Flight support for Float

Supporting Float methods such as ReactDOM.preload() are challenging for flight because it does not have an easy means to convey direct executions in other environments. Because the flight wire format is a JSON-like serialization that is expected to be rendered it currently only describes renderable elements. We need a way to convey a function invocation that gets run in the context of the client environment whether that is Fizz or Fiber.

Fiber is somewhat straightforward because the HostDispatcher is always active and we can just have the FlightClient dispatch the serialized directive.

Fizz is much more challenging becaue the dispatcher is always scoped but the specific request the dispatch belongs to is not readily available. Environments that support AsyncLocalStorage (or in the future AsyncContext) we will use this to be able to resolve directives in Fizz to the appropriate Request. For other environments directives will be elided. Right now this is pragmatic and non-breaking because all directives are opportunistic and non-critical. If this changes in the future we will need to reconsider how widespread support for async context tracking is.

For Flight, if AsyncLocalStorage is available Float methods can be called before and after await points and be expected to work. If AsyncLocalStorage is not available float methods called in the sync phase of a component render will be captured but anything after an await point will be a noop. If a float call is dropped in this manner a DEV warning should help you realize your code may need to be modified.

Author: gnoff

packages/react-client/src/ReactFlightClient.js

@@ -18,11 +18,14 @@ import type {
   SSRManifest,
 } from './ReactFlightClientConfig';
 
+import type {HintModel} from 'react-server/src/ReactFlightServerConfig';
+
 import {
   resolveClientReference,
   preloadModule,
   requireModule,
   parseModel,
+  dispatchHint,
 } from './ReactFlightClientConfig';
 
 import {knownServerReferences} from './ReactFlightServerReferenceRegistry';
@@ -778,6 +781,15 @@ export function resolveErrorDev(
   }
 }
 
+export function resolveHint(
+  response: Response,
+  code: string,
+  model: UninitializedModel,
+): void {
+  const hintModel = parseModel<HintModel>(response, model);
+  dispatchHint(code, hintModel);
+}
+
 export function close(response: Response): void {
   // In case there are any remaining unresolved chunks, they won't
   // be resolved now. So we need to issue an error to those.

packages/react-client/src/ReactFlightClientStream.js

@@ -16,6 +16,7 @@ import {
   resolveModel,
   resolveErrorProd,
   resolveErrorDev,
+  resolveHint,
   createResponse as createResponseBase,
   parseModelString,
   parseModelTuple,
@@ -46,6 +47,11 @@ function processFullRow(response: Response, row: string): void {
       resolveModule(response, id, row.slice(colon + 2));
       return;
     }
+    case 'H': {
+      const code = row[colon + 2];
+      resolveHint(response, code, row.slice(colon + 3));
+      return;
+    }
     case 'E': {
       const errorInfo = JSON.parse(row.slice(colon + 2));
       if (__DEV__) {

packages/react-client/src/forks/ReactFlightClientConfig.custom.js

@@ -35,6 +35,7 @@ export const resolveClientReference = $$$config.resolveClientReference;
 export const resolveServerReference = $$$config.resolveServerReference;
 export const preloadModule = $$$config.preloadModule;
 export const requireModule = $$$config.requireModule;
+export const dispatchHint = $$$config.dispatchHint;
 
 export opaque type Source = mixed;
 

packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js

@@ -7,6 +7,7 @@
  * @flow
  */
 
+import type {HostDispatcher} from 'react-dom/src/ReactDOMDispatcher';
 import type {EventPriority} from 'react-reconciler/src/ReactEventPriorities';
 import type {DOMEventName} from '../events/DOMEventNames';
 import type {Fiber, FiberRoot} from 'react-reconciler/src/ReactInternalTypes';
@@ -1868,10 +1869,6 @@ export function clearSingleton(instance: Instance): void {
 
 export const supportsResources = true;
 
-// The resource types we support. currently they match the form for the as argument.
-// In the future this may need to change, especially when modules / scripts are supported
-type ResourceType = 'style' | 'font' | 'script';
-
 type HoistableTagType = 'link' | 'meta' | 'title';
 type TResource<
   T: 'stylesheet' | 'style' | 'script' | 'void',
@@ -1962,7 +1959,7 @@ function getDocumentFromRoot(root: HoistableRoot): Document {
 // We want this to be the default dispatcher on ReactDOMSharedInternals but we don't want to mutate
 // internals in Module scope. Instead we export it and Internals will import it. There is already a cycle
 // from Internals -> ReactDOM -> HostConfig -> Internals so this doesn't introduce a new one.
-export const ReactDOMClientDispatcher = {
+export const ReactDOMClientDispatcher: HostDispatcher = {
   prefetchDNS,
   preconnect,
   preload,
@@ -2036,7 +2033,10 @@ function prefetchDNS(href: string, options?: mixed) {
   preconnectAs('dns-prefetch', null, href);
 }
 
