rewrite multiplayer sync to use generic callbacks from client/server for better extensibility (#480)

10 files changed, 383 insertions, 233 deletions

diff --git a/src/SMAPI/Events/ModMessageReceivedEventArgs.cs b/src/SMAPI/Events/ModMessageReceivedEventArgs.cs
index b1960a22..49366ec6 100644
--- a/src/SMAPI/Events/ModMessageReceivedEventArgs.cs
+++ b/src/SMAPI/Events/ModMessageReceivedEventArgs.cs
@@ -3,7 +3,7 @@ using StardewModdingAPI.Framework.Networking;
 
 namespace StardewModdingAPI.Events
 {
-    /// <summary>Event arguments when a mod receives a message over the network.</summary>
+    /// <summary>Event arguments for an <see cref="IMultiplayerEvents.ModMessageReceived"/> event.</summary>
     public class ModMessageReceivedEventArgs : EventArgs
     {
         /*********
diff --git a/src/SMAPI/Framework/Networking/MessageType.cs b/src/SMAPI/Framework/Networking/MessageType.cs
new file mode 100644
index 00000000..bd9acfa9
--- /dev/null
+++ b/src/SMAPI/Framework/Networking/MessageType.cs
@@ -0,0 +1,26 @@
+using StardewValley;
+
+namespace StardewModdingAPI.Framework.Networking
+{
+    /// <summary>Network message types recognised by SMAPI and Stardew Valley.</summary>
+    internal enum MessageType : byte
+    {
+        /*********
+        ** SMAPI
+        *********/
+        /// <summary>A data message intended for mods to consume.</summary>
+        ModMessage = 254,
+
+        /// <summary>Metadata context about a player synced by SMAPI.</summary>
+        ModContext = 255,
+
+        /*********
+        ** Vanilla
+        *********/
+        /// <summary>Metadata about the host server sent to a farmhand.</summary>
+        ServerIntroduction = Multiplayer.serverIntroduction,
+
+        /// <summary>Metadata about a player sent to a farmhand or server.</summary>
+        PlayerIntroduction = Multiplayer.playerIntroduction
+    }
+}
diff --git a/src/SMAPI/Framework/Networking/MultiplayerPeer.cs b/src/SMAPI/Framework/Networking/MultiplayerPeer.cs
index c7f8ffad..e703dbb1 100644
--- a/src/SMAPI/Framework/Networking/MultiplayerPeer.cs
+++ b/src/SMAPI/Framework/Networking/MultiplayerPeer.cs
@@ -2,7 +2,6 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using Lidgren.Network;
-using StardewValley;
 using StardewValley.Network;
 
 namespace StardewModdingAPI.Framework.Networking
@@ -95,7 +94,8 @@ namespace StardewModdingAPI.Framework.Networking
         /// <param name="playerID">The player's unique ID.</param>
         /// <param name="model">The metadata to copy.</param>
         /// <param name="client">The client through which to send messages.</param>
-        public static MultiplayerPeer ForConnectionToHost(long playerID, RemoteContextModel model, SLidgrenClient client)
+        /// <param name="isHost">Whether this connection is for the host player.</param>
+        public static MultiplayerPeer ForConnectionToHost(long playerID, RemoteContextModel model, SLidgrenClient client, bool isHost)
         {
             return new MultiplayerPeer(
                 playerID: playerID,
@@ -103,7 +103,7 @@ namespace StardewModdingAPI.Framework.Networking
                 server: null,
                 serverConnection: null,
                 client: client,
-                isHost: true
+                isHost: isHost
             );
         }
 
diff --git a/src/SMAPI/Framework/Networking/SLidgrenClient.cs b/src/SMAPI/Framework/Networking/SLidgrenClient.cs
index 9dfdba15..c05e6b76 100644
--- a/src/SMAPI/Framework/Networking/SLidgrenClient.cs
+++ b/src/SMAPI/Framework/Networking/SLidgrenClient.cs
@@ -1,44 +1,38 @@
 using System;
-using StardewValley;
 using StardewValley.Network;
 
 namespace StardewModdingAPI.Framework.Networking
 {
-    /// <summary>A multiplayer client used to connect to a hosted server. This is an implementation of <see cref="LidgrenClient"/> that adds support for SMAPI's metadata context exchange.</summary>
+    /// <summary>A multiplayer client used to connect to a hosted server. This is an implementation of <see cref="LidgrenClient"/> with callbacks for SMAPI functionality.</summary>
     internal class SLidgrenClient : LidgrenClient
     {
         /*********
         ** Properties
         *********/
-        /// <summary>Get the metadata to include in a metadata message sent to other players.</summary>
-        private readonly Func<object[]> GetMetadataMessageFields;
-
-        /// <summary>The method to call when receiving a custom SMAPI message from the server, which returns whether the message was processed.</summary>
-        private readonly Func<SLidgrenClient, IncomingMessage, bool> TryProcessMessage;
+        /// <summary>A callback to raise when receiving a message. This receives the client instance, incoming message, and a callback to run the default logic.</summary>
+        private readonly Action<SLidgrenClient, IncomingMessage, Action> OnProcessingMessage;
 
+        /// <summary>A callback to raise when sending a message. This receives the client instance, outgoing message, and a callback to run the default logic.</summary>
+        private readonly Action<SLidgrenClient, OutgoingMessage, Action> OnSendingMessage;
 
