Remove fakeCallbackNode (#20799)

Don't need this, because sync tasks are never cancelled. We can do the
same thing we do for microtask callbacks.

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

@@ -726,7 +726,8 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
       // TODO: Temporary until we confirm this warning is not fired.
       if (
         existingCallbackNode == null &&
-        existingCallbackPriority !== InputDiscreteLanePriority
+        existingCallbackPriority !== InputDiscreteLanePriority &&
+        existingCallbackPriority !== SyncLanePriority
       ) {
         console.error(
           'Expected scheduled callback to exist. This error is likely caused by a bug in React. Please file an issue.',
@@ -747,11 +748,8 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
   if (newCallbackPriority === SyncLanePriority) {
     // Special case: Sync React callbacks are scheduled on a special
     // internal queue
-
-    // TODO: After enableDiscreteEventMicroTasks lands, we can remove the fake node.
-    newCallbackNode = scheduleSyncCallback(
-      performSyncWorkOnRoot.bind(null, root),
-    );
+    scheduleSyncCallback(performSyncWorkOnRoot.bind(null, root));
+    newCallbackNode = null;
   } else if (newCallbackPriority === SyncBatchedLanePriority) {
     newCallbackNode = scheduleCallback(
       ImmediateSchedulerPriority,

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

@@ -726,7 +726,8 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
       // TODO: Temporary until we confirm this warning is not fired.
       if (
         existingCallbackNode == null &&
-        existingCallbackPriority !== InputDiscreteLanePriority
+        existingCallbackPriority !== InputDiscreteLanePriority &&
+        existingCallbackPriority !== SyncLanePriority
       ) {
         console.error(
           'Expected scheduled callback to exist. This error is likely caused by a bug in React. Please file an issue.',
@@ -747,11 +748,8 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
   if (newCallbackPriority === SyncLanePriority) {
     // Special case: Sync React callbacks are scheduled on a special
     // internal queue
-
-    // TODO: After enableDiscreteEventMicroTasks lands, we can remove the fake node.
-    newCallbackNode = scheduleSyncCallback(
-      performSyncWorkOnRoot.bind(null, root),
-    );
+    scheduleSyncCallback(performSyncWorkOnRoot.bind(null, root));
+    newCallbackNode = null;
   } else if (newCallbackPriority === SyncBatchedLanePriority) {
     newCallbackNode = scheduleCallback(
       ImmediateSchedulerPriority,

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

@@ -57,8 +57,6 @@ export type SchedulerCallback = (isSync: boolean) => SchedulerCallback | null;
 
 type SchedulerCallbackOptions = {timeout?: number, ...};
 
-const fakeCallbackNode = {};
-
 // Except for NoPriority, these correspond to Scheduler priorities. We use
 // ascending numbers so we can compare them like numbers. They start at 90 to
 // avoid clashing with Scheduler's priorities.
@@ -147,6 +145,8 @@ export function scheduleSyncCallback(callback: SchedulerCallback) {
   if (syncQueue === null) {
     syncQueue = [callback];
     // Flush the queue in the next tick, at the earliest.
+    // TODO: Figure out how to remove this It's only here as a last resort if we
+    // forget to explicitly flush.
     immediateQueueCallbackNode = Scheduler_scheduleCallback(
       Scheduler_ImmediatePriority,
       flushSyncCallbackQueueImpl,
@@ -156,13 +156,10 @@ export function scheduleSyncCallback(callback: SchedulerCallback) {
     // we already scheduled one when we created the queue.
     syncQueue.push(callback);
   }
-  return fakeCallbackNode;
 }
 
 export function cancelCallback(callbackNode: mixed) {
-  if (callbackNode !== fakeCallbackNode) {
-    Scheduler_cancelCallback(callbackNode);
-  }
+  Scheduler_cancelCallback(callbackNode);
 }
 
 export function flushSyncCallbackQueue() {

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

@@ -57,8 +57,6 @@ export type SchedulerCallback = (isSync: boolean) => SchedulerCallback | null;
 
 type SchedulerCallbackOptions = {timeout?: number, ...};
 
-const fakeCallbackNode = {};
-
 // Except for NoPriority, these correspond to Scheduler priorities. We use
 // ascending numbers so we can compare them like numbers. They start at 90 to
 // avoid clashing with Scheduler's priorities.
@@ -147,6 +145,8 @@ export function scheduleSyncCallback(callback: SchedulerCallback) {
   if (syncQueue === null) {
     syncQueue = [callback];
     // Flush the queue in the next tick, at the earliest.
+    // TODO: Figure out how to remove this It's only here as a last resort if we
+    // forget to explicitly flush.
     immediateQueueCallbackNode = Scheduler_scheduleCallback(
       Scheduler_ImmediatePriority,
       flushSyncCallbackQueueImpl,
@@ -156,13 +156,10 @@ export function scheduleSyncCallback(callback: SchedulerCallback) {
     // we already scheduled one when we created the queue.
     syncQueue.push(callback);
   }
-  return fakeCallbackNode;
 }
 
 export function cancelCallback(callbackNode: mixed) {
-  if (callbackNode !== fakeCallbackNode) {
-    Scheduler_cancelCallback(callbackNode);
-  }
+  Scheduler_cancelCallback(callbackNode);
 }
 
 export function flushSyncCallbackQueue() {