Hydration errors should force a client render

If something errors during hydration, we should try rendering again
without hydrating.

We'll find the nearest Suspense boundary and force it to client render,
discarding the server-rendered content.

packages/react-dom/src/__tests__/ReactDOMFizzServer-test.js

@@ -1783,4 +1783,263 @@ describe('ReactDOMFizzServer', () => {
     expect(getVisibleChildren(container)).toEqual(<div>client</div>);
     expect(ref.current).toEqual(serverRenderedDiv);
   });
+
+  // @gate supportsNativeUseSyncExternalStore
+  // @gate experimental
+  it(
+    'errors during hydration force a client render at the nearest Suspense ' +
+      'boundary, and during the client render it recovers',
+    async () => {
+      let isClient = false;
+
+      function subscribe() {
+        return () => {};
+      }
+      function getClientSnapshot() {
+        return 'Yay!';
+      }
+
+      // At the time of writing, the only API that exposes whether it's currently
+      // hydrating is the `getServerSnapshot` API, so I'm using that here to
+      // simulate an error during hydration.
+      function getServerSnapshot() {
+        if (isClient) {
+          throw new Error('Hydration error');
+        }
+        return 'Yay!';
+      }
+
+      function Child() {
+        const value = useSyncExternalStore(
+          subscribe,
+          getClientSnapshot,
+          getServerSnapshot,
+        );
+        Scheduler.unstable_yieldValue(value);
+        return value;
+      }
+
+      const span1Ref = React.createRef();
+      const span2Ref = React.createRef();
+      const span3Ref = React.createRef();
+
+      function App() {
+        return (
+          <div>
+            <span ref={span1Ref} />
+            <Suspense fallback="Loading...">
+              <span ref={span2Ref}>
+                <Child />
+              </span>
+            </Suspense>
+            <span ref={span3Ref} />
+          </div>
+        );
+      }
+
+      await act(async () => {
+        const {startWriting} = ReactDOMFizzServer.pipeToNodeWritable(
+          <App />,
+          writable,
+        );
+        startWriting();
+      });
+      expect(Scheduler).toHaveYielded(['Yay!']);
+
+      const [span1, span2, span3] = container.getElementsByTagName('span');
+
+      // Hydrate the tree. Child will throw during hydration, but not when it
+      // falls back to client rendering.
+      isClient = true;
+      ReactDOM.hydrateRoot(container, <App />);
+
+      expect(Scheduler).toFlushAndYield(['Yay!']);
+      expect(getVisibleChildren(container)).toEqual(
+        <div>
+          <span />
+          <span>Yay!</span>
+          <span />
+        </div>,
+      );
+
+      // The node that's inside the boundary that errored during hydration was
+      // not hydrated.
+      expect(span2Ref.current).not.toBe(span2);
+
+      // But the nodes outside the boundary were.
+      expect(span1Ref.current).toBe(span1);
+      expect(span3Ref.current).toBe(span3);
+    },
+  );
+
+  // @gate experimental
+  it(
+    'errors during hydration force a client render at the nearest Suspense ' +
+      'boundary, and during the client render it fails again',
+    async () => {
+      // Similar to previous test, but the client render errors, too. We should
+      // be able to capture it with an error boundary.
+
+      let isClient = false;
+
+      class ErrorBoundary extends React.Component {
+        state = {error: null};
+        static getDerivedStateFromError(error) {
+          return {error};
+        }
+        render() {
+          if (this.state.error !== null) {
+            return this.state.error.message;
+          }
+          return this.props.children;
+        }
+      }
+
+      function Child() {
+        if (isClient) {
+          throw new Error('Oops!');
+        }
+        Scheduler.unstable_yieldValue('Yay!');
+        return 'Yay!';
+      }
+
+      const span1Ref = React.createRef();
+      const span2Ref = React.createRef();
+      const span3Ref = React.createRef();
+
+      function App() {
+        return (
+          <ErrorBoundary>
+            <span ref={span1Ref} />
+            <Suspense fallback="Loading...">
+              <span ref={span2Ref}>
+                <Child />
+              </span>
+            </Suspense>
+            <span ref={span3Ref} />
+          </ErrorBoundary>
+        );
+      }
+
+      await act(async () => {
+        const {startWriting} = ReactDOMFizzServer.pipeToNodeWritable(
+          <App />,
+          writable,
+        );
+        startWriting();
+      });
+      expect(Scheduler).toHaveYielded(['Yay!']);
+
+      // Hydrate the tree. Child will throw during render.
+      isClient = true;
+      ReactDOM.hydrateRoot(container, <App />);
+
+      expect(Scheduler).toFlushAndYield([]);
+      expect(getVisibleChildren(container)).toEqual('Oops!');
+    },
+  );
+
+  // @gate supportsNativeUseSyncExternalStore
+  // @gate experimental
+  it(
+    'errors during hydration force a client render at the nearest Suspense ' +
+      'boundary, and during the client render it recovers, then a deeper ' +
+      'child suspends',
+    async () => {
+      let isClient = false;
+
+      function subscribe() {
+        return () => {};
+      }
+      function getClientSnapshot() {
+        return 'Yay!';
+      }
+
+      // At the time of writing, the only API that exposes whether it's currently
+      // hydrating is the `getServerSnapshot` API, so I'm using that here to
+      // simulate an error during hydration.
+      function getServerSnapshot() {
+        if (isClient) {
+          throw new Error('Hydration error');
+        }
+        return 'Yay!';
+      }
+
+      function Child() {
+        const value = useSyncExternalStore(
+          subscribe,
+          getClientSnapshot,
+          getServerSnapshot,
+        );
+        if (isClient) {
+          readText(value);
+        }
+        Scheduler.unstable_yieldValue(value);
+        return value;
+      }
+
+      const span1Ref = React.createRef();
+      const span2Ref = React.createRef();
+      const span3Ref = React.createRef();
+
+      function App() {
+        return (
+          <div>
+            <span ref={span1Ref} />
+            <Suspense fallback="Loading...">
+              <span ref={span2Ref}>
+                <Child />
+              </span>
+            </Suspense>
+            <span ref={span3Ref} />
+          </div>
+        );
+      }
+
+      await act(async () => {
+        const {startWriting} = ReactDOMFizzServer.pipeToNodeWritable(
+          <App />,
+          writable,
+        );
+        startWriting();
+      });
+      expect(Scheduler).toHaveYielded(['Yay!']);
+
+      const [span1, span2, span3] = container.getElementsByTagName('span');
+
+      // Hydrate the tree. Child will throw during hydration, but not when it
+      // falls back to client rendering.
+      isClient = true;
+      ReactDOM.hydrateRoot(container, <App />);
+
+      expect(Scheduler).toFlushAndYield([]);
+      expect(getVisibleChildren(container)).toEqual(
+        <div>
+          <span />
+          Loading...
+          <span />
+        </div>,
+      );
+
+      await act(async () => {
+        resolveText('Yay!');
+      });
+      expect(Scheduler).toFlushAndYield(['Yay!']);
+      expect(getVisibleChildren(container)).toEqual(
+        <div>
+          <span />
+          <span>Yay!</span>
+          <span />
+        </div>,
+      );
+
+      // The node that's inside the boundary that errored during hydration was
+      // not hydrated.
+      expect(span2Ref.current).not.toBe(span2);
+
+      // But the nodes outside the boundary were.
+      expect(span1Ref.current).toBe(span1);
+      expect(span3Ref.current).toBe(span3);
+    },
+  );
 });