         /*********
         ** Public methods
         *********/
         /// <summary>Construct an instance.</summary>
         /// <param name="address">The remote address being connected.</param>
-        /// <param name="getMetadataMessageFields">Get the metadata to include in a metadata message sent to other players.</param>
-        /// <param name="tryProcessMessage">The method to call when receiving a custom SMAPI message from the server, which returns whether the message was processed..</param>
-        public SLidgrenClient(string address, Func<object[]> getMetadataMessageFields, Func<SLidgrenClient, IncomingMessage, bool> tryProcessMessage)
+        /// <param name="onProcessingMessage">A callback to raise when receiving a message. This receives the client instance, incoming message, and a callback to run the default logic.</param>
+        /// <param name="onSendingMessage">A callback to raise when sending a message. This receives the client instance, outgoing message, and a callback to run the default logic.</param>
+        public SLidgrenClient(string address, Action<SLidgrenClient, IncomingMessage, Action> onProcessingMessage, Action<SLidgrenClient, OutgoingMessage, Action> onSendingMessage)
             : base(address)
         {
-            this.GetMetadataMessageFields = getMetadataMessageFields;
-            this.TryProcessMessage = tryProcessMessage;
+            this.OnProcessingMessage = onProcessingMessage;
+            this.OnSendingMessage = onSendingMessage;
         }
 
-        /// <summary>Send the metadata needed to connect with a remote server.</summary>
-        public override void sendPlayerIntroduction()
+        /// <summary>Send a message to the connected peer.</summary>
+        public override void sendMessage(OutgoingMessage message)
         {
-            // send custom intro
-            if (this.getUserID() != "")
-                Game1.player.userID.Value = this.getUserID();
-            this.sendMessage(SMultiplayer.ContextSyncMessageID, this.GetMetadataMessageFields());
-            base.sendPlayerIntroduction();
+            this.OnSendingMessage(this, message, () => base.sendMessage(message));
         }
 
 
@@ -49,10 +43,7 @@ namespace StardewModdingAPI.Framework.Networking
         /// <param name="message">The message to process.</param>
         protected override void processIncomingMessage(IncomingMessage message)
         {
-            if (this.TryProcessMessage(this, message))
-                return;
-
-            base.processIncomingMessage(message);
+            this.OnProcessingMessage(this, message, () => base.processIncomingMessage(message));
         }
     }
 }
diff --git a/src/SMAPI/Framework/Networking/SLidgrenServer.cs b/src/SMAPI/Framework/Networking/SLidgrenServer.cs
index 971eb66d..060b433b 100644
--- a/src/SMAPI/Framework/Networking/SLidgrenServer.cs
+++ b/src/SMAPI/Framework/Networking/SLidgrenServer.cs
@@ -1,5 +1,11 @@
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.IO;
 using System.Reflection;
 using Lidgren.Network;
+using StardewModdingAPI.Framework.Reflection;
+using StardewModdingAPI.Patches;
+using StardewValley;
 using StardewValley.Network;
 
 namespace StardewModdingAPI.Framework.Networking
@@ -10,8 +16,27 @@ namespace StardewModdingAPI.Framework.Networking
         /*********
         ** Properties
         *********/
-        /// <summary>A method which sends a message through a specific connection.</summary>
-        private readonly MethodInfo SendMessageToConnectionMethod;
+
+        /// <summary>The constructor for the internal <c>NetBufferReadStream</c> type.</summary>
+        private readonly ConstructorInfo NetBufferReadStreamConstructor = SLidgrenServer.GetNetBufferReadStreamConstructor();
+
+        /// <summary>The constructor for the internal <c>NetBufferWriteStream</c> type.</summary>
+        private readonly ConstructorInfo NetBufferWriteStreamConstructor = SLidgrenServer.GetNetBufferWriteStreamConstructor();
+
+        /// <summary>A method which reads farmer data from the given binary reader.</summary>
+        private readonly Func<BinaryReader, NetFarmerRoot> ReadFarmer;
+
+        /// <summary>A callback to raise when receiving a message. This receives the server instance, raw/parsed incoming message, and a callback to run the default logic.</summary>
+        private readonly Action<SLidgrenServer, NetIncomingMessage, IncomingMessage, Action> OnProcessingMessage;
+
+        /// <summary>A callback to raise when sending a message. This receives the server instance, outgoing connection, outgoing message, target player ID, and a callback to run the default logic.</summary>
+        private readonly Action<SLidgrenServer, NetConnection, OutgoingMessage, Action> OnSendingMessage;
+
+        /// <summary>The peer connections.</summary>
+        private readonly Bimap<long, NetConnection> Peers;
+
+        /// <summary>The underlying net server.</summary>
+        private readonly IReflectedField<NetServer> Server;
 
