Repository: facebook/react
Pull request: Update useMemoCache test to confirm that cache persists across errors

## Summary

Updates the `useMemoCache()` tests to validate that the memo cache persists when a component does a setState during render or throws during render. Forget's compilation output follows the general pattern used in this test and is resilient to rendering running partway and then again with different inputs.

## How did you test this change?

`yarn test` (this is a test-only change)

Changed files:
filename: packages/react-reconciler/src/__tests__/useMemoCache-test.js
patch: @@ -74,31 +74,37 @@ describe('useMemoCache()', () => {
     let setX;
     let forceUpdate;
     function Component(props) {
-      const cache = useMemoCache(4);
+      const cache = useMemoCache(5);
 
       // x is used to produce a `data` object passed to the child
       const [x, _setX] = useState(0);
       setX = _setX;
-      const c_x = x !== cache[0];
-      cache[0] = x;
 
       // n is passed as-is to the child as a cache breaker
       const [n, setN] = useState(0);
       forceUpdate = () => setN(a => a + 1);
-      const c_n = n !== cache[1];
-      cache[1] = n;
 
+      const c_0 = x !== cache[0];
       let data;
-      if (c_x) {
-        data = cache[2] = {text: `Count ${x}`};
+      if (c_0) {
+        data = {text: `Count ${x}`};
+        cache[0] = x;
+        cache[1] = data;
       } else {
-        data = cache[2];
+        data = cache[1];
       }
-      if (c_x || c_n) {
-        return (cache[3] = <Text data={data} n={n} />);
+      const c_2 = x !== cache[2];
+      const c_3 = n !== cache[3];
+      let t0;
+      if (c_2 || c_3) {
+        t0 = <Text data={data} n={n} />;
+        cache[2] = x;
+        cache[3] = n;
+        cache[4] = t0;
       } else {
-        return cache[3];
+        t0 = cache[4];
       }
+      return t0;
     }
     let data;
     const Text = jest.fn(function Text(props) {
@@ -135,132 +141,117 @@ describe('useMemoCache()', () => {
 
   // @gate enableUseMemoCacheHook
   test('update component using cache with setstate during render', async () => {
-    let setX;
     let setN;
     function Component(props) {
-      const cache = useMemoCache(4);
+      const cache = useMemoCache(5);
 
       // x is used to produce a `data` object passed to the child
-      const [x, _setX] = useState(0);
-      setX = _setX;
-      const c_x = x !== cache[0];
-      cache[0] = x;
+      const [x] = useState(0);
+
+      const c_0 = x !== cache[0];
+      let data;
+      if (c_0) {
+        data = {text: `Count ${x}`};
+        cache[0] = x;
+        cache[1] = data;
+      } else {
+        data = cache[1];
+      }
 
       // n is passed as-is to the child as a cache breaker
       const [n, _setN] = useState(0);
       setN = _setN;
-      const c_n = n !== cache[1];
-      cache[1] = n;
 
-      // NOTE: setstate and early return here means that x will update
-      // without the data value being updated. Subsequent renders could
-      // therefore think that c_x = false (hasn't changed) and skip updating
-      // data.
-      // The memoizing compiler will have to handle this case, but the runtime
-      // can help by falling back to resetting the cache if a setstate occurs
-      // during render (this mirrors what we do for useMemo and friends)
       if (n === 1) {
         setN(2);
         return;
       }
 
-      let data;
-      if (c_x) {
-        data = cache[2] = {text: `Count ${x}`};
+      const c_2 = x !== cache[2];
+      const c_3 = n !== cache[3];
+      let t0;
+      if (c_2 || c_3) {
+        t0 = <Text data={data} n={n} />;
+        cache[2] = x;
+        cache[3] = n;
+        cache[4] = t0;
       } else {
-        data = cache[2];
-      }
-      if (c_x || c_n) {
-        return (cache[3] = <Text data={data} n={n} />);
-      } else {
-        return cache[3];
+        t0 = cache[4];
       }
+      return t0;
     }
     let data;
     const Text = jest.fn(function Text(props) {
       data = props.data;
-      return data.text;
+      return `${data.text} (n=${props.n})`;
     });
 
     const root = ReactNoop.createRoot();
     await act(() => {
       root.render(<Component />);
     });
-    expect(root).toMatchRenderedOutput('Count 0');
+    expect(root).toMatchRenderedOutput('Count 0 (n=0)');
     expect(Text).toBeCalledTimes(1);
     const data0 = data;
 
