Emit Activity boundaries as comments in Fizz

packages/react-dom-bindings/src/server/ReactFizzConfigDOM.js

@@ -4087,6 +4087,28 @@ export function writePlaceholder(
   return writeChunkAndReturn(destination, placeholder2);
 }
 
+// Activity boundaries are encoded as comments.
+const startActivityBoundary = stringToPrecomputedChunk('<!--&-->');
+const endActivityBoundary = stringToPrecomputedChunk('<!--/&-->');
+
+export function pushStartActivityBoundary(
+  target: Array<Chunk | PrecomputedChunk>,
+  renderState: RenderState,
+): void {
+  target.push(startActivityBoundary);
+}
+
+export function pushEndActivityBoundary(
+  target: Array<Chunk | PrecomputedChunk>,
+  renderState: RenderState,
+  preambleState: null | PreambleState,
+): void {
+  if (preambleState) {
+    pushPreambleContribution(target, preambleState);
+  }
+  target.push(endActivityBoundary);
+}
+
 // Suspense boundaries are encoded as comments.
 const startCompletedSuspenseBoundary = stringToPrecomputedChunk('<!--$-->');
 const startPendingSuspenseBoundary1 = stringToPrecomputedChunk(
@@ -4225,6 +4247,23 @@ export function writeEndClientRenderedSuspenseBoundary(
 const boundaryPreambleContributionChunkStart = stringToPrecomputedChunk('<!--');
 const boundaryPreambleContributionChunkEnd = stringToPrecomputedChunk('-->');
 
+function pushPreambleContribution(
+  target: Array<Chunk | PrecomputedChunk>,
+  preambleState: PreambleState,
+) {
+  // Same as writePreambleContribution but for the render phase.
+  const contribution = preambleState.contribution;
+  if (contribution !== NoContribution) {
+    target.push(
+      boundaryPreambleContributionChunkStart,
+      // This is a number type so we can do the fast path without coercion checking
+      // eslint-disable-next-line react-internal/safe-string-coercion
+      stringToChunk('' + contribution),
+      boundaryPreambleContributionChunkEnd,
+    );
+  }
+}
+
 function writePreambleContribution(
   destination: Destination,
   preambleState: PreambleState,

packages/react-dom-bindings/src/server/ReactFizzConfigDOMLegacy.js

@@ -20,6 +20,8 @@ import {
   createRenderState as createRenderStateImpl,
   pushTextInstance as pushTextInstanceImpl,
   pushSegmentFinale as pushSegmentFinaleImpl,
+  pushStartActivityBoundary as pushStartActivityBoundaryImpl,
+  pushEndActivityBoundary as pushEndActivityBoundaryImpl,
   writeStartCompletedSuspenseBoundary as writeStartCompletedSuspenseBoundaryImpl,
   writeStartClientRenderedSuspenseBoundary as writeStartClientRenderedSuspenseBoundaryImpl,
   writeEndCompletedSuspenseBoundary as writeEndCompletedSuspenseBoundaryImpl,
@@ -207,6 +209,29 @@ export function pushSegmentFinale(
   }
 }
 
+export function pushStartActivityBoundary(
+  target: Array<Chunk | PrecomputedChunk>,
+  renderState: RenderState,
+): void {
+  if (renderState.generateStaticMarkup) {
+    // A completed boundary is done and doesn't need a representation in the HTML
+    // if we're not going to be hydrating it.
+    return;
+  }
+  pushStartActivityBoundaryImpl(target, renderState);
+}
+
+export function pushEndActivityBoundary(
+  target: Array<Chunk | PrecomputedChunk>,
+  renderState: RenderState,
+  preambleState: null | PreambleState,
+): void {
+  if (renderState.generateStaticMarkup) {
+    return;
+  }
+  pushEndActivityBoundaryImpl(target, renderState, preambleState);
+}
+
 export function writeStartCompletedSuspenseBoundary(
   destination: Destination,
   renderState: RenderState,

packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js

@@ -3669,7 +3669,7 @@ describe('ReactDOMServerPartialHydration', () => {
   });
 
   // @gate enableActivity
-  it('a visible Activity component acts like a fragment', async () => {
+  it('a visible Activity component is surrounded by comment markers', async () => {
     const ref = React.createRef();
 
     function App() {
@@ -3690,9 +3690,11 @@ describe('ReactDOMServerPartialHydration', () => {
     // pure indirection.
     expect(container).toMatchInlineSnapshot(`
       <div>
+        <!--&-->
         <span>
           Child
         </span>
+        <!--/&-->
       </div>
     `);
 