 
         /*********
@@ -19,18 +44,105 @@ namespace StardewModdingAPI.Framework.Networking
         *********/
         /// <summary>Construct an instance.</summary>
         /// <param name="gameServer">The underlying game server.</param>
-        public SLidgrenServer(IGameServer gameServer)
+        /// <param name="reflection">Simplifies access to private code.</param>
+        /// <param name="readFarmer">A method which reads farmer data from the given binary reader.</param>
+        /// <param name="onProcessingMessage">A callback to raise when receiving a message. This receives the server instance, raw/parsed incoming message, and a callback to run the default logic.</param>
+        /// <param name="onSendingMessage">A callback to raise when sending a message. This receives the server instance, outgoing connection, outgoing message, and a callback to run the default logic.</param>
+        public SLidgrenServer(IGameServer gameServer, Reflector reflection, Func<BinaryReader, NetFarmerRoot> readFarmer, Action<SLidgrenServer, NetIncomingMessage, IncomingMessage, Action> onProcessingMessage, Action<SLidgrenServer, NetConnection, OutgoingMessage, Action> onSendingMessage)
             : base(gameServer)
         {
-            this.SendMessageToConnectionMethod = typeof(LidgrenServer).GetMethod(nameof(LidgrenServer.sendMessage), BindingFlags.NonPublic | BindingFlags.Instance, null, new[] { typeof(NetConnection), typeof(OutgoingMessage) }, null);
+            this.ReadFarmer = readFarmer;
+            this.OnProcessingMessage = onProcessingMessage;
+            this.OnSendingMessage = onSendingMessage;
+            this.Peers = reflection.GetField<Bimap<long, NetConnection>>(this, "peers").GetValue();
+            this.Server = reflection.GetField<NetServer>(this, "server");
         }
 
         /// <summary>Send a message to a remote server.</summary>
         /// <param name="connection">The network connection.</param>
         /// <param name="message">The message to send.</param>
+        /// <remarks>This is an implementation of <see cref="LidgrenServer.sendMessage(NetConnection, OutgoingMessage)"/> which calls <see cref="OnSendingMessage"/>. This method is invoked via <see cref="LidgrenServerPatch.Prefix_LidgrenServer_SendMessage"/>.</remarks>
         public void SendMessage(NetConnection connection, OutgoingMessage message)
         {
-            this.SendMessageToConnectionMethod.Invoke(this, new object[] { connection, message });
+            this.OnSendingMessage(this, connection, message, () =>
+            {
+                NetServer server = this.Server.GetValue();
+                NetOutgoingMessage netMessage = server.CreateMessage();
+                using (Stream bufferWriteStream = (Stream)this.NetBufferWriteStreamConstructor.Invoke(new object[] { netMessage }))
+                using (BinaryWriter writer = new BinaryWriter(bufferWriteStream))
+                    message.Write(writer);
+
+                server.SendMessage(netMessage, connection, NetDeliveryMethod.ReliableOrdered);
+            });
+        }
+
+        /// <summary>Parse a data message from a client.</summary>
+        /// <param name="rawMessage">The raw network message to parse.</param>
+        /// <remarks>This is an implementation of <see cref="LidgrenServer.parseDataMessageFromClient"/> which calls <see cref="OnProcessingMessage"/>. This method is invoked via <see cref="LidgrenServerPatch.Prefix_LidgrenServer_ParseDataMessageFromClient"/>.</remarks>
+        [SuppressMessage("ReSharper", "AccessToDisposedClosure", Justification = "The callback is invoked synchronously.")]
+        public bool ParseDataMessageFromClient(NetIncomingMessage rawMessage)
+        {
+            // add hook to call multiplayer core
+            NetConnection peer = rawMessage.SenderConnection;
+            using (IncomingMessage message = new IncomingMessage())
+            using (Stream readStream = (Stream)this.NetBufferReadStreamConstructor.Invoke(new object[] { rawMessage }))
+            using (BinaryReader reader = new BinaryReader(readStream))
+            {
+                while (rawMessage.LengthBits - rawMessage.Position >= 8)
+                {
+                    message.Read(reader);
+                    this.OnProcessingMessage(this, rawMessage, message, () =>
+                    {
+                        if (this.Peers.ContainsLeft(message.FarmerID) && this.Peers[message.FarmerID] == peer)
+                            this.gameServer.processIncomingMessage(message);
+                        else if (message.MessageType == Multiplayer.playerIntroduction)
+                        {
+                            NetFarmerRoot farmer = this.ReadFarmer(message.Reader);
+                            this.gameServer.checkFarmhandRequest("", farmer, msg => this.SendMessage(peer, msg), () => this.Peers[farmer.Value.UniqueMultiplayerID] = peer);
+                        }
+                    });
+                }
+            }
+
+            return false;
+        }
+
+
+        /*********
+        ** Private methods
+        *********/
+        /// <summary>Get the constructor for the internal <c>NetBufferReadStream</c> type.</summary>
+        private static ConstructorInfo GetNetBufferReadStreamConstructor()
+        {
+            // get type
+            string typeName = $"StardewValley.Network.NetBufferReadStream, {Constants.GameAssemblyName}";
+            Type type = Type.GetType(typeName);
+            if (type == null)
+                throw new InvalidOperationException($"Can't find type: {typeName}");
+
+            // get constructor
+            ConstructorInfo constructor = type.GetConstructor(new[] { typeof(NetBuffer) });
+            if (constructor == null)
+                throw new InvalidOperationException($"Can't find constructor for type: {typeName}");
+
+            return constructor;
+        }
+
+        /// <summary>Get the constructor for the internal <c>NetBufferWriteStream</c> type.</summary>
+        private static ConstructorInfo GetNetBufferWriteStreamConstructor()
+        {
+            // get type
+            string typeName = $"StardewValley.Network.NetBufferWriteStream, {Constants.GameAssemblyName}";
+            Type type = Type.GetType(typeName);
+            if (type == null)
+                throw new InvalidOperationException($"Can't find type: {typeName}");
+
+            // get constructor
+            ConstructorInfo constructor = type.GetConstructor(new[] { typeof(NetBuffer) });
+            if (constructor == null)
+                throw new InvalidOperationException($"Can't find constructor for type: {typeName}");
+
+            return constructor;
         }
     }
 }
diff --git a/src/SMAPI/Framework/SCore.cs b/src/SMAPI/Framework/SCore.cs
index ca343389..f078acba 100644
--- a/src/SMAPI/Framework/SCore.cs
+++ b/src/SMAPI/Framework/SCore.cs
@@ -162,7 +162,7 @@ namespace StardewModdingAPI.Framework
             // apply game patches
             new GamePatcher(this.Monitor).Apply(
                 new DialogueErrorPatch(this.MonitorForGame, this.Reflection),
-                new NetworkingPatch()
+                new LidgrenServerPatch()
             );
         }
 
