Reduce allocations on InlineStreamMultiplexer/createStreamChannel (#450)

## Motivation
When creating stream channels on the `InlineStreamMultiplexer`, we are currently performing a `flatSubmit` on two different codepaths. This allocates two ELFs: one on `submit`, and one on `flatMap`. This is unnecessary, as we could do with just one allocation.

## Modifications
Create a single ELP and call `execute` on the EL instead of using `flatSubmit`.

## Result
Fewer allocations

Author: gjcairo

Sources/NIOHTTP2/HTTP2ChannelHandler+InlineStreamMultiplexer.swift

@@ -151,6 +151,11 @@ extension InlineStreamMultiplexer {
         self._commonStreamMultiplexer.createStreamChannel(multiplexer: .inline(self), streamStateInitializer)
     }
 
+    @inlinable
+    internal func createStreamChannel<Output: Sendable>(promise: EventLoopPromise<Output>?, _ initializer: @escaping NIOChannelInitializerWithOutput<Output>) {
+        self._commonStreamMultiplexer.createStreamChannel(multiplexer: .inline(self), promise: promise, initializer)
+    }
+
     @inlinable
     internal func createStreamChannel<Output: Sendable>(_ initializer: @escaping NIOChannelInitializerWithOutput<Output>) -> EventLoopFuture<Output> {
         self._commonStreamMultiplexer.createStreamChannel(multiplexer: .inline(self), initializer)
@@ -284,19 +289,23 @@ extension InlineStreamMultiplexer {
             // Always create streams channels on the next event loop tick. This avoids re-entrancy
             // issues where handlers interposed between the two HTTP/2 handlers could create streams
             // in channel active which become activated twice.
-            return self._eventLoop.flatSubmit {
-                self._inlineStreamMultiplexer.createStreamChannel(initializer)
+            let promise: EventLoopPromise<Channel> = self._eventLoop.makePromise()
+            self._eventLoop.execute {
+                self._inlineStreamMultiplexer.createStreamChannel(promise: promise, initializer)
             }
+            return promise.futureResult
         }
 
         @inlinable
         internal func createStreamChannel<Output: Sendable>(_ initializer: @escaping NIOChannelInitializerWithOutput<Output>) -> EventLoopFuture<Output> {
             // Always create streams channels on the next event loop tick. This avoids re-entrancy
             // issues where handlers interposed between the two HTTP/2 handlers could create streams
             // in channel active which become activated twice.
-            return self._eventLoop.flatSubmit {
-                self._inlineStreamMultiplexer.createStreamChannel(initializer)
+            let promise: EventLoopPromise<Output> = self._eventLoop.makePromise()
+            self._eventLoop.execute {
+                self._inlineStreamMultiplexer.createStreamChannel(promise: promise, initializer)
             }
+            return promise.futureResult
         }
 
         internal func setChannelContinuation(_ streamChannels: any AnyContinuation) {

docker/docker-compose.2204.510.yaml

@@ -40,7 +40,7 @@ services:
       - MAX_ALLOCS_ALLOWED_client_server_request_response_many_inline=889050
       - MAX_ALLOCS_ALLOWED_create_client_stream_channel=37050
       - MAX_ALLOCS_ALLOWED_create_client_stream_channel_inline=37050
-      - MAX_ALLOCS_ALLOWED_create_client_stream_channel_inline_no_promise_based_API=44050
+      - MAX_ALLOCS_ALLOWED_create_client_stream_channel_inline_no_promise_based_API=37050
       - MAX_ALLOCS_ALLOWED_create_client_stream_channel_no_promise_based_API=37050
       - MAX_ALLOCS_ALLOWED_get_100000_headers_canonical_form=200050
       - MAX_ALLOCS_ALLOWED_get_100000_headers_canonical_form_trimming_whitespace=200050

docker/docker-compose.2204.58.yaml

@@ -40,7 +40,7 @@ services:
       - MAX_ALLOCS_ALLOWED_client_server_request_response_many_inline=889050
       - MAX_ALLOCS_ALLOWED_create_client_stream_channel=37050
       - MAX_ALLOCS_ALLOWED_create_client_stream_channel_inline=37050
-      - MAX_ALLOCS_ALLOWED_create_client_stream_channel_inline_no_promise_based_API=44050
+      - MAX_ALLOCS_ALLOWED_create_client_stream_channel_inline_no_promise_based_API=37050
       - MAX_ALLOCS_ALLOWED_create_client_stream_channel_no_promise_based_API=37050
       - MAX_ALLOCS_ALLOWED_get_100000_headers_canonical_form=200050
       - MAX_ALLOCS_ALLOWED_get_100000_headers_canonical_form_trimming_whitespace=200050

docker/docker-compose.2204.59.yaml

@@ -40,7 +40,7 @@ services:
       - MAX_ALLOCS_ALLOWED_client_server_request_response_many_inline=889050
       - MAX_ALLOCS_ALLOWED_create_client_stream_channel=37050
       - MAX_ALLOCS_ALLOWED_create_client_stream_channel_inline=37050
-      - MAX_ALLOCS_ALLOWED_create_client_stream_channel_inline_no_promise_based_API=44050
+      - MAX_ALLOCS_ALLOWED_create_client_stream_channel_inline_no_promise_based_API=37050
       - MAX_ALLOCS_ALLOWED_create_client_stream_channel_no_promise_based_API=37050
       - MAX_ALLOCS_ALLOWED_get_100000_headers_canonical_form=200050
       - MAX_ALLOCS_ALLOWED_get_100000_headers_canonical_form_trimming_whitespace=200050

docker/docker-compose.2204.main.yaml

@@ -39,7 +39,7 @@ services:
       - MAX_ALLOCS_ALLOWED_client_server_request_response_many_inline=889050
       - MAX_ALLOCS_ALLOWED_create_client_stream_channel=37050
       - MAX_ALLOCS_ALLOWED_create_client_stream_channel_inline=37050
-      - MAX_ALLOCS_ALLOWED_create_client_stream_channel_inline_no_promise_based_API=44050
+      - MAX_ALLOCS_ALLOWED_create_client_stream_channel_inline_no_promise_based_API=37050
       - MAX_ALLOCS_ALLOWED_create_client_stream_channel_no_promise_based_API=37050
       - MAX_ALLOCS_ALLOWED_get_100000_headers_canonical_form=200050
       - MAX_ALLOCS_ALLOWED_get_100000_headers_canonical_form_trimming_whitespace=200050