Deprecate ambiguous task_size fields

proto/scheduler.proto

@@ -118,12 +118,24 @@ message TaskSize {
 
 // Next ID: 9
 message SchedulingMetadata {
-  // Task size used for scheduling purposes, when the scheduler is deciding
-  // which executors (if any) may execute a task, and also when an executor is
-  // deciding which task to dequeue. Executors may see a different value of this
-  // field than what the scheduler sees, depending on measured_task_size or
-  // predicted_task_size. See documentation of those fields for more info.
-  TaskSize task_size = 1;
+  // DEPRECATED. This field has two different meanings:
+  // 1. The execution server sets this to the task size "estimate" which only
+  //    incorporates the default task size and user-requested size, and not any
+  //    historical information or model predictions. This estimate is now split
+  //    into two separate fields: default_task_size and requested_size.
+  // 2. Just before scheduling, the scheduler sets this to the "final" task size
+  //    that should be used by the executor, which incorporates the measured
+  //    size or model-predicated size as applicable. This is replaced by
+  //    EnqueueTaskReservationRequest.task_size.
+  TaskSize task_size = 1 [deprecated = true];
+
+  // A default task size based on some hard-coded parameters. For example,
+  // non-test actions may be given some default task size, test actions
+  // declaring TEST_SIZE=large in their environment variables may be given a
+  // predefined number of resources, etc. Certain other rules may apply; e.g. VM
+  // isolation may require additional resources. If no better estimate is
+  // available, this is the size that is used.
+  TaskSize default_task_size = 13;
 
   // Task size measured from a previous task execution of a similar task, if
   // such data is available.
@@ -206,7 +218,13 @@ message ReEnqueueTaskResponse {
 
 message EnqueueTaskReservationRequest {
   string task_id = 1;
-  TaskSize task_size = 2;
+
+  // The size of the task that determines how many resources the executor should
+  // allocate for this task.
+  //
+  // DEPRECATED: use scheduling_metadata.scheduled_size instead.
+  TaskSize task_size = 2 [deprecated = true];
+
   SchedulingMetadata scheduling_metadata = 3;
 
   // If set, enqueue the task reservation on the given executor instance if it