diff --git a/src/SMAPI/Framework/SMultiplayer.cs b/src/SMAPI/Framework/SMultiplayer.cs
index e4f912d2..70f1a89a 100644
--- a/src/SMAPI/Framework/SMultiplayer.cs
+++ b/src/SMAPI/Framework/SMultiplayer.cs
@@ -14,6 +14,17 @@ using StardewValley.Network;
 namespace StardewModdingAPI.Framework
 {
     /// <summary>SMAPI's implementation of the game's core multiplayer logic.</summary>
+    /// <remarks>
+    /// SMAPI syncs mod context to all players through the host as such:
+    ///   1. Farmhand sends ModContext + PlayerIntro.
+    ///   2. If host receives ModContext: it stores the context, replies with known contexts, and forwards it to other farmhands.
+    ///   3. If host receives PlayerIntro before ModContext: it stores a 'vanilla player' context, and forwards it to other farmhands.
+    ///   4. If farmhand receives ModContext: it stores it.
+    ///   5. If farmhand receives ServerIntro without a preceding ModContext: it stores a 'vanilla host' context.
+    ///   6. If farmhand receives PlayerIntro without a preceding ModContext AND it's not the host peer: it stores a 'vanilla player' context.
+    ///
+    /// Once a farmhand/server stored a context, messages can be sent to that player through the SMAPI APIs.
+    /// </remarks>
     internal class SMultiplayer : Multiplayer
     {
         /*********
@@ -47,12 +58,6 @@ namespace StardewModdingAPI.Framework
         /*********
         ** Accessors
         *********/
-        /// <summary>The message ID for a SMAPI message containing context about a player.</summary>
-        public const byte ContextSyncMessageID = 255;
-
-        /// <summary>The message ID for a mod message.</summary>
-        public const byte ModMessageID = 254;
-
         /// <summary>The metadata for each connected peer.</summary>
         public IDictionary<long, MultiplayerPeer> Peers { get; } = new Dictionary<long, MultiplayerPeer>();
 
@@ -107,7 +112,7 @@ namespace StardewModdingAPI.Framework
             if (client is LidgrenClient)
             {
                 string address = this.Reflection.GetField<string>(client, "address").GetValue();
-                return new SLidgrenClient(address, this.GetContextSyncMessageFields, this.TryProcessMessageFromServer);
+                return new SLidgrenClient(address, this.OnClientProcessingMessage, this.OnClientSendingMessage);
             }
 
             return client;
@@ -120,56 +125,78 @@ namespace StardewModdingAPI.Framework
             if (server is LidgrenServer)
             {
                 IGameServer gameServer = this.Reflection.GetField<IGameServer>(server, "gameServer").GetValue();
-                return new SLidgrenServer(gameServer);
+                return new SLidgrenServer(gameServer, this.Reflection, this.readFarmer, this.OnServerProcessingMessage, this.OnServerSendingMessage);
             }
 
             return server;
         }
 
-        /// <summary>Process an incoming network message from an unknown farmhand, usually a player whose connection hasn't been approved yet.</summary>
+        /// <summary>A callback raised when sending a network message as the host player.</summary>
+        /// <param name="server">The server sending the message.</param>
+        /// <param name="connection">The connection to which a message is being sent.</param>
+        /// <param name="message">The message being sent.</param>
+        /// <param name="resume">Send the underlying message.</param>
+        protected void OnServerSendingMessage(SLidgrenServer server, NetConnection connection, OutgoingMessage message, Action resume)
+        {
+            resume();
+        }
+
+        /// <summary>A callback raised when sending a message as a farmhand.</summary>
+        /// <param name="client">The client sending the message.</param>
+        /// <param name="message">The message being sent.</param>
+        /// <param name="resume">Send the underlying message.</param>
+        protected void OnClientSendingMessage(SLidgrenClient client, OutgoingMessage message, Action resume)
+        {
+            switch (message.MessageType)
+            {
+                // sync mod context (step 1)
+                case (byte)MessageType.PlayerIntroduction:
+                    client.sendMessage((byte)MessageType.ModContext, this.GetContextSyncMessageFields());
+                    resume();
+                    break;
+
+                // run default logic
+                default:
+                    resume();
+                    break;
+            }
+        }
+
+        /// <summary>Process an incoming network message as the host player.</summary>
         /// <param name="server">The server instance that received the connection.</param>
         /// <param name="rawMessage">The raw network message that was received.</param>
         /// <param name="message">The message to process.</param>
-        public void ProcessMessageFromUnknownFarmhand(Server server, NetIncomingMessage rawMessage, IncomingMessage message)
+        /// <param name="resume">Process the message using the game's default logic.</param>
+        public void OnServerProcessingMessage(SLidgrenServer server, NetIncomingMessage rawMessage, IncomingMessage message, Action resume)
         {
-            // ignore invalid message (farmhands should only receive messages from the server)
-            if (!Game1.IsMasterGame)
-                return;
 
