Moar fuzziness

Adds more fuzziness to the generated tests. Specifcally, introduces
nested Suspense cases, where the fallback of a Suspense component
also suspends.

This flushed out a bug (yay!) whose test case I've hard coded.

packages/react-reconciler/src/ReactFiberBeginWork.js

@@ -1178,7 +1178,6 @@ function updateSuspenseComponent(
           currentPrimaryChildFragment.pendingProps,
           NoWork,
         );
-        primaryChildFragment.effectTag |= Placement;
 
         if ((workInProgress.mode & ConcurrentMode) === NoContext) {
           // Outside of concurrent mode, we commit the effects from the
@@ -1213,7 +1212,6 @@ function updateSuspenseComponent(
           nextFallbackChildren,
           currentFallbackChildFragment.expirationTime,
         ));
-        fallbackChildFragment.effectTag |= Placement;
         child = primaryChildFragment;
         primaryChildFragment.childExpirationTime = NoWork;
         // Skip the primary children, and continue working on the
@@ -1257,11 +1255,14 @@ function updateSuspenseComponent(
           NoWork,
           null,
         );
-
-        primaryChildFragment.effectTag |= Placement;
         primaryChildFragment.child = currentPrimaryChild;
         currentPrimaryChild.return = primaryChildFragment;
 
+        // Even though we're creating a new fiber, there are no new children,
+        // because we're reusing an already mounted tree. So we don't need to
+        // schedule a placement.
+        // primaryChildFragment.effectTag |= Placement;
+
         if ((workInProgress.mode & ConcurrentMode) === NoContext) {
           // Outside of concurrent mode, we commit the effects from the
           // partially completed, timed-out tree, too.

packages/react-reconciler/src/ReactFiberCommitWork.js

@@ -558,6 +558,16 @@ function hideOrUnhideAllChildren(finishedWork, isHidden) {
         } else {
           unhideTextInstance(instance, node.memoizedProps);
         }
+      } else if (
+        node.tag === SuspenseComponent &&
+        node.memoizedState !== null
+      ) {
+        // Found a nested Suspense component that timed out. Skip over the
+        // primary child fragment, which should remain hidden.
+        const fallbackChildFragment: Fiber = (node.child: any).sibling;
+        fallbackChildFragment.return = node;
+        node = fallbackChildFragment;
+        continue;
       } else if (node.child !== null) {
         node.child.return = node;
         node = node.child;

packages/react-reconciler/src/ReactFiberCompleteWork.js

@@ -45,6 +45,7 @@ import {
   Update,
   NoEffect,
   DidCapture,
+  Deletion,
 } from 'shared/ReactSideEffectTags';
 import invariant from 'shared/invariant';
 
@@ -82,7 +83,6 @@ import {
   popHydrationState,
 } from './ReactFiberHydrationContext';
 import {ConcurrentMode, NoContext} from './ReactTypeOfMode';
-import {reconcileChildFibers} from './ReactChildFiber';
 
 function markUpdate(workInProgress: Fiber) {
   // Tag the fiber with an update effect. This turns a Placement into
@@ -715,12 +715,16 @@ function completeWork(
         // the stateNode during the begin phase?
         const currentFallbackChild: Fiber | null = (current.child: any).sibling;
         if (currentFallbackChild !== null) {
-          reconcileChildFibers(
-            workInProgress,
-            currentFallbackChild,
-            null,
-            renderExpirationTime,
-          );
+          // Deletions go at the beginning of the return fiber's effect list
+          const first = workInProgress.firstEffect;
+          if (first !== null) {
+            workInProgress.firstEffect = currentFallbackChild;
+            currentFallbackChild.nextEffect = first;
+          } else {
+            workInProgress.firstEffect = workInProgress.lastEffect = currentFallbackChild;
+            currentFallbackChild.nextEffect = null;
+          }
+          currentFallbackChild.effectTag = Deletion;
         }
       }
 

packages/react-reconciler/src/__tests__/ReactSuspenseFuzz-test.internal.js

@@ -296,7 +296,28 @@ describe('ReactSuspenseFuzz', () => {
               {value: randomInteger(0, 5000), weight: 1},
             ]);
 
-            return React.createElement(Suspense, {maxDuration}, ...children);
+            const fallbackType = pickRandomWeighted([
+              {value: 'none', weight: 1},
+              {value: 'normal', weight: 1},
+              {value: 'nested suspense', weight: 1},
+            ]);
+
+            let fallback;
+            if (fallbackType === 'normal') {
+              fallback = 'Loading...';
+            } else if (fallbackType === 'nested suspense') {
+              fallback = React.createElement(
+                React.Fragment,
+                null,
+                ...createRandomChildren(3),
+              );
+            }
+
+            return React.createElement(
+              Suspense,
+              {maxDuration, fallback},
+              ...children,
+            );
           }
           case 'return':
           default:
@@ -333,6 +354,28 @@ describe('ReactSuspenseFuzz', () => {
     );
   });
 
+  it('hard-coded cases', () => {
+    const {Text, testResolvedOutput} = createFuzzer();
+
+    testResolvedOutput(
+      <React.Fragment>
+        <Text
+          initialDelay={20}
+          text="A"
+          updates={[{beginAfter: 10, suspendFor: 20}]}
+        />
+        <Suspense fallback="Loading... (B)">
+          <Text
+            initialDelay={10}
+            text="B"
+            updates={[{beginAfter: 30, suspendFor: 50}]}
+          />
+          <Text text="C" />
+        </Suspense>
+      </React.Fragment>,
+    );
+  });
+
   it('generative tests', () => {
     const {generateTestCase, testResolvedOutput} = createFuzzer();