Remove callbackId field from FiberRoot (#19458)

The old expiration times implementation used this field to infer when
the priority of a task had changed at a more granular level than a
Scheduler priority level.

Now that we have the LanePriority type, which is React-specific, we no
longer need the `callbackId` field.

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

@@ -38,7 +38,6 @@ function FiberRootNode(containerInfo, tag, hydrate) {
   this.pendingContext = null;
   this.hydrate = hydrate;
   this.callbackNode = null;
-  this.callbackId = NoLanes;
   this.callbackPriority = NoLanePriority;
   this.eventTimes = createLaneMap(NoLanes);
   this.expirationTimes = createLaneMap(NoTimestamp);

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

@@ -38,7 +38,6 @@ function FiberRootNode(containerInfo, tag, hydrate) {
   this.pendingContext = null;
   this.hydrate = hydrate;
   this.callbackNode = null;
-  this.callbackId = NoLanes;
   this.callbackPriority = NoLanePriority;
   this.eventTimes = createLaneMap(NoLanes);
   this.expirationTimes = createLaneMap(NoTimestamp);

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

@@ -677,10 +677,10 @@ function markUpdateLaneFromFiberToRoot(
 }
 
 // Use this function to schedule a task for a root. There's only one task per
-// root; if a task was already scheduled, we'll check to make sure the
-// expiration time of the existing task is the same as the expiration time of
-// the next level that the root has work on. This function is called on every
-// update, and right before exiting a task.
+// root; if a task was already scheduled, we'll check to make sure the priority
+// of the existing task is the same as the priority of the next level that the
+// root has work on. This function is called on every update, and right before
+// exiting a task.
 function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
   const existingCallbackNode = root.callbackNode;
 
@@ -689,37 +689,32 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
   markStarvedLanesAsExpired(root, currentTime);
 
   // Determine the next lanes to work on, and their priority.
-  const newCallbackId = getNextLanes(
+  const nextLanes = getNextLanes(
     root,
     root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes,
   );
   // This returns the priority level computed during the `getNextLanes` call.
   const newCallbackPriority = returnNextLanesPriority();
 
-  if (newCallbackId === NoLanes) {
+  if (nextLanes === NoLanes) {
     // Special case: There's nothing to work on.
     if (existingCallbackNode !== null) {
       cancelCallback(existingCallbackNode);
       root.callbackNode = null;
       root.callbackPriority = NoLanePriority;
-      root.callbackId = NoLanes;
     }
     return;
   }
 
   // Check if there's an existing task. We may be able to reuse it.
-  const existingCallbackId = root.callbackId;
-  const existingCallbackPriority = root.callbackPriority;
-  if (existingCallbackId !== NoLanes) {
-    if (newCallbackId === existingCallbackId) {
-      // This task is already scheduled. Let's check its priority.
-      if (existingCallbackPriority === newCallbackPriority) {
-        // The priority hasn't changed. Exit.
-        return;
-      }
-      // The task ID is the same but the priority changed. Cancel the existing
-      // callback. We'll schedule a new one below.
+  if (existingCallbackNode !== null) {
+    const existingCallbackPriority = root.callbackPriority;
+    if (existingCallbackPriority === newCallbackPriority) {
+      // The priority hasn't changed. We can reuse the existing task. Exit.
+      return;
     }
+    // The priority changed. Cancel the existing callback. We'll schedule a new
+    // one below.
     cancelCallback(existingCallbackNode);
   }
 
@@ -741,7 +736,6 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
     );
   }
 
-  root.callbackId = newCallbackId;
   root.callbackPriority = newCallbackPriority;
   root.callbackNode = newCallbackNode;
 }
@@ -2041,7 +2035,6 @@ function commitRootImpl(root, renderPriorityLevel) {
   // commitRoot never returns a continuation; it always finishes synchronously.
   // So we can clear these now to allow a new callback to be scheduled.
   root.callbackNode = null;
-  root.callbackId = NoLanes;
 
   // Update the first and last pending times on this root. The new first
   // pending time is whatever is left on the root fiber.

