Committing a update on one root should not flush work on another root

`forceExpire` is tracked per root rather than per scheduler.

src/renderers/shared/fiber/ReactFiberReconciler.js

@@ -343,7 +343,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       return;
     }
     processUpdateQueue(blockers, null, null, null, expirationTime);
-    expireWork(expirationTime);
+    expireWork(root, expirationTime);
   };
   WorkNode.prototype.then = function(callback) {
     const root = this._reactRootContainer;

src/renderers/shared/fiber/ReactFiberRoot.js

@@ -34,6 +34,9 @@ export type FiberRoot = {
   // A queue of callbacks that fire once their corresponding expiration time
   // has completed. Only fired once.
   completionCallbacks: UpdateQueue<null> | null,
+  // When set, indicates that all work in this tree with this time or earlier
+  // should be flushed by the end of the batch, as if it has task priority.
+  forceExpire: null | ExpirationTime,
   // The work schedule is a linked list.
   nextScheduledRoot: FiberRoot | null,
   // Top context object, used by renderSubtreeIntoContainer
@@ -64,6 +67,7 @@ exports.createFiberRoot = function(containerInfo: any): FiberRoot {
     completedAt: Done,
     blockers: null,
     completionCallbacks: null,
+    forceExpire: null,
     nextScheduledRoot: null,
     context: null,
     pendingContext: null,

src/renderers/shared/fiber/ReactFiberScheduler.js

@@ -223,9 +223,6 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   let nextUnitOfWork: Fiber | null = null;
   // The time at which we're currently rendering work.
   let nextRenderExpirationTime: ExpirationTime = Done;
-  // If not null, all work up to and including this time should be
-  // flushed before the end of the current batch.
-  let forceExpire: ExpirationTime | null = null;
 
   // The next fiber with an effect that we're currently committing.
   let nextEffect: Fiber | null = null;
@@ -305,7 +302,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         (earliestExpirationTime === Done ||
           earliestExpirationTime > rootExpirationTime)
       ) {
-        earliestExpirationTime = root.current.expirationTime;
+        earliestExpirationTime = rootExpirationTime;
         earliestExpirationRoot = root;
       }
       // We didn't find anything to do in this root, so let's try the next one.
@@ -1723,25 +1720,30 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   }
 
   function recalculateCurrentTime(): ExpirationTime {
-    if (forceExpire !== null) {
-      return forceExpire;
+    if (nextRenderedTree !== null) {
+      // Check if the current root is being force expired.
+      const forceExpire = nextRenderedTree.forceExpire;
+      if (forceExpire !== null) {
+        // Override the current time with the `forceExpire` time. This has the
+        // effect of expiring all work up to and including that time.
+        mostRecentCurrentTime = forceExpire;
+        return forceExpire;
+      }
     }
     mostRecentCurrentTime = msToExpirationTime(now());
     return mostRecentCurrentTime;
   }
 
-  function expireWork(expirationTime: ExpirationTime): void {
+  function expireWork(root: FiberRoot, expirationTime: ExpirationTime): void {
     invariant(
       !isPerformingWork,
       'Cannot commit while already performing work.',
     );
-    // Override the current time with the given time. This has the effect of
-    // expiring all work up to and including that time.
-    forceExpire = mostRecentCurrentTime = expirationTime;
+    root.forceExpire = expirationTime;
     try {
       performWork(TaskPriority, null);
     } finally {
-      forceExpire = null;
+      root.forceExpire = null;
       recalculateCurrentTime();
     }
   }

src/renderers/shared/fiber/__tests__/ReactIncrementalRoot-test.js

@@ -135,7 +135,7 @@ describe('ReactIncrementalRoot', () => {
     expect(root.getChildren()).toEqual([span('A')]);
   });
 
-  it('flushes ealier work if later work is committed', () => {
+  it('flushes earlier work if later work is committed', () => {
     let ops = [];
     const root = ReactNoop.createRoot();
     const work1 = root.prerender(<span prop="A" />);
@@ -153,4 +153,32 @@ describe('ReactIncrementalRoot', () => {
     expect(root.getChildren()).toEqual([span('B')]);
     expect(ops).toEqual(['complete 1', 'complete 2']);
   });
+
+  it('committing work on one tree does not commit or expire work in a separate tree', () => {
+    let ops = [];
+
+    const rootA = ReactNoop.createRoot('A');
+    const rootB = ReactNoop.createRoot('B');
+
+    function Foo(props) {
+      ops.push(props.label);
+      return <span prop={props.label} />;
+    }
+
+    // Prerender work on two separate roots
+    const workA = rootA.prerender(<Foo label="A" />);
+    rootB.prerender(<Foo label="B" />);
+
+    expect(rootA.getChildren()).toEqual([]);
+    expect(rootB.getChildren()).toEqual([]);
+    expect(ops).toEqual([]);
+
+    // Commit root A. This forces the remaining work on root A to expire, but
+    // should not expire work on root B.
+    workA.commit();
+
+    expect(rootA.getChildren()).toEqual([span('A')]);
+    expect(rootB.getChildren()).toEqual([]);
+    expect(ops).toEqual(['A']);
+  });
 });