-function preconnect(href: string, options?: {crossOrigin?: string}) {
+function preconnect(href: string, options: ?{crossOrigin?: string}) {
+  if (!enableFloat) {
+    return;
+  }
   if (__DEV__) {
     if (typeof href !== 'string' || !href) {
       console.error(
@@ -2064,9 +2064,8 @@ function preconnect(href: string, options?: {crossOrigin?: string}) {
   preconnectAs('preconnect', crossOrigin, href);
 }
 
-type PreloadAs = ResourceType;
 type PreloadOptions = {
-  as: PreloadAs,
+  as: string,
   crossOrigin?: string,
   integrity?: string,
   type?: string,
@@ -2115,7 +2114,7 @@ function preload(href: string, options: PreloadOptions) {
 
 function preloadPropsFromPreloadOptions(
   href: string,
-  as: ResourceType,
+  as: string,
   options: PreloadOptions,
 ): PreloadProps {
   return {
@@ -2128,9 +2127,8 @@ function preloadPropsFromPreloadOptions(
   };
 }
 
-type PreinitAs = 'style' | 'script';
 type PreinitOptions = {
-  as: PreinitAs,
+  as: string,
   precedence?: string,
   crossOrigin?: string,
   integrity?: string,

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

@@ -0,0 +1,70 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import type {
+  HostDispatcher,
+  PrefetchDNSOptions,
+  PreconnectOptions,
+  PreloadOptions,
+  PreinitOptions,
+} from 'react-dom/src/ReactDOMDispatcher';
+
+import {enableFloat} from 'shared/ReactFeatureFlags';
+
+import {emitHint} from 'react-server/src/ReactFlightServer';
+
+export const ReactDOMFlightServerDispatcher: HostDispatcher = {
+  prefetchDNS,
+  preconnect,
+  preload,
+  preinit,
+};
+
+function prefetchDNS(href: string, options?: ?PrefetchDNSOptions) {
+  if (enableFloat) {
+    if (typeof href === 'string') {
+      // We want to pass through options for validation purposes.
+      // We could __DEV__ gate this but currently consider keeping the serialization
+      // identical in prod safer
+      if (options) {
+        emitHint('D', [href, options]);
+      } else {
+        emitHint('D', href);
+      }
+    }
+  }
+}
+
+function preconnect(href: string, options: ?PreconnectOptions) {
+  if (enableFloat) {
+    if (typeof href === 'string') {
+      if (options) {
+        emitHint('C', [href, options]);
+      } else {
+        emitHint('C', href);
+      }
+    }
+  }
+}
+
+function preload(href: string, options: PreloadOptions) {
+  if (enableFloat) {
+    if (typeof href === 'string') {
+      emitHint('L', [href, options]);
+    }
+  }
+}
+
+function preinit(href: string, options: PreinitOptions): void {
+  if (enableFloat) {
+    if (typeof href === 'string') {
+      emitHint('I', [href, options]);
+    }
+  }
+}

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

@@ -21,9 +21,6 @@ export function scheduleWork(callback: () => void) {
 
 export function flushBuffered(destination: Destination) {}
 
-export const supportsRequestStorage = false;
-export const requestStorage: AsyncLocalStorage<any> = (null: any);
-
 export function beginWriting(destination: Destination) {}
 
 export function writeChunk(

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

@@ -38,6 +38,7 @@ import {
   stringToPrecomputedChunk,
   clonePrecomputedChunk,
 } from 'react-server/src/ReactServerStreamConfig';
+import {resolveResources, pingRequest} from 'react-server/src/ReactFizzServer';
 
 import isAttributeNameSafe from '../shared/isAttributeNameSafe';
 import isUnitlessNumber from '../shared/isUnitlessNumber';
@@ -79,30 +80,15 @@ import {
 import ReactDOMSharedInternals from 'shared/ReactDOMSharedInternals';
 const ReactDOMCurrentDispatcher = ReactDOMSharedInternals.Dispatcher;
 
-const ReactDOMServerDispatcher = enableFloat
-  ? {
-      prefetchDNS,
-      preconnect,
-      preload,
-      preinit,
-    }
-  : {};
-
-let currentResources: null | Resources = null;
-const currentResourcesStack = [];
-
-export function prepareToRender(resources: Resources): mixed {
-  currentResourcesStack.push(currentResources);
-  currentResources = resources;
+const ReactDOMServerDispatcher = {
+  prefetchDNS,
+  preconnect,
+  preload,
+  preinit,
+};
 
-  const previousHostDispatcher = ReactDOMCurrentDispatcher.current;
+export function prepareHostDispatcher() {
   ReactDOMCurrentDispatcher.current = ReactDOMServerDispatcher;
-  return previousHostDispatcher;
-}
-
-export function cleanupAfterRender(previousDispatcher: mixed) {
-  currentResources = currentResourcesStack.pop();
-  ReactDOMCurrentDispatcher.current = previousDispatcher;
 }
 