             switch (message.MessageType)
             {
-                // sync SMAPI context with connected instances
-                case SMultiplayer.ContextSyncMessageID:
+                // sync mod context (step 2)
+                case (byte)MessageType.ModContext:
                     {
-                        // get server
-                        if (!(server is SLidgrenServer customServer))
+                        // parse message
+                        RemoteContextModel model = this.ReadContext(message.Reader);
+                        this.Monitor.Log($"Received context for farmhand {message.FarmerID} running {(model != null ? $"SMAPI {model.ApiVersion} with {model.Mods.Length} mods" : "vanilla")}.", LogLevel.Trace);
+
+                        // store peer
+                        MultiplayerPeer newPeer = MultiplayerPeer.ForConnectionToFarmhand(message.FarmerID, model, server, rawMessage.SenderConnection);
+                        if (this.Peers.ContainsKey(message.FarmerID))
                         {
-                            this.Monitor.Log($"Received context from farmhand {message.FarmerID} via unknown client {server.GetType().FullName}. Mods will not be able to sync data to that player.", LogLevel.Warn);
+                            this.Monitor.Log($"Rejected mod context from farmhand {message.FarmerID}: already received context for that player.", LogLevel.Error);
                             return;
                         }
+                        this.Peers[message.FarmerID] = newPeer;
 
-                        // parse message
-                        string data = message.Reader.ReadString();
-                        RemoteContextModel model = this.JsonHelper.Deserialise<RemoteContextModel>(data);
-                        if (model.ApiVersion == null)
-                            model = null; // no data available for unmodded players
-
-                        // log info
-                        if (model != null)
-                            this.Monitor.Log($"Received context for farmhand {message.FarmerID} running SMAPI {model.ApiVersion} with {model.Mods.Length} mods{(this.VerboseLogging ? $": {data}" : "")}.", LogLevel.Trace);
-                        else
-                            this.Monitor.Log($"Received context for farmhand {message.FarmerID} running vanilla{(this.VerboseLogging ? $": {data}" : "")}.", LogLevel.Trace);
-
-                        // store peer
-                        MultiplayerPeer newPeer = this.Peers[message.FarmerID] = MultiplayerPeer.ForConnectionToFarmhand(message.FarmerID, model, customServer, rawMessage.SenderConnection);
+                        // reply with own context
+                        this.VerboseLog("   Replying with host context...");
+                        newPeer.SendMessage(new OutgoingMessage((byte)MessageType.ModContext, Game1.player.UniqueMultiplayerID, this.GetContextSyncMessageFields()));
 
-                        // reply with known contexts
-                        this.VerboseLog("   Replying with context for current player...");
-                        newPeer.SendMessage(new OutgoingMessage(SMultiplayer.ContextSyncMessageID, Game1.player.UniqueMultiplayerID, this.GetContextSyncMessageFields()));
+                        // reply with other players' context
                         foreach (MultiplayerPeer otherPeer in this.Peers.Values.Where(p => p.PlayerID != newPeer.PlayerID))
                         {
                             this.VerboseLog($"   Replying with context for player {otherPeer.PlayerID}...");
-                            newPeer.SendMessage(new OutgoingMessage(SMultiplayer.ContextSyncMessageID, otherPeer.PlayerID, this.GetContextSyncMessageFields(otherPeer)));
+                            newPeer.SendMessage(new OutgoingMessage((byte)MessageType.ModContext, otherPeer.PlayerID, this.GetContextSyncMessageFields(otherPeer)));
                         }
 
                         // forward to other peers
@@ -179,107 +206,103 @@ namespace StardewModdingAPI.Framework
                             foreach (MultiplayerPeer otherPeer in this.Peers.Values.Where(p => p.PlayerID != newPeer.PlayerID))
                             {
                                 this.VerboseLog($"   Forwarding context to player {otherPeer.PlayerID}...");
-                                otherPeer.SendMessage(new OutgoingMessage(SMultiplayer.ContextSyncMessageID, newPeer.PlayerID, fields));
+                                otherPeer.SendMessage(new OutgoingMessage((byte)MessageType.ModContext, newPeer.PlayerID, fields));
                             }
                         }
                     }
                     break;
 
-                // handle intro from unmodded player
-                case Multiplayer.playerIntroduction:
-                    if (!this.Peers.ContainsKey(message.FarmerID))
+                // handle player intro
+                case (byte)MessageType.PlayerIntroduction:
                     {
-                        // get server
-                        if (!(server is SLidgrenServer customServer))
+                        // get peer
+                        if (!this.Peers.TryGetValue(message.FarmerID, out MultiplayerPeer peer))
                         {
-                            this.Monitor.Log($"Received connection from farmhand {message.FarmerID} with unknown client {server.GetType().FullName}. Mods will not be able to sync data to that player.", LogLevel.Warn);
-                            return;
+                            this.Monitor.Log($"Received connection for vanilla player {message.FarmerID}.", LogLevel.Trace);
+                            this.Peers[message.FarmerID] = peer = MultiplayerPeer.ForConnectionToFarmhand(message.FarmerID, null, server, rawMessage.SenderConnection);
                         }
 
