[Flight Parcel] Pass import maps through client references (#32132)

Corresponding Parcel PR:
parcel-bundler/parcel#10073

Parcel avoids [cascading cache
invalidation](https://philipwalton.com/articles/cascading-cache-invalidation/)
by injecting a bundle manifest containing a mapping of stable bundle ids
to hashed URLs. When using an HTML entry point, this is done (as of the
above PR) via a native import map. This means that if a bundle's hash
changes, only that bundle will be invalidated (plus the HTML itself
which typically has a short caching policy), not any other bundles that
reference it.

For RSCs, we cannot currently use native import maps because of client
side navigations, where a new HTML file is not requested. Eventually,
multiple `<script type="importmap">` elements will be supported
(whatwg/html#10528) ([coming Chrome
133](https://chromestatus.com/feature/5121916248260608)), at which point
React could potentially inject them. In the meantime, I've added some
APIs to Parcel to polyfill this. With this change, an import map can be
sent along with a client reference, containing a mapping for any dynamic
imports and URL dependencies (e.g. images) that are referenced by the JS
bundles. On the client, the import map is extended with these new
mappings prior to executing the referenced bundles. This preserves the
caching advantages described above while supporting client navigations.

packages/react-server-dom-parcel/src/ReactFlightParcelReferences.js

@@ -22,6 +22,7 @@ export type ClientReference<T> = {
   $$id: string,
   $$name: string,
   $$bundles: Array<string>,
+  $$importMap?: ?{[string]: string},
 };
 
 const CLIENT_REFERENCE_TAG = Symbol.for('react.client.reference');
@@ -39,12 +40,14 @@ export function createClientReference<T>(
   id: string,
   exportName: string,
   bundles: Array<string>,
+  importMap?: ?{[string]: string},
 ): ClientReference<T> {
   return {
     $$typeof: CLIENT_REFERENCE_TAG,
     $$id: id,
     $$name: exportName,
     $$bundles: bundles,
+    $$importMap: importMap,
   };
 }
 

packages/react-server-dom-parcel/src/client/ReactFlightClientConfigBundlerParcel.js

@@ -11,7 +11,12 @@ import type {Thenable} from 'shared/ReactTypes';
 
 import type {ImportMetadata} from '../shared/ReactFlightImportMetadata';
 
-import {ID, NAME, BUNDLES} from '../shared/ReactFlightImportMetadata';
+import {
+  ID,
+  NAME,
+  BUNDLES,
+  IMPORT_MAP,
+} from '../shared/ReactFlightImportMetadata';
 import {prepareDestinationWithChunks} from 'react-client/src/ReactFlightClientConfig';
 
 export type ServerManifest = {
@@ -60,9 +65,14 @@ export function resolveServerReference<T>(
 export function preloadModule<T>(
   metadata: ClientReference<T>,
 ): null | Thenable<any> {
+  if (metadata[IMPORT_MAP]) {
+    parcelRequire.extendImportMap(metadata[IMPORT_MAP]);
+  }
+
   if (metadata[BUNDLES].length === 0) {
     return null;
   }
+
   return Promise.all(metadata[BUNDLES].map(url => parcelRequire.load(url)));
 }
 

packages/react-server-dom-parcel/src/server/ReactFlightServerConfigParcelBundler.js

@@ -37,6 +37,15 @@ export function resolveClientReferenceMetadata<T>(
   config: ClientManifest,
   clientReference: ClientReference<T>,
 ): ClientReferenceMetadata {
+  if (clientReference.$$importMap) {
+    return [
+      clientReference.$$id,
+      clientReference.$$name,
+      clientReference.$$bundles,
+      clientReference.$$importMap,
+    ];
+  }
+
   return [
     clientReference.$$id,
     clientReference.$$name,

packages/react-server-dom-parcel/src/shared/ReactFlightImportMetadata.js

@@ -10,11 +10,16 @@
 // This is the parsed shape of the wire format which is why it is
 // condensed to only the essentialy information
 export type ImportMetadata = [
-  /* id */ string,
-  /* name */ string,
-  /* bundles */ Array<string>,
+  // eslint does not understand Flow tuple syntax.
+  /* eslint-disable */
+  id: string,
+  name: string,
+  bundles: Array<string>,
+  importMap?: {[string]: string},
+  /* eslint-enable */
 ];
 
 export const ID = 0;
 export const NAME = 1;
 export const BUNDLES = 2;
+export const IMPORT_MAP = 3;

scripts/flow/environment.js

@@ -106,6 +106,7 @@ declare const __turbopack_require__: ((id: string) => any) & {
 declare var parcelRequire: {
   (id: string): any,
   load: (url: string) => Promise<mixed>,
+  extendImportMap: (importMap: {[string]: string}) => void,
   meta: {
     publicUrl: string,
   },