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

@@ -74,6 +74,7 @@ import {
   ForceUpdateForLegacySuspense,
   StaticMask,
   ShouldCapture,
+  ForceClientRender,
 } from './ReactFiberFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import {
@@ -2081,6 +2082,14 @@ function updateSuspenseComponent(current, workInProgress, renderLanes) {
               prevState,
               renderLanes,
             );
+          } else if (workInProgress.flags & ForceClientRender) {
+            // Something errored during hydration. Try again without hydrating.
+            workInProgress.flags &= ~ForceClientRender;
+            return retrySuspenseComponentWithoutHydrating(
+              current,
+              workInProgress,
+              renderLanes,
+            );
           } else if (
             (workInProgress.memoizedState: null | SuspenseState) !== null
           ) {

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

@@ -74,6 +74,7 @@ import {
   ForceUpdateForLegacySuspense,
   StaticMask,
   ShouldCapture,
+  ForceClientRender,
 } from './ReactFiberFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import {
@@ -2081,6 +2082,14 @@ function updateSuspenseComponent(current, workInProgress, renderLanes) {
               prevState,
               renderLanes,
             );
+          } else if (workInProgress.flags & ForceClientRender) {
+            // Something errored during hydration. Try again without hydrating.
+            workInProgress.flags &= ~ForceClientRender;
+            return retrySuspenseComponentWithoutHydrating(
+              current,
+              workInProgress,
+              renderLanes,
+            );
           } else if (
             (workInProgress.memoizedState: null | SuspenseState) !== null
           ) {

packages/react-reconciler/src/ReactFiberFlags.js

@@ -12,53 +12,54 @@ import {enableCreateEventHandleAPI} from 'shared/ReactFeatureFlags';
 export type Flags = number;
 
 // Don't change these two values. They're used by React Dev Tools.
-export const NoFlags = /*                      */ 0b000000000000000000000000;
-export const PerformedWork = /*                */ 0b000000000000000000000001;
+export const NoFlags = /*                      */ 0b0000000000000000000000000;
+export const PerformedWork = /*                */ 0b0000000000000000000000001;
 