-                        // store peer
-                        this.Monitor.Log($"Received connection for vanilla player {message.FarmerID}.", LogLevel.Trace);
-                        var peer = MultiplayerPeer.ForConnectionToFarmhand(message.FarmerID, null, customServer, rawMessage.SenderConnection);
-                        this.Peers[message.FarmerID] = peer;
-                        if (peer.IsHost)
-                            this.HostPeer = peer;
                     }
                     break;
 
                 // handle mod message
-                case SMultiplayer.ModMessageID:
-                    this.ReceiveModMessage(message);
-                    break;
-            }
-        }
-
-        /// <summary>Process an incoming message from an approved connection.</summary>
-        /// <param name="message">The message to process.</param>
-        public override void processIncomingMessage(IncomingMessage message)
-        {
-            switch (message.MessageType)
-            {
-                // handle mod message
-                case SMultiplayer.ModMessageID:
+                case (byte)MessageType.ModMessage:
                     this.ReceiveModMessage(message);
                     break;
 
-                // let game process message
                 default:
-                    base.processIncomingMessage(message);
+                    resume();
                     break;
             }
-            
         }
 
-        /// <summary>Process an incoming network message from the server.</summary>
+        /// <summary>Process an incoming network message as a farmhand.</summary>
         /// <param name="client">The client instance that received the connection.</param>
         /// <param name="message">The message to process.</param>
+        /// <param name="resume">Process the message using the game's default logic.</param>
         /// <returns>Returns whether the message was handled.</returns>
-        public bool TryProcessMessageFromServer(SLidgrenClient client, IncomingMessage message)
+        public void OnClientProcessingMessage(SLidgrenClient client, IncomingMessage message, Action resume)
         {
+            if (message.MessageType != Multiplayer.farmerDelta && message.MessageType != Multiplayer.locationDelta && message.MessageType != Multiplayer.teamDelta && message.MessageType != Multiplayer.worldDelta)
+                this.Monitor.Log($"CLIENT RECV {(MessageType)message.MessageType} {message.FarmerID}", LogLevel.Alert);
+
             switch (message.MessageType)
             {
-                // receive SMAPI context from a connected player
-                case SMultiplayer.ContextSyncMessageID:
+                // mod context sync (step 4)
+                case (byte)MessageType.ModContext:
                     {
                         // parse message
-                        string data = message.Reader.ReadString();
-                        RemoteContextModel model = this.JsonHelper.Deserialise<RemoteContextModel>(data);
-
-                        // log info
-                        if (model != null)
-                            this.Monitor.Log($"Received context for {(model.IsHost ? "host" : "farmhand")} {message.FarmerID} running SMAPI {model.ApiVersion} with {model.Mods.Length} mods{(this.VerboseLogging ? $": {data}" : "")}.", LogLevel.Trace);
-                        else
-                            this.Monitor.Log($"Received context for player {message.FarmerID} running vanilla{(this.VerboseLogging ? $": {data}" : "")}.", LogLevel.Trace);
+                        RemoteContextModel model = this.ReadContext(message.Reader);
+                        this.Monitor.Log($"Received context for {(model?.IsHost == true ? "host" : "farmhand")} {message.FarmerID} running {(model != null ? $"SMAPI {model.ApiVersion} with {model.Mods.Length} mods" : "vanilla")}.", LogLevel.Trace);
 
                         // store peer
-                        MultiplayerPeer peer = MultiplayerPeer.ForConnectionToHost(message.FarmerID, model, client);
+                        MultiplayerPeer peer = MultiplayerPeer.ForConnectionToHost(message.FarmerID, model, client, model?.IsHost ?? this.HostPeer == null);
+                        if (peer.IsHost && this.HostPeer != null)
+                        {
+                            this.Monitor.Log($"Rejected mod context from host player {peer.PlayerID}: already received host data from {(peer.PlayerID == this.HostPeer.PlayerID ? "that player" : $"player {peer.PlayerID}")}.", LogLevel.Error);
+                            return;
+                        }
                         this.Peers[message.FarmerID] = peer;
                         if (peer.IsHost)
                             this.HostPeer = peer;
                     }
-                    return true;
+                    break;
 
-                // handle intro from unmodded player
-                case Multiplayer.playerIntroduction:
-                    if (!this.Peers.ContainsKey(message.FarmerID))
+                // handle server intro
+                case (byte)MessageType.ServerIntroduction:
                     {
                         // store peer
-                        this.Monitor.Log($"Received connection for vanilla player {message.FarmerID}.", LogLevel.Trace);
-                        var peer = MultiplayerPeer.ForConnectionToHost(message.FarmerID, null, client);
-                        this.Peers[message.FarmerID] = peer;
-                        if (peer.IsHost)
-                            this.HostPeer = peer;
+                        if (!this.Peers.ContainsKey(message.FarmerID) && this.HostPeer == null)
+                        {
+                            this.Monitor.Log($"Received connection for vanilla host {message.FarmerID}.", LogLevel.Trace);
+                            this.Peers[message.FarmerID] = MultiplayerPeer.ForConnectionToHost(message.FarmerID, null, client, isHost: true);
+                        }
+                        resume();
+                        break;
+                    }
+
+                // handle player intro
+                case (byte)MessageType.PlayerIntroduction:
+                    {
+                        // store peer
+                        if (!this.Peers.TryGetValue(message.FarmerID, out MultiplayerPeer peer))
+                        {
+                            peer = MultiplayerPeer.ForConnectionToHost(message.FarmerID, null, client, isHost: this.HostPeer == null);
+                            this.Monitor.Log($"Received connection for vanilla {(peer.IsHost ? "host" : "farmhand")} {message.FarmerID}.", LogLevel.Trace);
+                            this.Peers[message.FarmerID] = peer;
+                            if (peer.IsHost)
+                                this.HostPeer = peer;
+                        }
                     }
-                    return false;
 
