Repository: facebook/react
Pull request: [Fiber][Float] Error when a host fiber changes "flavor"

Host Components can exist as four semantic types

1. regular Components (Vanilla obv)
2. singleton Components
2. hoistable components
3. resources

Each of these component types have their own rules related to mounting and reconciliation however they are not direclty modeled as their own unique fiber type. This is partly for code size but also because reconciling the inner type of these components would be in a very hot path in fiber creation and reconciliation and it's just not practical to do this logic check here.

Right now we have three Fiber types used to implement these 4 concepts but we probably need to reconsider the model and think of Host Components as a single fiber type with an inner implementation. Once we do this we can regularize things like transitioning between a resource and a regular component or a singleton and a hoistable instance. The cases where these transitions happen today aren't particularly common but they can be observed and currently the handling of these transitions is incomplete at best and buggy at worst. The most egregious case is the link type. This can be a regular component (stylesheet without precedence) a hoistable component (non stylesheet link tags) or a resource (stylesheet with a precedence) and if you have a single jsx slot that tries to reconcile transitions between these types it just doesn't work well.

This commit adds an error for when a Hoistable goes from Instance to Resource. Currently this is only possible for `<link>` elements going to and from stylesheets with precedence. Hopefully we'll be able to remove this error and implement as an inner type before we encounter new categories for the Hoistable types

detecting type shifting to and from regular components is harder to do efficiently because we don't want to reevaluate the type on every update for host components which is currently not required and would add overhead to a very hot path

singletons can't really type shift in their one practical implementation (DOM) so they are only a problem in theroy not practice

Changed files:
filename: packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js
patch: @@ -2358,6 +2358,7 @@ export function getResource(
   type: string,
   currentProps: any,
   pendingProps: any,
+  currentResource: null | Resource,
 ): null | Resource {
   const resourceRoot = getCurrentResourceRoot();
   if (!resourceRoot) {
@@ -2430,9 +2431,44 @@ export function getResource(
             );
           }
         }
+        if (currentProps && currentResource === null) {
+          // This node was previously an Instance type and is becoming a Resource type
+          // For now we error because we don't support flavor changes
+          let diff = '';
+          if (__DEV__) {
+            diff = `
+
+  - ${describeLinkForResourceErrorDEV(currentProps)}
+  + ${describeLinkForResourceErrorDEV(pendingProps)}`;
+          }
+          throw new Error(
+            'Expected <link> not to update to be updated to a stylehsheet with precedence.' +
+              ' Check the `rel`, `href`, and `precedence` props of this component.' +
+              ' Alternatively, check whether two different <link> components render in the same slot or share the same key.' +
+              diff,
+          );
+        }
         return resource;
+      } else {
+        if (currentProps && currentResource !== null) {
+          // This node was previously a Resource type and is becoming an Instance type
+          // For now we error because we don't support flavor changes
+          let diff = '';
+          if (__DEV__) {
+            diff = `
+
+  - ${describeLinkForResourceErrorDEV(currentProps)}
+  + ${describeLinkForResourceErrorDEV(pendingProps)}`;
+          }
+          throw new Error(
+            'Expected stylesheet with precedence to not be updated to a different kind of <link>.' +
+              ' Check the `rel`, `href`, and `precedence` props of this component.' +
+              ' Alternatively, check whether two different <link> components render in the same slot or share the same key.' +
+              diff,
+          );
+        }
+        return null;
       }
-      return null;
     }
     case 'script': {
       const async = pendingProps.async;
@@ -2473,6 +2509,49 @@ export function getResource(
   }
 }
 
+function describeLinkForResourceErrorDEV(props: any) {
+  if (__DEV__) {
+    let describedProps = 0;
+
+    let description = '<link';
+    if (typeof props.rel === 'string') {
+      describedProps++;
+      description += ` rel="${props.rel}"`;
+    } else if (hasOwnProperty.call(props, 'rel')) {
+      describedProps++;
+      description += ` rel="${
+        props.rel === null ? 'null' : 'invalid type ' + typeof props.rel
+      }"`;
+    }
+    if (typeof props.href === 'string') {
+      describedProps++;
+      description += ` href="${props.href}"`;
+    } else if (hasOwnProperty.call(props, 'href')) {
+      describedProps++;
+      description += ` href="${
+        props.href === null ? 'null' : 'invalid type ' + typeof props.href
+      }"`;
+    }
+    if (typeof props.precedence === 'string') {
+      describedProps++;
+      description += ` precedence="${props.precedence}"`;
+    } else if (hasOwnProperty.call(props, 'precedence')) {
+      describedProps++;
+      description += ` precedence={${
+        props.precedence === null
+          ? 'null'
+          : 'invalid type ' + typeof props.precedence
+      }}`;
+    }
+    if (Object.getOwnPropertyNames(props).length > describedProps) {
+      description += ' ...';
+    }
+    description += ' />';
+    return description;
+  }
+  return '';
+}
+
 function styleTagPropsFromRawProps(
   rawProps: StyleTagQualifyingProps,
 ): StyleTagProps {