@@ -3739,6 +3741,8 @@ describe('ReactDOMServerPartialHydration', () => {
         <span>
           Visible
         </span>
+        <!--&-->
+        <!--/&-->
       </div>
     `);
 
@@ -3760,6 +3764,8 @@ describe('ReactDOMServerPartialHydration', () => {
         <span>
           Visible
         </span>
+        <!--&-->
+        <!--/&-->
         <span
           style="display: none;"
         >

packages/react-markup/src/ReactFizzConfigMarkup.js

@@ -151,13 +151,31 @@ export function pushSegmentFinale(
   return;
 }
 
+export function pushStartActivityBoundary(
+  target: Array<Chunk | PrecomputedChunk>,
+  renderState: RenderState,
+): void {
+  // Markup doesn't have any instructions.
+  return;
+}
+
+export function pushEndActivityBoundary(
+  target: Array<Chunk | PrecomputedChunk>,
+  renderState: RenderState,
+  preambleState: null | PreambleState,
+): void {
+  // Markup doesn't have any instructions.
+  return;
+}
+
 export function writeStartCompletedSuspenseBoundary(
   destination: Destination,
   renderState: RenderState,
 ): boolean {
   // Markup doesn't have any instructions.
   return true;
 }
+
 export function writeStartClientRenderedSuspenseBoundary(
   destination: Destination,
   renderState: RenderState,

packages/react-noop-renderer/src/ReactNoopServer.js

@@ -30,6 +30,10 @@ type TextInstance = {
   hidden: boolean,
 };
 
+type ActivityInstance = {
+  children: Array<Instance | TextInstance | SuspenseInstance>,
+};
+
 type SuspenseInstance = {
   state: 'pending' | 'complete' | 'client-render',
   children: Array<Instance | TextInstance | SuspenseInstance>,
@@ -164,44 +168,74 @@ const ReactNoopServer = ReactFizzServer({
     });
   },
 
+  pushStartActivityBoundary(
+    target: Array<Uint8Array>,
+    renderState: RenderState,
+  ): void {
+    const activityInstance: ActivityInstance = {
+      children: [],
+    };
+    target.push(Buffer.from(JSON.stringify(activityInstance), 'utf8'));
+  },
+
+  pushEndActivityBoundary(
+    target: Array<Uint8Array>,
+    renderState: RenderState,
+    preambleState: null | PreambleState,
+  ): void {
+    target.push(POP);
+  },
+
   writeStartCompletedSuspenseBoundary(
     destination: Destination,
     renderState: RenderState,
-    suspenseInstance: SuspenseInstance,
   ): boolean {
-    suspenseInstance.state = 'complete';
+    const suspenseInstance: SuspenseInstance = {
+      state: 'complete',
+      children: [],
+    };
     const parent = destination.stack[destination.stack.length - 1];
     parent.children.push(suspenseInstance);
     destination.stack.push(suspenseInstance);
+    return true;
   },
   writeStartPendingSuspenseBoundary(
     destination: Destination,
     renderState: RenderState,
-    suspenseInstance: SuspenseInstance,
   ): boolean {
-    suspenseInstance.state = 'pending';
+    const suspenseInstance: SuspenseInstance = {
+      state: 'pending',
+      children: [],
+    };
     const parent = destination.stack[destination.stack.length - 1];
     parent.children.push(suspenseInstance);
     destination.stack.push(suspenseInstance);
+    return true;
   },
   writeStartClientRenderedSuspenseBoundary(
     destination: Destination,
     renderState: RenderState,
-    suspenseInstance: SuspenseInstance,
   ): boolean {
-    suspenseInstance.state = 'client-render';
+    const suspenseInstance: SuspenseInstance = {
+      state: 'client-render',
+      children: [],
+    };
     const parent = destination.stack[destination.stack.length - 1];
     parent.children.push(suspenseInstance);
     destination.stack.push(suspenseInstance);
+    return true;
   },
   writeEndCompletedSuspenseBoundary(destination: Destination): boolean {
     destination.stack.pop();
+    return true;
   },
   writeEndPendingSuspenseBoundary(destination: Destination): boolean {
     destination.stack.pop();
+    return true;
   },
   writeEndClientRenderedSuspenseBoundary(destination: Destination): boolean {
     destination.stack.pop();
+    return true;
   },
 
   writeStartSegment(
@@ -218,9 +252,11 @@ const ReactNoopServer = ReactFizzServer({
       throw new Error('Segments are only expected at the root of the stack.');
     }
     destination.stack.push(segment);
+    return true;
   },
   writeEndSegment(destination: Destination, formatContext: null): boolean {
     destination.stack.pop();
+    return true;
   },
 