                 // handle mod message
-                case SMultiplayer.ModMessageID:
+                case (byte)MessageType.ModMessage:
                     this.ReceiveModMessage(message);
-                    return true;
+                    break;
 
                 default:
-                    return false;
+                    resume();
+                    break;
             }
         }
 
@@ -352,12 +375,12 @@ namespace StardewModdingAPI.Framework
                     if (playerIDs == null || playerIDs.Contains(peer.PlayerID))
                     {
                         model.ToPlayerIDs = new[] { peer.PlayerID };
-                        peer.SendMessage(new OutgoingMessage(SMultiplayer.ModMessageID, peer.PlayerID, data));
+                        peer.SendMessage(new OutgoingMessage((byte)MessageType.ModMessage, peer.PlayerID, data));
                     }
                 }
             }
             else if (this.HostPeer != null && this.HostPeer.HasSmapi)
-                this.HostPeer.SendMessage(new OutgoingMessage(SMultiplayer.ModMessageID, this.HostPeer.PlayerID, data));
+                this.HostPeer.SendMessage(new OutgoingMessage((byte)MessageType.ModMessage, this.HostPeer.PlayerID, data));
             else
                 this.VerboseLog("  Can't send message because no valid connections were found.");
 
@@ -367,6 +390,17 @@ namespace StardewModdingAPI.Framework
         /*********
         ** Private methods
         *********/
+        /// <summary>Read the metadata context for a player.</summary>
+        /// <param name="reader">The stream reader.</param>
+        private RemoteContextModel ReadContext(BinaryReader reader)
+        {
+            string data = reader.ReadString();
+            RemoteContextModel model = this.JsonHelper.Deserialise<RemoteContextModel>(data);
+            return model.ApiVersion != null
+                ? model
+                : null; // no data available for unmodded players
+        }
+
         /// <summary>Receive a mod message sent from another player's mods.</summary>
         /// <param name="message">The raw message to parse.</param>
         private void ReceiveModMessage(IncomingMessage message)
@@ -392,7 +426,7 @@ namespace StardewModdingAPI.Framework
                     {
                         newModel.ToPlayerIDs = new[] { peer.PlayerID };
                         this.VerboseLog($"  Forwarding message to player {peer.PlayerID}.");
-                        peer.SendMessage(new OutgoingMessage(SMultiplayer.ModMessageID, peer.PlayerID, this.JsonHelper.Serialise(newModel, Formatting.None)));
+                        peer.SendMessage(new OutgoingMessage((byte)MessageType.ModMessage, peer.PlayerID, this.JsonHelper.Serialise(newModel, Formatting.None)));
                     }
                 }
             }
diff --git a/src/SMAPI/Patches/LidgrenServerPatch.cs b/src/SMAPI/Patches/LidgrenServerPatch.cs
new file mode 100644
index 00000000..6f937665
--- /dev/null
+++ b/src/SMAPI/Patches/LidgrenServerPatch.cs
@@ -0,0 +1,89 @@
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.IO;
+using System.Reflection;
+using Harmony;
+using Lidgren.Network;
+using StardewModdingAPI.Framework;
+using StardewModdingAPI.Framework.Networking;
+using StardewModdingAPI.Framework.Patching;
+using StardewValley;
+using StardewValley.Network;
+
+namespace StardewModdingAPI.Patches
+{
+    /// <summary>A Harmony patch to let SMAPI override <see cref="LidgrenServer"/> methods.</summary>
+    internal class LidgrenServerPatch : IHarmonyPatch
+    {
+        /*********
+        ** Accessors
+        *********/
+        /// <summary>A unique name for this patch.</summary>
+        public string Name => $"{nameof(LidgrenServerPatch)}";
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Apply the Harmony patch.</summary>
+        /// <param name="harmony">The Harmony instance.</param>
+        public void Apply(HarmonyInstance harmony)
+        {
+            // override parseDataMessageFromClient
+            {
+                MethodInfo method = AccessTools.Method(typeof(LidgrenServer), "parseDataMessageFromClient");
+                MethodInfo prefix = AccessTools.Method(this.GetType(), nameof(LidgrenServerPatch.Prefix_LidgrenServer_ParseDataMessageFromClient));
+                harmony.Patch(method, new HarmonyMethod(prefix), null);
+            }
+
+            // override sendMessage
+            {
+                MethodInfo method = typeof(LidgrenServer).GetMethod("sendMessage", BindingFlags.NonPublic | BindingFlags.Instance, null, new [] { typeof(NetConnection), typeof(OutgoingMessage) }, null);
+                MethodInfo prefix = AccessTools.Method(this.GetType(), nameof(LidgrenServerPatch.Prefix_LidgrenServer_SendMessage));
+                harmony.Patch(method, new HarmonyMethod(prefix), null);
+            }
+        }
+
+
+        /*********
+        ** Private methods
+        *********/
+        /// <summary>The method to call instead of the <see cref="LidgrenServer.parseDataMessageFromClient"/> method.</summary>
+        /// <param name="__instance">The instance being patched.</param>
+        /// <param name="dataMsg">The raw network message to parse.</param>
+        /// <param name="___peers">The private <c>peers</c> field on the <paramref name="__instance"/> instance.</param>
