From 1a8ee88914d0362b82d9718462ffa00e37d5a062 Mon Sep 17 00:00:00 2001
From: Brian Vaughn <bvaughn@fb.com>
Date: Mon, 9 Nov 2020 10:46:24 -0500
Subject: [PATCH] Added failing tests for mutable source mutate during render

---
 .../useMutableSource-test.internal.js         | 35 ++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/packages/react-reconciler/src/__tests__/useMutableSource-test.internal.js b/packages/react-reconciler/src/__tests__/useMutableSource-test.internal.js
index 9b0d868568e6d..c5ba1eb93fbe1 100644
--- a/packages/react-reconciler/src/__tests__/useMutableSource-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/useMutableSource-test.internal.js
@@ -25,9 +25,9 @@ function loadModules() {
   jest.useFakeTimers();
 
   ReactFeatureFlags = require('shared/ReactFeatureFlags');
-
   ReactFeatureFlags.enableSchedulerTracing = true;
   ReactFeatureFlags.enableProfilerTimer = true;
+
   React = require('react');
   ReactNoop = require('react-noop-renderer');
   Scheduler = require('scheduler');
@@ -1718,6 +1718,39 @@ describe('useMutableSource', () => {
     });
   });
 
+  // See https://github.com/facebook/react/issues/19948
+  it('should recover from mutations during read', () => {
+    const source = createSource('initial');
+    const mutableSource = createMutableSource(source, param => param.version);
+
+    function MutateDuringRead() {
+      const value = useMutableSource(
+        mutableSource,
+        defaultGetSnapshot,
+        defaultSubscribe,
+      );
+      Scheduler.unstable_yieldValue('MutateDuringRead:' + value);
+      // This is not a supported/recommended pattern.
+      if (value === 'initial') {
+        source.value = 'updated';
+      }
+      return null;
+    }
+
+    act(() => {
+      ReactNoop.renderLegacySyncRoot(
+        <React.StrictMode>
+          <MutateDuringRead />
+        </React.StrictMode>,
+      );
+    });
+
+    expect(Scheduler).toHaveYielded([
+      'MutateDuringRead:initial',
+      'MutateDuringRead:updated',
+    ]);
+  });
+
   if (__DEV__) {
     describe('dev warnings', () => {
       // @gate experimental