Introduce a connection pool for Mojang's session servers.

This has the potential to cut the time that players spend at the
"logging in..." (or "encrypting..." for 1.13+) screen by a fair
amount (gains of 200+ ms were noted for my own home connection).

While this sounds minor, I really do like to aim for all the details
and this is one of them.

proxy/src/main/java/com/velocitypowered/proxy/connection/http/NettyHttpClient.java

@@ -1,21 +1,50 @@
 package com.velocitypowered.proxy.connection.http;
 
 import com.velocitypowered.proxy.VelocityServer;
+import io.netty.bootstrap.Bootstrap;
 import io.netty.channel.*;
+import io.netty.channel.pool.*;
 import io.netty.handler.codec.http.*;
 import io.netty.handler.ssl.SslContext;
 import io.netty.handler.ssl.SslContextBuilder;
 import io.netty.handler.ssl.SslHandler;
 
 import javax.net.ssl.SSLEngine;
+import java.net.InetSocketAddress;
 import java.net.URL;
 import java.util.concurrent.CompletableFuture;
 
 public class NettyHttpClient {
-    private final VelocityServer server;
+    private final ChannelPoolMap<InetSocketAddress, SimpleChannelPool> poolMap;
 
     public NettyHttpClient(VelocityServer server) {
-        this.server = server;
+        Bootstrap bootstrap = server.initializeGenericBootstrap();
+        this.poolMap = new AbstractChannelPoolMap<InetSocketAddress, SimpleChannelPool>() {
+            @Override
+            protected SimpleChannelPool newPool(InetSocketAddress key) {
+                return new FixedChannelPool(bootstrap.remoteAddress(key), new ChannelPoolHandler() {
+                    @Override
+                    public void channelReleased(Channel channel) throws Exception {
+                        channel.pipeline().remove("collector");
+                    }
+
+                    @Override
+                    public void channelAcquired(Channel channel) throws Exception {
+                        System.out.println("ACQUIRED");
+                    }
+
+                    @Override
+                    public void channelCreated(Channel channel) throws Exception {
+                        if (key.getPort() == 443) {
+                            SslContext context = SslContextBuilder.forClient().build();
+                            SSLEngine engine = context.newEngine(channel.alloc());
+                            channel.pipeline().addLast("ssl", new SslHandler(engine));
+                        }
+                        channel.pipeline().addLast("http", new HttpClientCodec());
+                    }
+                }, 8);
+            }
+        };
     }
 
     public CompletableFuture<String> get(URL url) {
@@ -27,36 +56,26 @@ public class NettyHttpClient {
         }
 
         CompletableFuture<String> reply = new CompletableFuture<>();
-        server.initializeGenericBootstrap()
-                .handler(new ChannelInitializer<Channel>() {
-                    @Override
-                    protected void initChannel(Channel ch) throws Exception {
-                        if (ssl) {
-                            SslContext context = SslContextBuilder.forClient().build();
-                            SSLEngine engine = context.newEngine(ch.alloc());
-                            ch.pipeline().addLast(new SslHandler(engine));
-                        }
-                        ch.pipeline().addLast(new HttpClientCodec());
-                        ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
-                            @Override
-                            public void channelActive(ChannelHandlerContext ctx) throws Exception {
+        InetSocketAddress address = new InetSocketAddress(host, port);
+        poolMap.get(address)
+                .acquire()
+                .addListener(future -> {
+                    if (future.isSuccess()) {
+                        Channel channel = (Channel) future.getNow();
+                        channel.pipeline().addLast("collector", new SimpleHttpResponseCollector(reply));
+
                         DefaultFullHttpRequest request = new DefaultFullHttpRequest(HttpVersion.HTTP_1_1, HttpMethod.GET, url.getPath() + "?" + url.getQuery());
                         request.headers().add(HttpHeaderNames.HOST, url.getHost());
                         request.headers().add(HttpHeaderNames.USER_AGENT, "Velocity");
-                                ctx.writeAndFlush(request);
-                            }
+                        channel.writeAndFlush(request);
+
+                        reply.whenComplete((resp, err) -> {
+                            // Make sure to release this connection
+                            poolMap.get(address).release(channel, channel.voidPromise());
                         });
-                        ch.pipeline().addLast(new SimpleHttpResponseCollector(reply));
-                    }
-                })
-                .connect(host, port)
-                .addListener(new ChannelFutureListener() {
-                    @Override
-                    public void operationComplete(ChannelFuture future) throws Exception {
-                        if (!future.isSuccess()) {
+                    } else {
                         reply.completeExceptionally(future.cause());
                     }
-                    }
                 });
         return reply;
     }

proxy/src/main/java/com/velocitypowered/proxy/connection/http/SimpleHttpResponseCollector.java

@@ -2,10 +2,7 @@ package com.velocitypowered.proxy.connection.http;
 
 import io.netty.channel.ChannelHandlerContext;
 import io.netty.channel.ChannelInboundHandlerAdapter;
-import io.netty.handler.codec.http.HttpContent;
-import io.netty.handler.codec.http.HttpResponse;
-import io.netty.handler.codec.http.HttpResponseStatus;
-import io.netty.handler.codec.http.LastHttpContent;
+import io.netty.handler.codec.http.*;
 import io.netty.util.ReferenceCountUtil;
 
 import java.nio.charset.StandardCharsets;
@@ -14,6 +11,7 @@ import java.util.concurrent.CompletableFuture;
 class SimpleHttpResponseCollector extends ChannelInboundHandlerAdapter {
     private final StringBuilder buffer = new StringBuilder(1024);
     private final CompletableFuture<String> reply;
+    private boolean canKeepAlive;
 
     SimpleHttpResponseCollector(CompletableFuture<String> reply) {
         this.reply = reply;
@@ -23,18 +21,23 @@ class SimpleHttpResponseCollector extends ChannelInboundHandlerAdapter {
     public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
         try {
             if (msg instanceof HttpResponse) {
-                HttpResponseStatus status = ((HttpResponse) msg).status();
+                HttpResponse response = (HttpResponse) msg;
+                HttpResponseStatus status = response.status();
                 if (status != HttpResponseStatus.OK) {
-                    ctx.close();
                     reply.completeExceptionally(new RuntimeException("Unexpected status code " + status.code()));
+                    return;
                 }
+
+                this.canKeepAlive = HttpUtil.isKeepAlive(response);
             }
 
             if (msg instanceof HttpContent) {
                 buffer.append(((HttpContent) msg).content().toString(StandardCharsets.UTF_8));
 
                 if (msg instanceof LastHttpContent) {
+                    if (!canKeepAlive) {
                         ctx.close();
+                    }
                     reply.complete(buffer.toString());
                 }
             }
@@ -42,4 +45,10 @@ class SimpleHttpResponseCollector extends ChannelInboundHandlerAdapter {
             ReferenceCountUtil.release(msg);
         }
     }
+
+    @Override
+    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
+        ctx.close();
+        reply.completeExceptionally(cause);
+    }
 }