From 3fac6a780b999fcd50007bf3c3e3c7e9314f290a Mon Sep 17 00:00:00 2001
From: Andrew Clark <git@andrewclark.io>
Date: Wed, 12 May 2021 12:26:06 -0500
Subject: [PATCH 1/2] Failing test: Class callback fired multiple times

Happens during a rebase (low priority update followed by high priority
update). The high priority callback gets fired twice.
---
 .../ReactClassSetStateCallback-test.js        | 47 +++++++++++++++++++
 1 file changed, 47 insertions(+)
 create mode 100644 packages/react-reconciler/src/__tests__/ReactClassSetStateCallback-test.js

diff --git a/packages/react-reconciler/src/__tests__/ReactClassSetStateCallback-test.js b/packages/react-reconciler/src/__tests__/ReactClassSetStateCallback-test.js
new file mode 100644
index 0000000000000..13d9ce838c03b
--- /dev/null
+++ b/packages/react-reconciler/src/__tests__/ReactClassSetStateCallback-test.js
@@ -0,0 +1,47 @@
+let React;
+let ReactNoop;
+let Scheduler;
+
+describe('ReactClassSetStateCallback', () => {
+  beforeEach(() => {
+    jest.resetModules();
+
+    React = require('react');
+    ReactNoop = require('react-noop-renderer');
+    Scheduler = require('scheduler');
+  });
+
+  function Text({text}) {
+    Scheduler.unstable_yieldValue(text);
+    return text;
+  }
+
+  test('regression: setState callback (2nd arg) should only fire once, even after a rebase', async () => {
+    let app;
+    class App extends React.Component {
+      state = {step: 0};
+      render() {
+        app = this;
+        return <Text text={this.state.step} />;
+      }
+    }
+
+    const root = ReactNoop.createRoot();
+    await ReactNoop.act(async () => {
+      root.render(<App />);
+    });
+    expect(Scheduler).toHaveYielded([0]);
+
+    await ReactNoop.act(async () => {
+      app.setState({step: 1}, () =>
+        Scheduler.unstable_yieldValue('Callback 1'),
+      );
+      ReactNoop.flushSync(() => {
+        app.setState({step: 2}, () =>
+          Scheduler.unstable_yieldValue('Callback 2'),
+        );
+      });
+    });
+    expect(Scheduler).toHaveYielded([2, 'Callback 2', 2, 'Callback 1']);
+  });
+});

From a807e67a599dbe42c3369e6d2304937153423ba8 Mon Sep 17 00:00:00 2001
From: Andrew Clark <git@andrewclark.io>
Date: Wed, 12 May 2021 12:27:06 -0500
Subject: [PATCH 2/2] Prevent setState callback firing during rebase

Before enqueueing the effect, adds a guard to check if the update was
already committed.
---
 packages/react-reconciler/src/ReactUpdateQueue.new.js | 7 ++++++-
 packages/react-reconciler/src/ReactUpdateQueue.old.js | 7 ++++++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/packages/react-reconciler/src/ReactUpdateQueue.new.js b/packages/react-reconciler/src/ReactUpdateQueue.new.js
index 0bdcbf580764a..06ee95a92c705 100644
--- a/packages/react-reconciler/src/ReactUpdateQueue.new.js
+++ b/packages/react-reconciler/src/ReactUpdateQueue.new.js
@@ -580,7 +580,12 @@ export function processUpdateQueue<State>(
           instance,
         );
         const callback = update.callback;
-        if (callback !== null) {
+        if (
+          callback !== null &&
+          // If the update was already committed, we should not queue its
+          // callback again.
+          update.lane !== NoLane
+        ) {
           workInProgress.flags |= Callback;
           const effects = queue.effects;
           if (effects === null) {
diff --git a/packages/react-reconciler/src/ReactUpdateQueue.old.js b/packages/react-reconciler/src/ReactUpdateQueue.old.js
index 209abfb32e743..520f993fbd81f 100644
--- a/packages/react-reconciler/src/ReactUpdateQueue.old.js
+++ b/packages/react-reconciler/src/ReactUpdateQueue.old.js
@@ -580,7 +580,12 @@ export function processUpdateQueue<State>(
           instance,
         );
         const callback = update.callback;
-        if (callback !== null) {
+        if (
+          callback !== null &&
+          // If the update was already committed, we should not queue its
+          // callback again.
+          update.lane !== NoLane
+        ) {
           workInProgress.flags |= Callback;
           const effects = queue.effects;
           if (effects === null) {