   writeCompletedSegmentInstruction(
@@ -241,6 +277,7 @@ const ReactNoopServer = ReactFizzServer({
       0,
       ...segment.children,
     );
+    return true;
   },
 
   writeCompletedBoundaryInstruction(
@@ -255,6 +292,7 @@ const ReactNoopServer = ReactFizzServer({
     }
     boundary.children = segment.children;
     boundary.state = 'complete';
+    return true;
   },
 
   writeClientRenderBoundaryInstruction(
@@ -263,6 +301,7 @@ const ReactNoopServer = ReactFizzServer({
     boundary: SuspenseInstance,
   ): boolean {
     boundary.status = 'client-render';
+    return true;
   },
 
   writePreambleStart() {},

packages/react-server/src/ReactFizzServer.js

@@ -51,6 +51,8 @@ import {
 import {
   writeCompletedRoot,
   writePlaceholder,
+  pushStartActivityBoundary,
+  pushEndActivityBoundary,
   writeStartCompletedSuspenseBoundary,
   writeStartPendingSuspenseBoundary,
   writeStartClientRenderedSuspenseBoundary,
@@ -2200,23 +2202,50 @@ function renderLazyComponent(
   renderElement(request, task, keyPath, Component, resolvedProps, ref);
 }
 
-function renderOffscreen(
+function renderActivity(
   request: Request,
   task: Task,
   keyPath: KeyNode,
   props: Object,
 ): void {
-  const mode: ?OffscreenMode = (props.mode: any);
-  if (mode === 'hidden') {
-    // A hidden Offscreen boundary is not server rendered. Prerendering happens
-    // on the client.
+  const segment = task.blockedSegment;
+  if (segment === null) {
+    // Replay
+    const mode: ?OffscreenMode = (props.mode: any);
+    if (mode === 'hidden') {
+      // A hidden Activity boundary is not server rendered. Prerendering happens
+      // on the client.
+    } else {
+      // A visible Activity boundary has its children rendered inside the boundary.
+      const prevKeyPath = task.keyPath;
+      task.keyPath = keyPath;
+      renderNode(request, task, props.children, -1);
+      task.keyPath = prevKeyPath;
+    }
   } else {
-    // A visible Offscreen boundary is treated exactly like a fragment: a
-    // pure indirection.
-    const prevKeyPath = task.keyPath;
-    task.keyPath = keyPath;
-    renderNodeDestructive(request, task, props.children, -1);
-    task.keyPath = prevKeyPath;
+    // Render
+    // An Activity boundary is delimited so that we can hydrate it separately.
+    pushStartActivityBoundary(segment.chunks, request.renderState);
+    segment.lastPushedText = false;
+    const mode: ?OffscreenMode = (props.mode: any);
+    if (mode === 'hidden') {
+      // A hidden Activity boundary is not server rendered. Prerendering happens
+      // on the client.
+    } else {
+      // A visible Activity boundary has its children rendered inside the boundary.
+      const prevKeyPath = task.keyPath;
+      task.keyPath = keyPath;
+      // We use the non-destructive form because if something suspends, we still
+      // need to pop back up and finish the end comment.
+      renderNode(request, task, props.children, -1);
+      task.keyPath = prevKeyPath;
+    }
+    pushEndActivityBoundary(
+      segment.chunks,
+      request.renderState,
+      task.blockedPreamble,
+    );
+    segment.lastPushedText = false;
   }
 }
 
@@ -2291,7 +2320,7 @@ function renderElement(
       return;
     }
     case REACT_ACTIVITY_TYPE: {
-      renderOffscreen(request, task, keyPath, props);
+      renderActivity(request, task, keyPath, props);
       return;
     }
     case REACT_SUSPENSE_LIST_TYPE: {

packages/react-server/src/forks/ReactFizzConfig.custom.js

@@ -59,6 +59,8 @@ export const pushFormStateMarkerIsNotMatching =
   $$$config.pushFormStateMarkerIsNotMatching;
 export const writeCompletedRoot = $$$config.writeCompletedRoot;
 export const writePlaceholder = $$$config.writePlaceholder;
+export const pushStartActivityBoundary = $$$config.pushStartActivityBoundary;
+export const pushEndActivityBoundary = $$$config.pushEndActivityBoundary;
 export const writeStartCompletedSuspenseBoundary =
   $$$config.writeStartCompletedSuspenseBoundary;
 export const writeStartPendingSuspenseBoundary =