NetworkSession: immediate-send now causes a buffer flush when the packet is ready

instead of skipping queues and forcing sync compression as previously seen.

this maintains proper packet order and allows immediate-flush to be used to reduce latency in-game.

Small servers won't notice any difference, but for larger ones it may make a difference, since the buffer time effectively depends on the amount of load RakLib is under.
closes #3325

src/network/mcpe/NetworkSession.php

@@ -174,7 +174,7 @@ class NetworkSession{
 	 */
 	private array $sendBufferAckPromises = [];
 
-	/** @phpstan-var \SplQueue<array{CompressBatchPromise|string, list<PromiseResolver<true>>}> */
+	/** @phpstan-var \SplQueue<array{CompressBatchPromise|string, list<PromiseResolver<true>>, bool}> */
 	private \SplQueue $compressedQueue;
 	private bool $forceAsyncCompression = true;
 	private bool $enableCompression = false; //disabled until handshake completed
@@ -235,7 +235,7 @@ class NetworkSession{
 
 	private function onSessionStartSuccess() : void{
 		$this->logger->debug("Session start handshake completed, awaiting login packet");
-		$this->flushSendBuffer(true);
+		$this->flushGamePacketQueue();
 		$this->enableCompression = true;
 		$this->setHandler(new LoginPacketHandler(
 			$this->server,
@@ -529,7 +529,7 @@ class NetworkSession{
 				$this->addToSendBuffer(self::encodePacketTimed(PacketSerializer::encoder(), $evPacket));
 			}
 			if($immediate){
-				$this->flushSendBuffer(true);
+				$this->flushGamePacketQueue();
 			}
 
 			return true;
@@ -577,14 +577,12 @@ class NetworkSession{
 		$this->sendBuffer[] = $buffer;
 	}
 
-	private function flushSendBuffer(bool $immediate = false) : void{
+	private function flushGamePacketQueue() : void{
 		if(count($this->sendBuffer) > 0){
 			Timings::$playerNetworkSend->startTiming();
 			try{
 				$syncMode = null; //automatic
-				if($immediate){
+				if($this->forceAsyncCompression){
-					$syncMode = true;
-				}elseif($this->forceAsyncCompression){
 					$syncMode = false;
 				}
 
@@ -599,7 +597,9 @@ class NetworkSession{
 				$this->sendBuffer = [];
 				$ackPromises = $this->sendBufferAckPromises;
 				$this->sendBufferAckPromises = [];
-				$this->queueCompressedNoBufferFlush($batch, $immediate, $ackPromises);
+				//these packets were already potentially buffered for up to 50ms - make sure the transport layer doesn't
+				//delay them any longer
+				$this->queueCompressedNoGamePacketFlush($batch, networkFlush: true, ackPromises: $ackPromises);
 			}finally{
 				Timings::$playerNetworkSend->stopTiming();
 			}
@@ -619,8 +619,10 @@ class NetworkSession{
 	public function queueCompressed(CompressBatchPromise|string $payload, bool $immediate = false) : void{
 		Timings::$playerNetworkSend->startTiming();
 		try{
-			$this->flushSendBuffer($immediate); //Maintain ordering if possible
+			//if the next packet causes a flush, avoid unnecessarily flushing twice
-			$this->queueCompressedNoBufferFlush($payload, $immediate);
+			//however, if the next packet does *not* cause a flush, game packets should be flushed to avoid delays
+			$this->flushGamePacketQueue();
+			$this->queueCompressedNoGamePacketFlush($payload, $immediate);
 		}finally{
 			Timings::$playerNetworkSend->stopTiming();
 		}
@@ -631,22 +633,13 @@ class NetworkSession{
 	 *
 	 * @phpstan-param list<PromiseResolver<true>> $ackPromises
 	 */
-	private function queueCompressedNoBufferFlush(CompressBatchPromise|string $batch, bool $immediate = false, array $ackPromises = []) : void{
+	private function queueCompressedNoGamePacketFlush(CompressBatchPromise|string $batch, bool $networkFlush = false, array $ackPromises = []) : void{
 		Timings::$playerNetworkSend->startTiming();
 		try{
+			$this->compressedQueue->enqueue([$batch, $ackPromises, $networkFlush]);
 			if(is_string($batch)){
-				if($immediate){
-					//Skips all queues
-					$this->sendEncoded($batch, true, $ackPromises);
-				}else{
-					$this->compressedQueue->enqueue([$batch, $ackPromises]);
 				$this->flushCompressedQueue();
-				}
-			}elseif($immediate){
-				//Skips all queues
-				$this->sendEncoded($batch->getResult(), true, $ackPromises);
 			}else{
-				$this->compressedQueue->enqueue([$batch, $ackPromises]);
 				$batch->onResolve(function() : void{
 					if($this->connected){
 						$this->flushCompressedQueue();
@@ -663,14 +656,14 @@ class NetworkSession{
 		try{
 			while(!$this->compressedQueue->isEmpty()){
 				/** @var CompressBatchPromise|string $current */
-				[$current, $ackPromises] = $this->compressedQueue->bottom();
+				[$current, $ackPromises, $networkFlush] = $this->compressedQueue->bottom();
 				if(is_string($current)){
 					$this->compressedQueue->dequeue();
-					$this->sendEncoded($current, false, $ackPromises);
+					$this->sendEncoded($current, $networkFlush, $ackPromises);
 
 				}elseif($current->hasResult()){
 					$this->compressedQueue->dequeue();
-					$this->sendEncoded($current->getResult(), false, $ackPromises);
+					$this->sendEncoded($current->getResult(), $networkFlush, $ackPromises);
 
 				}else{
 					//can't send any more queued until this one is ready
@@ -710,7 +703,7 @@ class NetworkSession{
 			$this->disconnectGuard = true;
 			$func();
 			$this->disconnectGuard = false;
-			$this->flushSendBuffer(true);
+			$this->flushGamePacketQueue();
 			$this->sender->close("");
 			foreach($this->disposeHooks as $callback){
 				$callback();
@@ -1345,6 +1338,6 @@ class NetworkSession{
 			Timings::$playerNetworkSendInventorySync->stopTiming();
 		}
 
-		$this->flushSendBuffer();
+		$this->flushGamePacketQueue();
 	}
 }