Suspense fuzz tester (facebook#14147)

* Don't warn if an unmounted component is pinged

* Suspense fuzz tester

The fuzzer works by generating a random tree of React elements. The tree
two types of custom components:

- A Text component suspends rendering on initial mount for a fuzzy
  duration of time. It may update a fuzzy number of times; each update
  supsends for a fuzzy duration of time.
- A Container component wraps some children. It may remount its children
  a fuzzy number of times, by updating its key.

The tree may also include nested Suspense components.

After this tree is generated, the tester sets a flag to temporarily
disable Text components from suspending. The tree is rendered
synchronously. The output of this render is the expected output.

Then the tester flips the flag back to enable suspending. It renders the
tree again. This time the Text components will suspend for the amount of
time configured by the props. The tester waits until everything has
resolved. The resolved output is then compared to the expected output
generated in the previous step.

Finally, we render once more, but this time in concurrent mode. Once
again, the resolved output is compared to the expected output.

I tested by commenting out various parts of the Suspense implementation
to see if broke in the expected way. I also confirmed that it would have
caught facebook#14133, a recent bug related to deletions.

* When a generated test case fails, log its input

* 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.

* Use seeded random number generator

So if there's a failure, we can bisect.

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;
         }
       }