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

@@ -670,10 +670,10 @@ function markUpdateLaneFromFiberToRoot(
 }
 
 // Use this function to schedule a task for a root. There's only one task per
-// root; if a task was already scheduled, we'll check to make sure the
-// expiration time of the existing task is the same as the expiration time of
-// the next level that the root has work on. This function is called on every
-// update, and right before exiting a task.
+// root; if a task was already scheduled, we'll check to make sure the priority
+// of the existing task is the same as the priority of the next level that the
+// root has work on. This function is called on every update, and right before
+// exiting a task.
 function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
   const existingCallbackNode = root.callbackNode;
 
@@ -682,37 +682,32 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
   markStarvedLanesAsExpired(root, currentTime);
 
   // Determine the next lanes to work on, and their priority.
-  const newCallbackId = getNextLanes(
+  const nextLanes = getNextLanes(
     root,
     root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes,
   );
   // This returns the priority level computed during the `getNextLanes` call.
   const newCallbackPriority = returnNextLanesPriority();
 
-  if (newCallbackId === NoLanes) {
+  if (nextLanes === NoLanes) {
     // Special case: There's nothing to work on.
     if (existingCallbackNode !== null) {
       cancelCallback(existingCallbackNode);
       root.callbackNode = null;
       root.callbackPriority = NoLanePriority;
-      root.callbackId = NoLanes;
     }
     return;
   }
 
   // Check if there's an existing task. We may be able to reuse it.
-  const existingCallbackId = root.callbackId;
-  const existingCallbackPriority = root.callbackPriority;
-  if (existingCallbackId !== NoLanes) {
-    if (newCallbackId === existingCallbackId) {
-      // This task is already scheduled. Let's check its priority.
-      if (existingCallbackPriority === newCallbackPriority) {
-        // The priority hasn't changed. Exit.
-        return;
-      }
-      // The task ID is the same but the priority changed. Cancel the existing
-      // callback. We'll schedule a new one below.
+  if (existingCallbackNode !== null) {
+    const existingCallbackPriority = root.callbackPriority;
+    if (existingCallbackPriority === newCallbackPriority) {
+      // The priority hasn't changed. We can reuse the existing task. Exit.
+      return;
     }
+    // The priority changed. Cancel the existing callback. We'll schedule a new
+    // one below.
     cancelCallback(existingCallbackNode);
   }
 
@@ -734,7 +729,6 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
     );
   }
 
-  root.callbackId = newCallbackId;
   root.callbackPriority = newCallbackPriority;
   root.callbackNode = newCallbackNode;
 }
@@ -1959,7 +1953,6 @@ function commitRootImpl(root, renderPriorityLevel) {
   // commitRoot never returns a continuation; it always finishes synchronously.
   // So we can clear these now to allow a new callback to be scheduled.
   root.callbackNode = null;
-  root.callbackId = NoLanes;
 
   // Update the first and last pending times on this root. The new first
   // pending time is whatever is left on the root fiber.

packages/react-reconciler/src/ReactInternalTypes.js

@@ -205,17 +205,15 @@ type BaseFiberRootProperties = {|
   pendingContext: Object | null,
   // Determines if we should attempt to hydrate on the initial mount
   +hydrate: boolean,
-  // Node returned by Scheduler.scheduleCallback
-  callbackNode: *,
 
   // Used by useMutableSource hook to avoid tearing during hydration.
   mutableSourceEagerHydrationData?: Array<
     MutableSource<any> | MutableSourceVersion,
   > | null,
 
-  // Represents the next task that the root should work on, or the current one
-  // if it's already working.
-  callbackId: Lanes,
+  // Node returned by Scheduler.scheduleCallback. Represents the next rendering
+  // task that the root will work on.
+  callbackNode: *,
   callbackPriority: LanePriority,
   eventTimes: LaneMap<number>,
   expirationTimes: LaneMap<number>,