Move lazy ID initialization to the core implementation

We never need an ID before we write a pending boundary. This also ensures
that ID generation is deterministic by moving it to the write phase.

packages/react-dom/src/server/ReactDOMServerFormatConfig.js

@@ -168,14 +168,17 @@ export function getChildFormatContext(
   return parentContext;
 }
 
-export type SuspenseBoundaryID = {
-  formattedID: null | PrecomputedChunk,
-};
+export type SuspenseBoundaryID = null | PrecomputedChunk;
+
+export const UNINITIALIZED_SUSPENSE_BOUNDARY_ID: SuspenseBoundaryID = null;
 
-export function createSuspenseBoundaryID(
+export function assignSuspenseBoundaryID(
   responseState: ResponseState,
 ): SuspenseBoundaryID {
-  return {formattedID: null};
+  const generatedID = responseState.nextSuspenseID++;
+  return stringToPrecomputedChunk(
+    responseState.boundaryPrefix + generatedID.toString(16),
+  );
 }
 
 export type OpaqueIDType = string;
@@ -198,16 +201,6 @@ function encodeHTMLTextNode(text: string): string {
   return escapeTextForBrowser(text);
 }
 
-function assignAnID(
-  responseState: ResponseState,
-  id: SuspenseBoundaryID,
-): PrecomputedChunk {
-  const generatedID = responseState.nextSuspenseID++;
-  return (id.formattedID = stringToPrecomputedChunk(
-    responseState.boundaryPrefix + generatedID.toString(16),
-  ));
-}
-
 const textSeparator = stringToPrecomputedChunk('<!-- -->');
 
 export function pushTextInstance(
@@ -1377,8 +1370,11 @@ export function writeStartPendingSuspenseBoundary(
   id: SuspenseBoundaryID,
 ): boolean {
   writeChunk(destination, startPendingSuspenseBoundary1);
-  const formattedID = assignAnID(responseState, id);
-  writeChunk(destination, formattedID);
+  invariant(
+    id !== null,
+    'An ID must have been assigned before we can complete the boundary.',
+  );
+  writeChunk(destination, id);
   return writeChunk(destination, startPendingSuspenseBoundary2);
 }
 export function writeStartClientRenderedSuspenseBoundary(
@@ -1708,13 +1704,12 @@ export function writeCompletedBoundaryInstruction(
     // Future calls can just reuse the same function.
     writeChunk(destination, completeBoundaryScript1Partial);
   }
-  const formattedBoundaryID = boundaryID.formattedID;
   invariant(
-    formattedBoundaryID !== null,
+    boundaryID !== null,
     'An ID must have been assigned before we can complete the boundary.',
   );
   const formattedContentID = stringToChunk(contentSegmentID.toString(16));
-  writeChunk(destination, formattedBoundaryID);
+  writeChunk(destination, boundaryID);
   writeChunk(destination, completeBoundaryScript2);
   writeChunk(destination, responseState.segmentPrefix);
   writeChunk(destination, formattedContentID);
@@ -1740,11 +1735,10 @@ export function writeClientRenderBoundaryInstruction(
     // Future calls can just reuse the same function.
     writeChunk(destination, clientRenderScript1Partial);
   }
-  const formattedBoundaryID = boundaryID.formattedID;
   invariant(
-    formattedBoundaryID !== null,
+    boundaryID !== null,
     'An ID must have been assigned before we can complete the boundary.',
   );
-  writeChunk(destination, formattedBoundaryID);
+  writeChunk(destination, boundaryID);
   return writeChunk(destination, clientRenderScript2);
 }

packages/react-dom/src/server/ReactDOMServerLegacyFormatConfig.js

@@ -78,7 +78,8 @@ export type {
 
 export {
   getChildFormatContext,
-  createSuspenseBoundaryID,
+  UNINITIALIZED_SUSPENSE_BOUNDARY_ID,
+  assignSuspenseBoundaryID,
   makeServerID,
   pushStartInstance,
   pushEndInstance,

packages/react-native-renderer/src/server/ReactNativeServerFormatConfig.js

@@ -103,10 +103,9 @@ export function getChildFormatContext(
 // This is very specific to DOM where we can't assign an ID to.
 export type SuspenseBoundaryID = number;
 
-export function createSuspenseBoundaryID(
+export function assignSuspenseBoundaryID(
   responseState: ResponseState,
 ): SuspenseBoundaryID {
-  // TODO: This is not deterministic since it's created during render.
   return responseState.nextSuspenseID++;
 }
 

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

@@ -81,7 +81,9 @@ const ReactNoopServer = ReactFizzServer({
   closeWithError(destination: Destination, error: mixed): void {},
   flushBuffered(destination: Destination): void {},
 
-  createSuspenseBoundaryID(): SuspenseInstance {
+  UNINITIALIZED_SUSPENSE_BOUNDARY_ID: null,
+
+  assignSuspenseBoundaryID(): SuspenseInstance {
     // The ID is a pointer to the boundary itself.
     return {state: 'pending', children: []};
   },

packages/react-server/src/ReactFizzServer.js

@@ -53,7 +53,8 @@ import {
   pushEndInstance,
   pushStartCompletedSuspenseBoundary,
   pushEndCompletedSuspenseBoundary,
-  createSuspenseBoundaryID,
+  UNINITIALIZED_SUSPENSE_BOUNDARY_ID,
+  assignSuspenseBoundaryID,
   getChildFormatContext,
 } from './ReactServerFormatConfig';
 import {
@@ -123,7 +124,7 @@ type LegacyContext = {
 };
 
 type SuspenseBoundary = {
-  +id: SuspenseBoundaryID,
+  id: SuspenseBoundaryID,
   rootSegmentID: number,
   forceClientRender: boolean, // if it errors or infinitely suspends
   parentFlushed: boolean,
@@ -281,7 +282,7 @@ function createSuspenseBoundary(
   fallbackAbortableTasks: Set<Task>,
 ): SuspenseBoundary {
   return {
-    id: createSuspenseBoundaryID(request.responseState),
+    id: UNINITIALIZED_SUSPENSE_BOUNDARY_ID,
     rootSegmentID: -1,
     parentFlushed: false,
     pendingTasks: 0,
@@ -1595,11 +1596,10 @@ function flushSegment(
       request.partialBoundaries.push(boundary);
     }
 
-    writeStartPendingSuspenseBoundary(
-      destination,
-      request.responseState,
-      boundary.id,
-    );
+    /// This is the first time we should have referenced this ID.
+    const id = (boundary.id = assignSuspenseBoundaryID(request.responseState));
+
+    writeStartPendingSuspenseBoundary(destination, request.responseState, id);
 
     // Flush the fallback.
     flushSubtree(request, destination, segment);

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

@@ -33,7 +33,9 @@ export opaque type OpaqueIDType = mixed;
 export const isPrimaryRenderer = false;
 
 export const getChildFormatContext = $$$hostConfig.getChildFormatContext;
-export const createSuspenseBoundaryID = $$$hostConfig.createSuspenseBoundaryID;
+export const UNINITIALIZED_SUSPENSE_BOUNDARY_ID =
+  $$$hostConfig.UNINITIALIZED_SUSPENSE_BOUNDARY_ID;
+export const assignSuspenseBoundaryID = $$$hostConfig.assignSuspenseBoundaryID;
 export const makeServerID = $$$hostConfig.makeServerID;
 export const pushTextInstance = $$$hostConfig.pushTextInstance;
 export const pushStartInstance = $$$hostConfig.pushStartInstance;