filename: packages/react-dom/src/__tests__/ReactDOMHostComponentTransitions-test.js
patch: @@ -0,0 +1,123 @@
+/**
+ * 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.
+ *
+ * @emails react-core
+ * @jest-environment ./scripts/jest/ReactDOMServerIntegrationEnvironment
+ */
+
+'use strict';
+
+let JSDOM;
+let React;
+let ReactDOMClient;
+let container;
+let waitForAll;
+
+describe('ReactDOM HostSingleton', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    JSDOM = require('jsdom').JSDOM;
+    // Test Environment
+    const jsdom = new JSDOM(
+      '<!DOCTYPE html><html><head></head><body><div id="container">',
+      {
+        runScripts: 'dangerously',
+      },
+    );
+    global.window = jsdom.window;
+    global.document = jsdom.window.document;
+    container = global.document.getElementById('container');
+
+    React = require('react');
+    ReactDOMClient = require('react-dom/client');
+
+    const InternalTestUtils = require('internal-test-utils');
+    waitForAll = InternalTestUtils.waitForAll;
+  });
+
+  it('errors when a hoistable component becomes a Resource', async () => {
+    const errors = [];
+    function onError(e) {
+      errors.push(e.message);
+    }
+    const root = ReactDOMClient.createRoot(container, {
+      onUncaughtError: onError,
+    });
+
+    root.render(
+      <div>
+        <link rel="preload" href="bar" as="style" />
+      </div>,
+    );
+    await waitForAll([]);
+
+    root.render(
+      <div>
+        <link rel="stylesheet" href="bar" precedence="default" />
+      </div>,
+    );
+    await waitForAll([]);
+    if (__DEV__) {
+      expect(errors).toEqual([
+        `Expected <link> not to update to be updated to a stylehsheet with precedence. Check the \`rel\`, \`href\`, and \`precedence\` props of this component. Alternatively, check whether two different <link> components render in the same slot or share the same key.
+
+  - <link rel=\"preload\" href=\"bar\" ... />
+  + <link rel=\"stylesheet\" href=\"bar\" precedence=\"default\" />`,
+      ]);
+    } else {
+      expect(errors).toEqual([
+        'Expected <link> not to update to be updated to a stylehsheet with precedence. Check the `rel`, `href`, and `precedence` props of this component. Alternatively, check whether two different <link> components render in the same slot or share the same key.',
+      ]);
+    }
+  });
+
+  it('errors when a hoistable Resource becomes an instance', async () => {
+    const errors = [];
+    function onError(e) {
+      errors.push(e.message);
+    }
+    const root = ReactDOMClient.createRoot(container, {
+      onUncaughtError: onError,
+    });
+
+    root.render(
+      <div>
+        <link rel="stylesheet" href="bar" precedence="default" />
+      </div>,
+    );
+    await waitForAll([]);
+    const event = new window.Event('load');
+    const preloads = document.querySelectorAll('link[rel="preload"]');
+    for (let i = 0; i < preloads.length; i++) {
+      const node = preloads[i];
+      node.dispatchEvent(event);
+    }
+    const stylesheets = document.querySelectorAll('link[rel="preload"]');
+    for (let i = 0; i < stylesheets.length; i++) {
+      const node = stylesheets[i];
+      node.dispatchEvent(event);
+    }
+
+    root.render(
+      <div>
+        <link rel="foo" href="bar" />
+      </div>,
+    );
+    await waitForAll([]);
+    if (__DEV__) {
+      expect(errors).toEqual([
+        `Expected stylesheet with precedence to not be updated to a different kind of <link>. Check the \`rel\`, \`href\`, and \`precedence\` props of this component. Alternatively, check whether two different <link> components render in the same slot or share the same key.
+
+  - <link rel=\"stylesheet\" href=\"bar\" precedence=\"default\" />
+  + <link rel=\"foo\" href=\"bar\" />`,
+      ]);
+    } else {
+      expect(errors).toEqual([
+        'Expected stylesheet with precedence to not be updated to a different kind of <link>. Check the `rel`, `href`, and `precedence` props of this component. Alternatively, check whether two different <link> components render in the same slot or share the same key.',
+      ]);
+    }
+  });
+});

