Swapped first/last logic in setPendingExpirationTime and removed no longer in use "first" time value

Author: Brian Vaughn

packages/react-reconciler/src/ReactFiberHooks.js

@@ -75,7 +75,7 @@ import {
   getCurrentPriorityLevel,
 } from './SchedulerWithReactIntegration';
 import {
-  getFirstPendingExpirationTime,
+  getLastPendingExpirationTime,
   getWorkInProgressVersion,
   markSourceAsDirty,
   setPendingExpirationTime,
@@ -916,7 +916,7 @@ function readFromUnsubcribedMutableSource<Source, Snapshot>(
     isSafeToReadFromSource = currentRenderVersion === version;
   } else {
     // If there's no version, then we should fallback to checking the update time.
-    const pendingExpirationTime = getFirstPendingExpirationTime(root);
+    const pendingExpirationTime = getLastPendingExpirationTime(root);
 
     if (pendingExpirationTime === NoWork) {
       isSafeToReadFromSource = true;
@@ -1040,7 +1040,7 @@ function useMutableSource<Source, Snapshot>(
           // There is no mechanism currently to associate these updates though,
           // so for now we fall back to synchronously flushing all pending updates.
           // TODO: Improve this later.
-          markRootExpiredAtTime(root, getFirstPendingExpirationTime(root));
+          markRootExpiredAtTime(root, getLastPendingExpirationTime(root));
         }
       }
     }
@@ -1108,7 +1108,7 @@ function useMutableSource<Source, Snapshot>(
         // We missed a mutation before committing.
         // It's possible that other components using this source also have pending updates scheduled.
         // In that case, we should ensure they all commit together.
-        markRootExpiredAtTime(root, getFirstPendingExpirationTime(root));
+        markRootExpiredAtTime(root, getLastPendingExpirationTime(root));
       }
     }
 

packages/react-reconciler/src/ReactFiberRoot.js

@@ -79,7 +79,6 @@ type BaseFiberRootProperties = {|
   lastExpiredTime: ExpirationTime,
   // Used by useMutableSource hook to avoid tearing within this root
   // when external, mutable sources are read from during render.
-  mutableSourceFirstPendingUpdateTime: ExpirationTime,
   mutableSourceLastPendingUpdateTime: ExpirationTime,
 |};
 

packages/react-reconciler/src/ReactMutableSource.js

@@ -32,19 +32,10 @@ export function clearPendingUpdates(
 ): void {
   if (expirationTime <= root.mutableSourceLastPendingUpdateTime) {
     // All updates for this source have been processed.
-    root.mutableSourceFirstPendingUpdateTime = NoWork;
     root.mutableSourceLastPendingUpdateTime = NoWork;
-  } else if (expirationTime <= root.mutableSourceFirstPendingUpdateTime) {
-    // The highest priority pending updates have been processed,
-    // but we don't how many updates remain between the current and lowest priority.
-    root.mutableSourceFirstPendingUpdateTime = expirationTime - 1;
   }
 }
 
-export function getFirstPendingExpirationTime(root: FiberRoot): ExpirationTime {
-  return root.mutableSourceFirstPendingUpdateTime;
-}
-
 export function getLastPendingExpirationTime(root: FiberRoot): ExpirationTime {
   return root.mutableSourceLastPendingUpdateTime;
 }
@@ -53,18 +44,14 @@ export function setPendingExpirationTime(
   root: FiberRoot,
   expirationTime: ExpirationTime,
 ): void {
-  // Because we only track one pending update time per root,
-  // track the lowest priority update.
-  // It's inclusive of all other pending updates.
-  if (expirationTime > root.mutableSourceLastPendingUpdateTime) {
+  const mutableSourceLastPendingUpdateTime =
+    root.mutableSourceLastPendingUpdateTime;
+  if (
+    mutableSourceLastPendingUpdateTime === NoWork ||
+    expirationTime < mutableSourceLastPendingUpdateTime
+  ) {
     root.mutableSourceLastPendingUpdateTime = expirationTime;
   }
-
-  if (root.mutableSourceFirstPendingUpdateTime === NoWork) {
-    root.mutableSourceFirstPendingUpdateTime = expirationTime;
-  } else if (expirationTime < root.mutableSourceFirstPendingUpdateTime) {
-    root.mutableSourceFirstPendingUpdateTime = expirationTime;
-  }
 }
 
 export function markSourceAsDirty(mutableSource: MutableSource<any>): void {