+              'If this is due to a subscription please re-write it to use React provided hooks. ' +
+              'Otherwise concurrent mode guarantees are off the table.',
+          );
+        }
+        ReactCurrentBatchConfig._updatedFibers.clear();
+      }
+    }
   }
 }

packages/react/src/__tests__/ReactStartTransition-test.js

@@ -0,0 +1,91 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ */
+
+'use strict';
+
+let React;
+let ReactTestRenderer;
+let act;
+let useState;
+let useTransition;
+
+const SUSPICIOUS_NUMBER_OF_FIBERS_UPDATED = 10;
+
+describe('ReactStartTransition', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    React = require('react');
+    ReactTestRenderer = require('react-test-renderer');
+    act = require('jest-react').act;
+    useState = React.useState;
+    useTransition = React.useTransition;
+  });
+
+  // @gate warnOnSubscriptionInsideStartTransition || !__DEV__
+  it('Warns if a suspicious number of fibers are updated inside startTransition', () => {
+    const subs = new Set();
+    const useUserSpaceSubscription = () => {
+      const setState = useState(0)[1];
+      subs.add(setState);
+    };
+
+    let triggerHookTransition;
+
+    const Component = ({level}) => {
+      useUserSpaceSubscription();
+      if (level === 0) {
+        triggerHookTransition = useTransition()[1];
+      }
+      if (level < SUSPICIOUS_NUMBER_OF_FIBERS_UPDATED) {
+        return <Component level={level + 1} />;
+      }
+      return null;
+    };
+
+    act(() => {
+      ReactTestRenderer.create(<Component level={0} />, {
+        unstable_isConcurrent: true,
+      });
+    });
+
+    expect(() => {
+      act(() => {
+        React.startTransition(() => {
+          subs.forEach(setState => {
+            setState(state => state + 1);
+          });
+        });
+      });
+    }).toWarnDev(
+      [
+        'Detected a large number of updates inside startTransition. ' +
+          'If this is due to a subscription please re-write it to use React provided hooks. ' +
+          'Otherwise concurrent mode guarantees are off the table.',
+      ],
+      {withoutStack: true},
+    );
+
+    expect(() => {
+      act(() => {
+        triggerHookTransition(() => {
+          subs.forEach(setState => {
+            setState(state => state + 1);
+          });
+        });
+      });
+    }).toWarnDev(
+      [
+        'Detected a large number of updates inside startTransition. ' +
+          'If this is due to a subscription please re-write it to use React provided hooks. ' +
+          'Otherwise concurrent mode guarantees are off the table.',
+      ],
+      {withoutStack: true},
+    );
+  });
+});

packages/shared/ReactFeatureFlags.js

@@ -99,6 +99,8 @@ export const enableTrustedTypesIntegration = false;
 // a deprecated pattern we want to get rid of in the future
 export const warnAboutSpreadingKeyToJSX = false;
 
+export const warnOnSubscriptionInsideStartTransition = false;
+
 export const enableComponentStackLocations = true;
 
 export const enableNewReconciler = false;

packages/shared/forks/ReactFeatureFlags.native-fb.js

@@ -48,6 +48,7 @@ export const disableTextareaChildren = false;
 export const disableModulePatternComponents = false;
 export const warnUnstableRenderSubtreeIntoContainer = false;
 export const warnAboutSpreadingKeyToJSX = false;
+export const warnOnSubscriptionInsideStartTransition = false;
 export const enableComponentStackLocations = false;
 export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;

packages/shared/forks/ReactFeatureFlags.native-oss.js

@@ -39,6 +39,7 @@ export const disableTextareaChildren = false;
 export const disableModulePatternComponents = false;
 export const warnUnstableRenderSubtreeIntoContainer = false;
 export const warnAboutSpreadingKeyToJSX = false;
+export const warnOnSubscriptionInsideStartTransition = false;
 export const enableComponentStackLocations = false;
 export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.js

@@ -39,6 +39,7 @@ export const disableTextareaChildren = false;
 export const disableModulePatternComponents = false;
 export const warnUnstableRenderSubtreeIntoContainer = false;
 export const warnAboutSpreadingKeyToJSX = false;
+export const warnOnSubscriptionInsideStartTransition = false;
 export const enableComponentStackLocations = true;
 export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.native.js

@@ -49,7 +49,7 @@ export const enableSuspenseLayoutEffectSemantics = false;
 export const enableGetInspectorDataForInstanceInProduction = false;
 export const enableNewReconciler = false;
 export const deferRenderPhaseUpdateToNextBatch = false;
-
+export const warnOnSubscriptionInsideStartTransition = false;
 export const enableStrictEffects = false;
 export const createRootStrictEffectsByDefault = false;
 export const enableUseRefAccessWarning = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.www.js

@@ -39,6 +39,7 @@ export const disableTextareaChildren = false;
 export const disableModulePatternComponents = true;
 export const warnUnstableRenderSubtreeIntoContainer = false;
 export const warnAboutSpreadingKeyToJSX = false;
+export const warnOnSubscriptionInsideStartTransition = false;
 export const enableComponentStackLocations = true;
 export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;

packages/shared/forks/ReactFeatureFlags.testing.js

@@ -39,6 +39,7 @@ export const disableTextareaChildren = false;
 export const disableModulePatternComponents = false;
 export const warnUnstableRenderSubtreeIntoContainer = false;
 export const warnAboutSpreadingKeyToJSX = false;
+export const warnOnSubscriptionInsideStartTransition = false;
 export const enableComponentStackLocations = true;
 export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;

packages/shared/forks/ReactFeatureFlags.testing.www.js

@@ -39,6 +39,7 @@ export const disableTextareaChildren = __EXPERIMENTAL__;
 export const disableModulePatternComponents = true;
 export const warnUnstableRenderSubtreeIntoContainer = false;
 export const warnAboutSpreadingKeyToJSX = false;
+export const warnOnSubscriptionInsideStartTransition = false;
 export const enableComponentStackLocations = true;
 export const enableLegacyFBSupport = !__EXPERIMENTAL__;
 export const enableFilterEmptyStringAttributesDOM = false;

packages/shared/forks/ReactFeatureFlags.www-dynamic.js

@@ -25,6 +25,7 @@ export const disableSchedulerTimeoutInWorkLoop = __VARIANT__;
 export const enableLazyContextPropagation = __VARIANT__;
 export const enableSyncDefaultUpdates = __VARIANT__;
 export const consoleManagedByDevToolsDuringStrictMode = __VARIANT__;
+export const warnOnSubscriptionInsideStartTransition = __VARIANT__;
 
 // Enable this flag to help with concurrent mode debugging.
 // It logs information to the console about React scheduling, rendering, and commit phases.

packages/shared/forks/ReactFeatureFlags.www.js

@@ -31,6 +31,7 @@ export const {
   disableSchedulerTimeoutInWorkLoop,
   enableLazyContextPropagation,
   enableSyncDefaultUpdates,
+  warnOnSubscriptionInsideStartTransition,
 } = dynamicFeatureFlags;
 
 // On WWW, __EXPERIMENTAL__ is used for a new modern build.
@@ -56,7 +57,6 @@ export const enableSchedulingProfiler =
 // For now, we'll turn it on for everyone because it's *already* on for everyone in practice.
 // At least this will let us stop shipping <Profiler> implementation to all users.
 export const enableSchedulerDebugging = true;
-
 export const warnAboutDeprecatedLifecycles = true;
 export const disableLegacyContext = __EXPERIMENTAL__;
 export const warnAboutStringRefs = false;