Fix: flushSync changes priority inside effect

When called from inside an effect, flushSync cannot synchronously flush
its updates because React is already working. So we fire a warning.

However, we should still change the priority of the updates to sync so
that they flush at the end of the current task.

This only affects useEffect because updates inside useLayoutEffect (and
the rest of the commit phase, like ref callbacks) are already sync.

packages/react-reconciler/src/ReactFiberWorkLoop.new.js

@@ -1166,16 +1166,6 @@ export function unbatchedUpdates<A, R>(fn: (a: A) => R, a: A): R {
 
 export function flushSync<A, R>(fn: A => R, a: A): R {
   const prevExecutionContext = executionContext;
-  if ((prevExecutionContext & (RenderContext | CommitContext)) !== NoContext) {
-    if (__DEV__) {
-      console.error(
-        'flushSync was called from inside a lifecycle method. React cannot ' +
-          'flush when React is already rendering. Consider moving this call to ' +
-          'a scheduler task or micro task.',
-      );
-    }
-    return fn(a);
-  }
   executionContext |= BatchedContext;
 
   const previousPriority = getCurrentUpdatePriority();
@@ -1192,7 +1182,17 @@ export function flushSync<A, R>(fn: A => R, a: A): R {
     // Flush the immediate callbacks that were scheduled during this batch.
     // Note that this will happen even if batchedUpdates is higher up
     // the stack.
-    flushSyncCallbackQueue();
+    if ((executionContext & (RenderContext | CommitContext)) === NoContext) {
+      flushSyncCallbackQueue();
+    } else {
+      if (__DEV__) {
+        console.error(
+          'flushSync was called from inside a lifecycle method. React cannot ' +
+            'flush when React is already rendering. Consider moving this call to ' +
+            'a scheduler task or micro task.',
+        );
+      }
+    }
   }
 }
 

packages/react-reconciler/src/ReactFiberWorkLoop.old.js

@@ -1166,16 +1166,6 @@ export function unbatchedUpdates<A, R>(fn: (a: A) => R, a: A): R {
 
 export function flushSync<A, R>(fn: A => R, a: A): R {
   const prevExecutionContext = executionContext;
-  if ((prevExecutionContext & (RenderContext | CommitContext)) !== NoContext) {
-    if (__DEV__) {
-      console.error(
-        'flushSync was called from inside a lifecycle method. React cannot ' +
-          'flush when React is already rendering. Consider moving this call to ' +
-          'a scheduler task or micro task.',
-      );
-    }
-    return fn(a);
-  }
   executionContext |= BatchedContext;
 
   const previousPriority = getCurrentUpdatePriority();
@@ -1192,7 +1182,17 @@ export function flushSync<A, R>(fn: A => R, a: A): R {
     // Flush the immediate callbacks that were scheduled during this batch.
     // Note that this will happen even if batchedUpdates is higher up
     // the stack.
-    flushSyncCallbackQueue();
+    if ((executionContext & (RenderContext | CommitContext)) === NoContext) {
+      flushSyncCallbackQueue();
+    } else {
+      if (__DEV__) {
+        console.error(
+          'flushSync was called from inside a lifecycle method. React cannot ' +
+            'flush when React is already rendering. Consider moving this call to ' +
+            'a scheduler task or micro task.',
+        );
+      }
+    }
   }
 }
 

packages/react-reconciler/src/__tests__/ReactFlushSync-test.js

@@ -0,0 +1,58 @@
+let React;
+let ReactNoop;
+let Scheduler;
+let useState;
+let useEffect;
+
+describe('ReactFlushSync', () => {
+  beforeEach(() => {
+    jest.resetModules();
+
+    React = require('react');
+    ReactNoop = require('react-noop-renderer');
+    Scheduler = require('scheduler');
+    useState = React.useState;
+    useEffect = React.useEffect;
+  });
+
+  function Text({text}) {
+    Scheduler.unstable_yieldValue(text);
+    return text;
+  }
+
+  // @gate experimental
+  test('changes priority of updates in useEffect', async () => {
+    function App() {
+      const [syncState, setSyncState] = useState(0);
+      const [state, setState] = useState(0);
+      useEffect(() => {
+        if (syncState !== 1) {
+          setState(1);
+          ReactNoop.flushSync(() => setSyncState(1));
+        }
+      }, [syncState, state]);
+      return <Text text={`${syncState}, ${state}`} />;
+    }
+
+    const root = ReactNoop.createRoot();
+    await ReactNoop.act(async () => {
+      root.render(<App />);
+      // This will yield right before the passive effect fires
+      expect(Scheduler).toFlushUntilNextPaint(['0, 0']);
+
+      // The passive effect will schedule a sync update and a normal update.
+      // They should commit in two separate batches. First the sync one.
+      expect(() =>
+        expect(Scheduler).toFlushUntilNextPaint(['1, 0']),
+      ).toErrorDev('flushSync was called from inside a lifecycle method');
+
+      // The remaining update is not sync
+      ReactNoop.flushSync();
+      expect(Scheduler).toHaveYielded([]);
+
+      // Now flush it.
+      expect(Scheduler).toFlushUntilNextPaint(['1, 1']);
+    });
+    expect(root).toMatchRenderedOutput('1, 1');
+  });
+});