 // Used to distinguish these contexts from ones used in other renderers.
@@ -4804,16 +4790,18 @@ function getResourceKey(as: string, href: string): string {
 }
 
 export function prefetchDNS(href: string, options?: mixed) {
-  if (!currentResources) {
-    // While we expect that preconnect calls are primarily going to be observed
-    // during render because effects and events don't run on the server it is
-    // still possible that these get called in module scope. This is valid on
-    // the client since there is still a document to interact with but on the
-    // server we need a request to associate the call to. Because of this we
-    // simply return and do not warn.
+  if (!enableFloat) {
+    return;
+  }
+  const resources = resolveResources();
+  if (!resources) {
+    // In async contexts we can sometimes resolve resources from AsyncLocalStorage. If we can't we can also
+    // possibly get them from the stack if we are not in an async context. Since we were not able to resolve
+    // the resources for this call in either case we opt to do nothing. We can consider making this a warning
+    // but there may be times where calling a function outside of render is intentional (i.e. to warm up data
+    // fetching) and we don't want to warn in those cases.
     return;
   }
-  const resources = currentResources;
   if (__DEV__) {
     if (typeof href !== 'string' || !href) {
       console.error(
@@ -4858,17 +4846,19 @@ export function prefetchDNS(href: string, options?: mixed) {
   }
 }
 
-export function preconnect(href: string, options?: {crossOrigin?: string}) {
-  if (!currentResources) {
-    // While we expect that preconnect calls are primarily going to be observed
-    // during render because effects and events don't run on the server it is
-    // still possible that these get called in module scope. This is valid on
-    // the client since there is still a document to interact with but on the
-    // server we need a request to associate the call to. Because of this we
-    // simply return and do not warn.
+export function preconnect(href: string, options?: ?{crossOrigin?: string}) {
+  if (!enableFloat) {
+    return;
+  }
+  const resources = resolveResources();
+  if (!resources) {
+    // In async contexts we can sometimes resolve resources from AsyncLocalStorage. If we can't we can also
+    // possibly get them from the stack if we are not in an async context. Since we were not able to resolve
+    // the resources for this call in either case we opt to do nothing. We can consider making this a warning
+    // but there may be times where calling a function outside of render is intentional (i.e. to warm up data
+    // fetching) and we don't want to warn in those cases.
     return;
   }
-  const resources = currentResources;
   if (__DEV__) {
     if (typeof href !== 'string' || !href) {
       console.error(
@@ -4917,24 +4907,25 @@ export function preconnect(href: string, options?: {crossOrigin?: string}) {
   }
 }
 
-type PreloadAs = 'style' | 'font' | 'script';
 type PreloadOptions = {
-  as: PreloadAs,
+  as: string,
   crossOrigin?: string,
   integrity?: string,
   type?: string,
 };
 export function preload(href: string, options: PreloadOptions) {
-  if (!currentResources) {
-    // While we expect that preload calls are primarily going to be observed
-    // during render because effects and events don't run on the server it is
-    // still possible that these get called in module scope. This is valid on
-    // the client since there is still a document to interact with but on the
-    // server we need a request to associate the call to. Because of this we
-    // simply return and do not warn.
+  if (!enableFloat) {
+    return;
+  }
+  const resources = resolveResources();
+  if (!resources) {
+    // In async contexts we can sometimes resolve resources from AsyncLocalStorage. If we can't we can also
+    // possibly get them from the stack if we are not in an async context. Since we were not able to resolve
+    // the resources for this call in either case we opt to do nothing. We can consider making this a warning
+    // but there may be times where calling a function outside of render is intentional (i.e. to warm up data
+    // fetching) and we don't want to warn in those cases.
     return;
   }
-  const resources = currentResources;
   if (__DEV__) {
     if (typeof href !== 'string' || !href) {
       console.error(
@@ -5055,27 +5046,30 @@ export function preload(href: string, options: PreloadOptions) {
         resources.explicitOtherPreloads.add(resource);
       }
     }
+    pingRequest();
   }
 }
 
-type PreinitAs = 'style' | 'script';
 type PreinitOptions = {
-  as: PreinitAs,
+  as: string,
   precedence?: string,
   crossOrigin?: string,
   integrity?: string,
 };
 export function preinit(href: string, options: PreinitOptions): void {
-  if (!currentResources) {
-    // While we expect that preinit calls are primarily going to be observed
-    // during render because effects and events don't run on the server it is
-    // still possible that these get called in module scope. This is valid on
-    // the client since there is still a document to interact with but on the
-    // server we need a request to associate the call to. Because of this we
-    // simply return and do not warn.
+  if (!enableFloat) {
+    return;
+  }
+  const resources = resolveResources();
+  if (!resources) {
+    // In async contexts we can sometimes resolve resources from AsyncLocalStorage. If we can't we can also
+    // possibly get them from the stack if we are not in an async context. Since we were not able to resolve
+    // the resources for this call in either case we opt to do nothing. We can consider making this a warning
+    // but there may be times where calling a function outside of render is intentional (i.e. to warm up data
+    // fetching) and we don't want to warn in those cases.
     return;
   }
-  preinitImpl(currentResources, href, options);
+  preinitImpl(resources, href, options);
 }
 
 // On the server, preinit may be called outside of render when sending an
@@ -5297,7 +5291,7 @@ function preinitImpl(
 
 function preloadPropsFromPreloadOptions(
   href: string,
-  as: PreloadAs,
+  as: string,
   options: PreloadOptions,
 ): PreloadProps {
   return {