filename: packages/react-reconciler/src/ReactFiberBeginWork.js
patch: @@ -1689,22 +1689,36 @@ function updateHostHoistable(
   renderLanes: Lanes,
 ) {
   markRef(current, workInProgress);
-  const currentProps = current === null ? null : current.memoizedProps;
-  const resource = (workInProgress.memoizedState = getResource(
-    workInProgress.type,
-    currentProps,
-    workInProgress.pendingProps,
-  ));
+
   if (current === null) {
-    if (!getIsHydrating() && resource === null) {
-      // This is not a Resource Hoistable and we aren't hydrating so we construct the instance.
-      workInProgress.stateNode = createHoistableInstance(
-        workInProgress.type,
-        workInProgress.pendingProps,
-        getRootHostContainer(),
-        workInProgress,
-      );
+    const resource = getResource(
+      workInProgress.type,
+      null,
+      workInProgress.pendingProps,
+      null,
+    );
+    if (resource) {
+      workInProgress.memoizedState = resource;
+    } else {
+      if (!getIsHydrating()) {
+        // This is not a Resource Hoistable and we aren't hydrating so we construct the instance.
+        workInProgress.stateNode = createHoistableInstance(
+          workInProgress.type,
+          workInProgress.pendingProps,
+          getRootHostContainer(),
+          workInProgress,
+        );
+      }
     }
+  } else {
+    // Get Resource may or may not return a resource. either way we stash the result
+    // on memoized state.
+    workInProgress.memoizedState = getResource(
+      workInProgress.type,
+      current.memoizedProps,
+      workInProgress.pendingProps,
+      current.memoizedState,
+    );
   }
 
   // Resources never have reconciler managed children. It is possible for

filename: packages/react-reconciler/src/ReactFiberCompleteWork.js
patch: @@ -1052,7 +1052,6 @@ function completeWork(
             return null;
           } else {
             // This is a Hoistable Instance
-
             // This must come at the very end of the complete phase.
             bubbleProperties(workInProgress);
             preloadInstanceAndSuspendIfNeeded(
@@ -1064,32 +1063,34 @@ function completeWork(
             return null;
           }
         } else {
-          // We are updating.
-          const currentResource = current.memoizedState;
-          if (nextResource !== currentResource) {
-            // We are transitioning to, from, or between Hoistable Resources
-            // and require an update
-            markUpdate(workInProgress);
-          }
-          if (nextResource !== null) {
-            // This is a Hoistable Resource
-            // This must come at the very end of the complete phase.
-
-            bubbleProperties(workInProgress);
-            if (nextResource === currentResource) {
-              workInProgress.flags &= ~MaySuspendCommit;
-            } else {
+          // This is an update.
+          if (nextResource) {
+            // This is a Resource
+            if (nextResource !== current.memoizedState) {
+              // we have a new Resource. we need to update
+              markUpdate(workInProgress);
+              // This must come at the very end of the complete phase.
+              bubbleProperties(workInProgress);
+              // This must come at the very end of the complete phase, because it might
+              // throw to suspend, and if the resource immediately loads, the work loop
+              // will resume rendering as if the work-in-progress completed. So it must
+              // fully complete.
               preloadResourceAndSuspendIfNeeded(
                 workInProgress,
                 nextResource,
                 type,
                 newProps,
                 renderLanes,
               );
+              return null;
+            } else {
+              // This must come at the very end of the complete phase.
+              bubbleProperties(workInProgress);
+              workInProgress.flags &= ~MaySuspendCommit;
+              return null;
             }
-            return null;
           } else {
-            // This is a Hoistable Instance
+            // This is an Instance
             // We may have props to update on the Hoistable instance.
             if (supportsMutation) {
               const oldProps = current.memoizedProps;
@@ -1107,7 +1108,6 @@ function completeWork(
                 renderLanes,
               );
             }
-
             // This must come at the very end of the complete phase.
             bubbleProperties(workInProgress);
             preloadInstanceAndSuspendIfNeeded(

filename: scripts/error-codes/codes.json
patch: @@ -512,5 +512,7 @@
   "524": "Values cannot be passed to next() of AsyncIterables passed to Client Components.",
   "525": "A React Element from an older version of React was rendered. This is not supported. It can happen if:\n- Multiple copies of the \"react\" package is used.\n- A library pre-bundled an old copy of \"react\" or \"react/jsx-runtime\".\n- A compiler tries to \"inline\" JSX instead of using the runtime.",
   "526": "Could not reference an opaque temporary reference. This is likely due to misconfiguring the temporaryReferences options on the server.",
-  "527": "Incompatible React versions: The \"react\" and \"react-dom\" packages must have the exact same version. Instead got:\n  - react:      %s\n  - react-dom:  %s\nLearn more: https://react.dev/warnings/version-mismatch"
+  "527": "Incompatible React versions: The \"react\" and \"react-dom\" packages must have the exact same version. Instead got:\n  - react:      %s\n  - react-dom:  %s\nLearn more: https://react.dev/warnings/version-mismatch",
+  "528": "Expected <link> not to update to be updated to a stylehsheet with precedence. Check the `rel`, `href`, and `precedence` props of this component. Alternatively, check whether two different <link> components render in the same slot or share the same key.%s",
+  "529": "Expected stylesheet with precedence to not be updated to a different kind of <link>. Check the `rel`, `href`, and `precedence` props of this component. Alternatively, check whether two different <link> components render in the same slot or share the same key.%s"
 }