-    // Simultaneously trigger an update to x (should create a new data value)
-    // and trigger the setState+early return. The runtime should reset the cache
-    // to avoid an inconsistency
+    // Trigger an update that will cause a setState during render. The `data` prop
+    // does not depend on `n`, and should remain cached.
     await act(() => {
-      setX(1);
       setN(1);
     });
-    expect(root).toMatchRenderedOutput('Count 1');
+    expect(root).toMatchRenderedOutput('Count 0 (n=2)');
     expect(Text).toBeCalledTimes(2);
-    expect(data).not.toBe(data0);
-    const data1 = data;
-
-    // Forcing an unrelated update shouldn't recreate the
-    // data object.
-    await act(() => {
-      setN(3);
-    });
-    expect(root).toMatchRenderedOutput('Count 1');
-    expect(Text).toBeCalledTimes(3);
-    expect(data).toBe(data1); // confirm that the cache persisted across renders
+    expect(data).toBe(data0);
   });
 
   // @gate enableUseMemoCacheHook
   test('update component using cache with throw during render', async () => {
-    let setX;
     let setN;
     let shouldFail = true;
     function Component(props) {
-      const cache = useMemoCache(4);
+      const cache = useMemoCache(5);
 
       // x is used to produce a `data` object passed to the child
-      const [x, _setX] = useState(0);
-      setX = _setX;
-      const c_x = x !== cache[0];
-      cache[0] = x;
+      const [x] = useState(0);
+
+      const c_0 = x !== cache[0];
+      let data;
+      if (c_0) {
+        data = {text: `Count ${x}`};
+        cache[0] = x;
+        cache[1] = data;
+      } else {
+        data = cache[1];
+      }
 
       // n is passed as-is to the child as a cache breaker
       const [n, _setN] = useState(0);
       setN = _setN;
-      const c_n = n !== cache[1];
-      cache[1] = n;
 
-      // NOTE the initial failure will trigger a re-render, after which the function
-      // will early return. This validates that the runtime resets the cache on error:
-      // if it doesn't the cache will be corrupt, with the cached version of data
-      // out of data from the cached version of x.
       if (n === 1) {
         if (shouldFail) {
           shouldFail = false;
           throw new Error('failed');
         }
-        setN(2);
-        return;
       }
 
-      let data;
-      if (c_x) {
-        data = cache[2] = {text: `Count ${x}`};
+      const c_2 = x !== cache[2];
+      const c_3 = n !== cache[3];
+      let t0;
+      if (c_2 || c_3) {
+        t0 = <Text data={data} n={n} />;
+        cache[2] = x;
+        cache[3] = n;
+        cache[4] = t0;
       } else {
-        data = cache[2];
-      }
-      if (c_x || c_n) {
-        return (cache[3] = <Text data={data} n={n} />);
-      } else {
-        return cache[3];
+        t0 = cache[4];
       }
+      return t0;
     }
     let data;
     const Text = jest.fn(function Text(props) {
       data = props.data;
-      return data.text;
+      return `${data.text} (n=${props.n})`;
     });
 
     spyOnDev(console, 'error');
@@ -273,30 +264,25 @@ describe('useMemoCache()', () => {
         </ErrorBoundary>,
       );
     });
-    expect(root).toMatchRenderedOutput('Count 0');
+    expect(root).toMatchRenderedOutput('Count 0 (n=0)');
     expect(Text).toBeCalledTimes(1);
     const data0 = data;
 
-    // Simultaneously trigger an update to x (should create a new data value)
-    // and trigger the setState+early return. The runtime should reset the cache
-    // to avoid an inconsistency
     await act(() => {
-      // this update bumps the count
-      setX(1);
       // this triggers a throw.
       setN(1);
     });
-    expect(root).toMatchRenderedOutput('Count 1');
+    expect(root).toMatchRenderedOutput('Count 0 (n=1)');
     expect(Text).toBeCalledTimes(2);
-    expect(data).not.toBe(data0);
+    expect(data).toBe(data0);
     const data1 = data;
 
     // Forcing an unrelated update shouldn't recreate the
     // data object.
     await act(() => {
-      setN(3);
+      setN(2);
     });
-    expect(root).toMatchRenderedOutput('Count 1');
+    expect(root).toMatchRenderedOutput('Count 0 (n=2)');
     expect(Text).toBeCalledTimes(3);
     expect(data).toBe(data1); // confirm that the cache persisted across renders
   });