Fix a crash in Suspense with findDOMNode

packages/react-dom/src/__tests__/ReactDOMSuspensePlaceholder-test.js

@@ -225,4 +225,48 @@ describe('ReactDOMSuspensePlaceholder', () => {
     await Lazy;
     expect(log).toEqual(['cDU first', 'cDU second']);
   });
+
+  // Regression test for https://github.com/facebook/react/issues/14188
+  it('can call findDOMNode() in a suspended component commit phase (#2)', () => {
+    let suspendOnce = Promise.resolve();
+    function Suspend() {
+      if (suspendOnce) {
+        let promise = suspendOnce;
+        suspendOnce = null;
+        throw promise;
+      }
+      return null;
+    }
+
+    const log = [];
+    class Child extends React.Component {
+      componentDidMount() {
+        log.push('cDM');
+        ReactDOM.findDOMNode(this);
+      }
+
+      componentDidUpdate() {
+        log.push('cDU');
+        ReactDOM.findDOMNode(this);
+      }
+
+      render() {
+        return null;
+      }
+    }
+
+    function App() {
+      return (
+        <Suspense fallback="Loading">
+          <Suspend />
+          <Child />
+        </Suspense>
+      );
+    }
+
+    ReactDOM.render(<App />, container);
+    expect(log).toEqual(['cDM']);
+    ReactDOM.render(<App />, container);
+    expect(log).toEqual(['cDM', 'cDU']);
+  });
 });

packages/react-reconciler/src/ReactFiberTreeReflection.js

@@ -21,6 +21,8 @@ import {
   HostRoot,
   HostPortal,
   HostText,
+  Fragment,
+  SuspenseComponent,
 } from 'shared/ReactWorkTags';
 import {NoEffect, Placement} from 'shared/ReactSideEffectTags';
 
@@ -119,8 +121,27 @@ export function findCurrentFiberUsingSlowPath(fiber: Fiber): Fiber | null {
     let parentA = a.return;
     let parentB = parentA ? parentA.alternate : null;
     if (!parentA || !parentB) {
-      // We're at the root.
-      break;
+      // We're either at the root, or we're in a special Fragment
+      // with no alternate, which is how Suspense (un)hiding works.
+      let maybeSuspenseFragment = parentA || parentB;
+      if (maybeSuspenseFragment && maybeSuspenseFragment.tag === Fragment) {
+        const maybeSuspense = maybeSuspenseFragment.return;
+        if (
+          maybeSuspense &&
+          maybeSuspense.tag === SuspenseComponent &&
+          // If state isn't null, it timed out and we have two Fragment children.
+          maybeSuspense.memoizedState !== null
+        ) {
+          parentA = maybeSuspense;
+          parentB = maybeSuspense;
+          a = maybeSuspenseFragment;
+          b = maybeSuspenseFragment;
+        } else {
+          break;
+        }
+      } else {
+        break;
+      }
     }
 
     // If both copies of the parent fiber point to the same child, we can