Improve flatScheduleTask (#3079)

Motivation:

As pointed out in #3071, the `flatScheduleTask` implementations can be
improved.

Modifications:

- Refactor the `flatScheduleTask` implementations to skip `flatMap`
calls, which avoids creating an extra promise.
- As there is now a lower number of allocations, reduce the necessary
thresholds for the allocation tests.

Result:

Reduction in the number of allocations in the package.

---------

Co-authored-by: George Barnett <[email protected]>

IntegrationTests/tests_04_performance/Thresholds/5.10.json

@@ -33,8 +33,8 @@
     "encode_1000_ws_frames_new_buffer_with_space": 3050,
     "encode_1000_ws_frames_new_buffer_with_space_with_mask": 5050,
     "execute_hop_10000_tasks": 0,
-    "flat_schedule_10000_tasks": 130100,
-    "flat_schedule_assume_isolated_10000_tasks": 100100,
+    "flat_schedule_10000_tasks": 100100,
+    "flat_schedule_assume_isolated_10000_tasks": 90100,
     "future_assume_isolated_lots_of_callbacks": 74050,
     "future_erase_result": 4050,
     "future_lots_of_callbacks": 74050,

IntegrationTests/tests_04_performance/Thresholds/5.9.json

@@ -33,8 +33,8 @@
     "encode_1000_ws_frames_new_buffer_with_space": 3050,
     "encode_1000_ws_frames_new_buffer_with_space_with_mask": 5050,
     "execute_hop_10000_tasks": 0,
-    "flat_schedule_10000_tasks": 130100,
-    "flat_schedule_assume_isolated_10000_tasks": 100100,
+    "flat_schedule_10000_tasks": 100100,
+    "flat_schedule_assume_isolated_10000_tasks": 90100,
     "future_assume_isolated_lots_of_callbacks": 74050,
     "future_erase_result": 4050,
     "future_lots_of_callbacks": 74050,

IntegrationTests/tests_04_performance/Thresholds/6.0.json

@@ -19,7 +19,7 @@
     "1000_udp_reqs": 6050,
     "1000_udpbootstraps": 2050,
     "1000_udpconnections": 75050,
-    "1_reqs_1000_conn": 384050,
+    "1_reqs_1000_conn": 384000,
     "assume_isolated_scheduling_10000_executions": 89,
     "bytebuffer_lots_of_rw": 2050,
     "creating_10000_headers": 0,
@@ -33,8 +33,8 @@
     "encode_1000_ws_frames_new_buffer_with_space": 3050,
     "encode_1000_ws_frames_new_buffer_with_space_with_mask": 5050,
     "execute_hop_10000_tasks": 0,
-    "flat_schedule_10000_tasks": 130100,
-    "flat_schedule_assume_isolated_10000_tasks": 100100,
+    "flat_schedule_10000_tasks": 100100,
+    "flat_schedule_assume_isolated_10000_tasks": 90100,
     "future_assume_isolated_lots_of_callbacks": 74050,
     "future_erase_result": 4050,
     "future_lots_of_callbacks": 74050,

IntegrationTests/tests_04_performance/Thresholds/nightly-6.1.json

@@ -33,8 +33,8 @@
     "encode_1000_ws_frames_new_buffer_with_space": 3050,
     "encode_1000_ws_frames_new_buffer_with_space_with_mask": 5050,
     "execute_hop_10000_tasks": 0,
-    "flat_schedule_10000_tasks": 130100,
-    "flat_schedule_assume_isolated_10000_tasks": 100100,
+    "flat_schedule_10000_tasks": 100100,
+    "flat_schedule_assume_isolated_10000_tasks": 90100,
     "future_assume_isolated_lots_of_callbacks": 74050,
     "future_erase_result": 4050,
     "future_lots_of_callbacks": 74050,

IntegrationTests/tests_04_performance/Thresholds/nightly-main.json

@@ -33,8 +33,8 @@
     "encode_1000_ws_frames_new_buffer_with_space": 3050,
     "encode_1000_ws_frames_new_buffer_with_space_with_mask": 5050,
     "execute_hop_10000_tasks": 0,
-    "flat_schedule_10000_tasks": 130100,
-    "flat_schedule_assume_isolated_10000_tasks": 100100,
+    "flat_schedule_10000_tasks": 100100,
+    "flat_schedule_assume_isolated_10000_tasks": 80100,
     "future_assume_isolated_lots_of_callbacks": 74050,
     "future_erase_result": 4050,
     "future_lots_of_callbacks": 74050,

Sources/NIOCore/EventLoop.swift

@@ -1009,7 +1009,15 @@ extension EventLoop {
         let promise: EventLoopPromise<T> = self.makePromise(file: file, line: line)
         let scheduled = self.scheduleTask(deadline: deadline, task)
 
-        scheduled.futureResult.flatMap { $0 }.cascade(to: promise)
+        scheduled.futureResult.whenComplete { result in
+            switch result {
+            case .success(let futureResult):
+                promise.completeWith(futureResult)
+            case .failure(let error):
+                promise.fail(error)
+            }
+        }
+
         return .init(promise: promise, cancellationTask: { scheduled.cancel() })
     }
 
@@ -1051,7 +1059,15 @@ extension EventLoop {
         let promise: EventLoopPromise<T> = self.makePromise(file: file, line: line)
         let scheduled = self.scheduleTask(in: delay, task)
 
-        scheduled.futureResult.flatMap { $0 }.cascade(to: promise)
+        scheduled.futureResult.whenComplete { result in
+            switch result {
+            case .success(let futureResult):
+                promise.completeWith(futureResult)
+            case .failure(let error):
+                promise.fail(error)
+            }
+        }
+
         return .init(promise: promise, cancellationTask: { scheduled.cancel() })
     }
 

Sources/NIOCore/EventLoopFuture+AssumeIsolated.swift

@@ -113,7 +113,15 @@ public struct NIOIsolatedEventLoop {
         let promise: EventLoopPromise<T> = self._wrapped.makePromise(file: file, line: line)
         let scheduled = self.scheduleTask(deadline: deadline, task)
 
-        scheduled.futureResult.flatMap { $0 }.cascade(to: promise)
+        scheduled.futureResult.whenComplete { result in
+            switch result {
+            case .success(let futureResult):
+                promise.completeWith(futureResult)
+            case .failure(let error):
+                promise.fail(error)
+            }
+        }
+
         return .init(promise: promise, cancellationTask: { scheduled.cancel() })
     }