Correctly handle rapid disconnects. Fixes #31

proxy/src/main/java/com/velocitypowered/proxy/connection/MinecraftConnection.java

@@ -104,19 +104,22 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
     }
 
     public void write(Object msg) {
-        ensureOpen();
+        if (channel.isActive()) {
             channel.writeAndFlush(msg, channel.voidPromise());
         }
+    }
 
     public void delayedWrite(Object msg) {
-        ensureOpen();
+        if (channel.isActive()) {
             channel.write(msg, channel.voidPromise());
         }
+    }
 
     public void flush() {
-        ensureOpen();
+        if (channel.isActive()) {
             channel.flush();
         }
+    }
 
     public void closeWith(Object msg) {
         if (channel.isActive()) {

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -18,6 +18,13 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
 
     @Override
     public void handle(MinecraftPacket packet) {
+        if (!connection.getProxyPlayer().isActive()) {
+            // Connection was left open accidentally. Close it so as to avoid "You logged in from another location"
+            // errors.
+            connection.getMinecraftConnection().close();
+            return;
+        }
+
         ClientPlaySessionHandler playerHandler =
                 (ClientPlaySessionHandler) connection.getProxyPlayer().getConnection().getSessionHandler();
         if (packet instanceof KeepAlive) {
@@ -69,6 +76,13 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
 
     @Override
     public void handleUnknown(ByteBuf buf) {
+        if (!connection.getProxyPlayer().isActive()) {
+            // Connection was left open accidentally. Close it so as to avoid "You logged in from another location"
+            // errors.
+            connection.getMinecraftConnection().close();
+            return;
+        }
+
         ClientPlaySessionHandler playerHandler =
                 (ClientPlaySessionHandler) connection.getProxyPlayer().getConnection().getSessionHandler();
         ByteBuf remapped = playerHandler.getIdRemapper().remap(buf, ProtocolConstants.Direction.CLIENTBOUND);

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ConnectedPlayer.java

@@ -132,7 +132,6 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
         String error = ThrowableUtils.briefDescription(throwable);
         String userMessage;
         if (connectedServer != null && connectedServer.getServerInfo().equals(info)) {
-            logger.error("{}: exception occurred in connection to {}", this, info.getName(), throwable);
             userMessage = "Exception in server " + info.getName();
         } else {
             logger.error("{}: unable to connect to server {}", this, info.getName(), throwable);