From 57c32b148fd7f140393adb8ca7fba28ce40e0647 Mon Sep 17 00:00:00 2001
From: Brian Vaughn <bvaughn@fb.com>
Date: Mon, 10 Feb 2020 14:21:55 -0800
Subject: [PATCH] Fixed high pri updates erasing pending lower pri subscription
 updates

---
 packages/react-reconciler/src/ReactFiberHooks.js      |  6 ++++++
 .../ReactHooksWithNoopRenderer-test.internal.js       | 11 +++++++----
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/packages/react-reconciler/src/ReactFiberHooks.js b/packages/react-reconciler/src/ReactFiberHooks.js
index b3c4443b8590f..21d3fa3518333 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.js
@@ -900,6 +900,12 @@ function useMutableSourceImpl<S>(
     isSafeToReadFromSource =
       pendingExpirationTime === NoWork ||
       pendingExpirationTime >= expirationTime;
+
+    if (!isSafeToReadFromSource) {
+      // Preserve the pending update if the current priority doesn't include it.
+      currentlyRenderingFiber.expirationTime = pendingExpirationTime;
+      markUnprocessedUpdateTime(pendingExpirationTime);
+    }
   }
 
   let prevMemoizedState = ((hook.memoizedState: any): ?MutableSourceState<S>);
diff --git a/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.internal.js b/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.internal.js
index b887f20b736ad..52f5acaba6676 100644
--- a/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.internal.js
@@ -2016,7 +2016,6 @@ describe('ReactHooksWithNoopRenderer', () => {
         () => Scheduler.unstable_yieldValue('Sync effect'),
       );
       expect(Scheduler).toFlushAndYield(['a:one', 'b:one', 'Sync effect']);
-      ReactNoop.flushPassiveEffects();
 
       // Changing values should schedule an update with React.
       // Start working on this update but don't finish it.
@@ -2036,9 +2035,7 @@ describe('ReactHooksWithNoopRenderer', () => {
       });
       expect(Scheduler).toHaveYielded(['a:one', 'b:one', 'Sync effect']);
 
-      // TODO (useMutableSource) Re-enable the assertion below; it fails now for reasons unknown.
-      // Once the update is processed, the new value should be used
-      // expect(Scheduler).toFlushAndYield(['a:two', 'b:two']);
+      expect(Scheduler).toFlushAndYield(['a:two', 'b:two']);
     });
 
     it('should read from source on newly mounted subtree if no pending updates are scheduled for source', () => {
@@ -2098,6 +2095,12 @@ describe('ReactHooksWithNoopRenderer', () => {
       });
       expect(Scheduler).toHaveYielded(['a:two', 'b:two', 'Sync effect']);
     });
+
+    // TODO (useMutableSource) Test case for a scoped subscription,
+    // followed by a render that reads from a different part of the store,
+    // with a mutation between its first and second read,
+    // that isn't picked up on by the scoped subscription,
+    // to verify that we also use the version number to protect against this case.
   });
 
   describe('useCallback', () => {