 // You can change the rest (and add more).
-export const Placement = /*                    */ 0b000000000000000000000010;
-export const Update = /*                       */ 0b000000000000000000000100;
+export const Placement = /*                    */ 0b0000000000000000000000010;
+export const Update = /*                       */ 0b0000000000000000000000100;
 export const PlacementAndUpdate = /*           */ Placement | Update;
-export const Deletion = /*                     */ 0b000000000000000000001000;
-export const ChildDeletion = /*                */ 0b000000000000000000010000;
-export const ContentReset = /*                 */ 0b000000000000000000100000;
-export const Callback = /*                     */ 0b000000000000000001000000;
-export const DidCapture = /*                   */ 0b000000000000000010000000;
-export const Ref = /*                          */ 0b000000000000000100000000;
-export const Snapshot = /*                     */ 0b000000000000001000000000;
-export const Passive = /*                      */ 0b000000000000010000000000;
-export const Hydrating = /*                    */ 0b000000000000100000000000;
+export const Deletion = /*                     */ 0b0000000000000000000001000;
+export const ChildDeletion = /*                */ 0b0000000000000000000010000;
+export const ContentReset = /*                 */ 0b0000000000000000000100000;
+export const Callback = /*                     */ 0b0000000000000000001000000;
+export const DidCapture = /*                   */ 0b0000000000000000010000000;
+export const ForceClientRender = /*            */ 0b0000000000000000100000000;
+export const Ref = /*                          */ 0b0000000000000001000000000;
+export const Snapshot = /*                     */ 0b0000000000000010000000000;
+export const Passive = /*                      */ 0b0000000000000100000000000;
+export const Hydrating = /*                    */ 0b0000000000001000000000000;
 export const HydratingAndUpdate = /*           */ Hydrating | Update;
-export const Visibility = /*                   */ 0b000000000001000000000000;
-export const StoreConsistency = /*             */ 0b000000000010000000000000;
+export const Visibility = /*                   */ 0b0000000000010000000000000;
+export const StoreConsistency = /*             */ 0b0000000000100000000000000;
 
 export const LifecycleEffectMask =
   Passive | Update | Callback | Ref | Snapshot | StoreConsistency;
 
 // Union of all commit flags (flags with the lifetime of a particular commit)
-export const HostEffectMask = /*               */ 0b000000000011111111111111;
+export const HostEffectMask = /*               */ 0b0000000000111111111111111;
 
 // These are not really side effects, but we still reuse this field.
-export const Incomplete = /*                   */ 0b000000000100000000000000;
-export const ShouldCapture = /*                */ 0b000000001000000000000000;
-export const ForceUpdateForLegacySuspense = /* */ 0b000000010000000000000000;
-export const DidPropagateContext = /*          */ 0b000000100000000000000000;
-export const NeedsPropagation = /*             */ 0b000001000000000000000000;
+export const Incomplete = /*                   */ 0b0000000001000000000000000;
+export const ShouldCapture = /*                */ 0b0000000010000000000000000;
+export const ForceUpdateForLegacySuspense = /* */ 0b0000000100000000000000000;
+export const DidPropagateContext = /*          */ 0b0000001000000000000000000;
+export const NeedsPropagation = /*             */ 0b0000010000000000000000000;
 
 // Static tags describe aspects of a fiber that are not specific to a render,
 // e.g. a fiber uses a passive effect (even if there are no updates on this particular render).
 // This enables us to defer more work in the unmount case,
 // since we can defer traversing the tree during layout to look for Passive effects,
 // and instead rely on the static flag as a signal that there may be cleanup work.
-export const RefStatic = /*                    */ 0b000010000000000000000000;
-export const LayoutStatic = /*                 */ 0b000100000000000000000000;
-export const PassiveStatic = /*                */ 0b001000000000000000000000;
+export const RefStatic = /*                    */ 0b0000100000000000000000000;
+export const LayoutStatic = /*                 */ 0b0001000000000000000000000;
+export const PassiveStatic = /*                */ 0b0010000000000000000000000;
 
 // These flags allow us to traverse to fibers that have effects on mount
 // without traversing the entire tree after every commit for
 // double invoking
-export const MountLayoutDev = /*               */ 0b010000000000000000000000;
-export const MountPassiveDev = /*              */ 0b100000000000000000000000;
+export const MountLayoutDev = /*               */ 0b0100000000000000000000000;
+export const MountPassiveDev = /*              */ 0b1000000000000000000000000;
 
 // Groups of flags that are used in the commit phase to skip over trees that
 // don't contain effects, by checking subtreeFlags.