[Scheduler] Support inferring priority from stack (facebook#16105)

When executing a task, wraps the callback in an extra function whose
name includes the current priority level. Profiling tools with access
to function names can use this to determine the priority of the task.

Author: acdlite

packages/scheduler/src/Scheduler.js

@@ -56,6 +56,22 @@ var isPerformingWork = false;
 var isHostCallbackScheduled = false;
 var isHostTimeoutScheduled = false;
 
+function scheduler_flushTaskAtPriority_Immediate(callback, didTimeout) {
+  return callback(didTimeout);
+}
+function scheduler_flushTaskAtPriority_UserBlocking(callback, didTimeout) {
+  return callback(didTimeout);
+}
+function scheduler_flushTaskAtPriority_Normal(callback, didTimeout) {
+  return callback(didTimeout);
+}
+function scheduler_flushTaskAtPriority_Low(callback, didTimeout) {
+  return callback(didTimeout);
+}
+function scheduler_flushTaskAtPriority_Idle(callback, didTimeout) {
+  return callback(didTimeout);
+}
+
 function flushTask(task, currentTime) {
   // Remove the task from the list before calling the callback. That way the
   // list is in a consistent state even if the callback throws.
@@ -83,7 +99,40 @@ function flushTask(task, currentTime) {
   var continuationCallback;
   try {
     var didUserCallbackTimeout = task.expirationTime <= currentTime;
-    continuationCallback = callback(didUserCallbackTimeout);
+    // Add an extra function to the callstack. Profiling tools can use this
+    // to infer the priority of work that appears higher in the stack.
+    switch (currentPriorityLevel) {
+      case ImmediatePriority:
+        continuationCallback = scheduler_flushTaskAtPriority_Immediate(
+          callback,
+          didUserCallbackTimeout,
+        );
+        break;
+      case UserBlockingPriority:
+        continuationCallback = scheduler_flushTaskAtPriority_UserBlocking(
+          callback,
+          didUserCallbackTimeout,
+        );
+        break;
+      case NormalPriority:
+        continuationCallback = scheduler_flushTaskAtPriority_Normal(
+          callback,
+          didUserCallbackTimeout,
+        );
+        break;
+      case LowPriority:
+        continuationCallback = scheduler_flushTaskAtPriority_Low(
+          callback,
+          didUserCallbackTimeout,
+        );
+        break;
+      case IdlePriority:
+        continuationCallback = scheduler_flushTaskAtPriority_Idle(
+          callback,
+          didUserCallbackTimeout,
+        );
+        break;
+    }
   } catch (error) {
     throw error;
   } finally {

packages/scheduler/src/__tests__/Scheduler-test.js

@@ -14,6 +14,8 @@ let runWithPriority;
 let ImmediatePriority;
 let UserBlockingPriority;
 let NormalPriority;
+let LowPriority;
+let IdlePriority;
 let scheduleCallback;
 let cancelCallback;
 let wrapCallback;
@@ -31,6 +33,8 @@ describe('Scheduler', () => {
     ImmediatePriority = Scheduler.unstable_ImmediatePriority;
     UserBlockingPriority = Scheduler.unstable_UserBlockingPriority;
     NormalPriority = Scheduler.unstable_NormalPriority;
+    LowPriority = Scheduler.unstable_LowPriority;
+    IdlePriority = Scheduler.unstable_IdlePriority;
     scheduleCallback = Scheduler.unstable_scheduleCallback;
     cancelCallback = Scheduler.unstable_cancelCallback;
     wrapCallback = Scheduler.unstable_wrapCallback;
@@ -414,6 +418,73 @@ describe('Scheduler', () => {
     ]);
   });
 
+  if (__DEV__) {
+    // Function names are minified in prod, though you could still infer the
+    // priority if you have sourcemaps.
+    it('adds extra function to the JS stack whose name includes the priority level', () => {
+      function inferPriorityFromCallstack() {
+        try {
+          throw Error();
+        } catch (e) {
+          const stack = e.stack;
+          const lines = stack.split('\n');
+          for (let i = lines.length - 1; i >= 0; i--) {
+            const line = lines[i];
+            const found = line.match(
+              /scheduler_flushTaskAtPriority_([A-Za-z]+)/,
+            );
+            if (found !== null) {
+              const priorityStr = found[1];
+              switch (priorityStr) {
+                case 'Immediate':
+                  return ImmediatePriority;
+                case 'UserBlocking':
+                  return UserBlockingPriority;
+                case 'Normal':
+                  return NormalPriority;
+                case 'Low':
+                  return LowPriority;
+                case 'Idle':
+                  return IdlePriority;
+              }
+            }
+          }
+          return null;
+        }
+      }
+
+      scheduleCallback(ImmediatePriority, () =>
+        Scheduler.unstable_yieldValue(
+          'Immediate: ' + inferPriorityFromCallstack(),
+        ),
+      );
+      scheduleCallback(UserBlockingPriority, () =>
+        Scheduler.unstable_yieldValue(
+          'UserBlocking: ' + inferPriorityFromCallstack(),
+        ),
+      );
+      scheduleCallback(NormalPriority, () =>
+        Scheduler.unstable_yieldValue(
+          'Normal: ' + inferPriorityFromCallstack(),
+        ),
+      );
+      scheduleCallback(LowPriority, () =>
+        Scheduler.unstable_yieldValue('Low: ' + inferPriorityFromCallstack()),
+      );
+      scheduleCallback(IdlePriority, () =>
+        Scheduler.unstable_yieldValue('Idle: ' + inferPriorityFromCallstack()),
+      );
+
+      expect(Scheduler).toFlushAndYield([
+        'Immediate: ' + ImmediatePriority,
+        'UserBlocking: ' + UserBlockingPriority,
+        'Normal: ' + NormalPriority,
+        'Low: ' + LowPriority,
+        'Idle: ' + IdlePriority,
+      ]);
+    });
+  }
+
   describe('delayed tasks', () => {
     it('schedules a delayed task', () => {
       scheduleCallback(