+        /// <param name="___gameServer">The private <c>gameServer</c> field on the <paramref name="__instance"/> instance.</param>
+        /// <returns>Returns whether to execute the original method.</returns>
+        /// <remarks>This method must be static for Harmony to work correctly. See the Harmony documentation before renaming arguments.</remarks>
+        [SuppressMessage("ReSharper", "InconsistentNaming", Justification = "Argument names are defined by Harmony.")]
+        private static bool Prefix_LidgrenServer_ParseDataMessageFromClient(LidgrenServer __instance, NetIncomingMessage dataMsg, Bimap<long, NetConnection> ___peers, IGameServer ___gameServer)
+        {
+            if (__instance is SLidgrenServer smapiServer)
+            {
+                smapiServer.ParseDataMessageFromClient(dataMsg);
+                return false;
+            }
+
+            return true;
+        }
+
+        /// <summary>The method to call instead of the <see cref="LidgrenServer.sendMessage"/> method.</summary>
+        /// <param name="__instance">The instance being patched.</param>
+        /// <param name="connection">The connection to which to send the message.</param>
+        /// <param name="___peers">The private <c>peers</c> field on the <paramref name="__instance"/> instance.</param>
+        /// <param name="___gameServer">The private <c>gameServer</c> field on the <paramref name="__instance"/> instance.</param>
+        /// <returns>Returns whether to execute the original method.</returns>
+        /// <remarks>This method must be static for Harmony to work correctly. See the Harmony documentation before renaming arguments.</remarks>
+        [SuppressMessage("ReSharper", "InconsistentNaming", Justification = "Argument names are defined by Harmony.")]
+        private static bool Prefix_LidgrenServer_SendMessage(LidgrenServer __instance, NetConnection connection, OutgoingMessage message, Bimap<long, NetConnection> ___peers, IGameServer ___gameServer)
+        {
+            if (__instance is SLidgrenServer smapiServer)
+            {
+                smapiServer.SendMessage(connection, message);
+                return false;
+            }
+
+            return true;
+        }
+    }
+}
diff --git a/src/SMAPI/Patches/NetworkingPatch.cs b/src/SMAPI/Patches/NetworkingPatch.cs
deleted file mode 100644
index 12ccf84c..00000000
--- a/src/SMAPI/Patches/NetworkingPatch.cs
+++ /dev/null
@@ -1,103 +0,0 @@
-using System;
-using System.Diagnostics.CodeAnalysis;
-using System.IO;
-using System.Reflection;
-using Harmony;
-using Lidgren.Network;
-using StardewModdingAPI.Framework;
-using StardewModdingAPI.Framework.Networking;
-using StardewModdingAPI.Framework.Patching;
-using StardewValley;
-using StardewValley.Network;
-
-namespace StardewModdingAPI.Patches
-{
-    /// <summary>A Harmony patch to enable the SMAPI multiplayer metadata handshake.</summary>
-    internal class NetworkingPatch : IHarmonyPatch
-    {
-        /*********
-        ** Properties
-        *********/
-        /// <summary>The constructor for the internal <c>NetBufferReadStream</c> type.</summary>
-        private static readonly ConstructorInfo NetBufferReadStreamConstructor = NetworkingPatch.GetNetBufferReadStreamConstructor();
-
-
-        /*********
-        ** Accessors
-        *********/
-        /// <summary>A unique name for this patch.</summary>
-        public string Name => $"{nameof(NetworkingPatch)}";
-
-
-        /*********
-        ** Public methods
-        *********/
-        /// <summary>Apply the Harmony patch.</summary>
-        /// <param name="harmony">The Harmony instance.</param>
-        public void Apply(HarmonyInstance harmony)
-        {
-            MethodInfo method = AccessTools.Method(typeof(LidgrenServer), "parseDataMessageFromClient");
-            MethodInfo prefix = AccessTools.Method(this.GetType(), nameof(NetworkingPatch.Prefix_LidgrenServer_ParseDataMessageFromClient));
-            harmony.Patch(method, new HarmonyMethod(prefix), null);
-        }
-
-
-        /*********
-        ** Private methods
-        *********/
-        /// <summary>The method to call instead of the <see cref="LidgrenServer.parseDataMessageFromClient"/> method.</summary>
-        /// <param name="__instance">The instance being patched.</param>
-        /// <param name="dataMsg">The raw network message to parse.</param>
-        /// <param name="___peers">The private <c>peers</c> field on the <paramref name="__instance"/> instance.</param>
-        /// <param name="___gameServer">The private <c>gameServer</c> field on the <paramref name="__instance"/> instance.</param>
-        /// <returns>Returns whether to execute the original method.</returns>
-        /// <remarks>This method must be static for Harmony to work correctly. See the Harmony documentation before renaming arguments.</remarks>
-        [SuppressMessage("ReSharper", "InconsistentNaming", Justification = "Argument names are defined by Harmony.")]
-        private static bool Prefix_LidgrenServer_ParseDataMessageFromClient(LidgrenServer __instance, NetIncomingMessage dataMsg, Bimap<long, NetConnection> ___peers, IGameServer ___gameServer)
-        {
-            // get SMAPI overrides
-            SMultiplayer multiplayer = ((SGame)Game1.game1).Multiplayer;
-            SLidgrenServer server = (SLidgrenServer)__instance;
-
-            // add hook to call multiplayer core
-            NetConnection peer = dataMsg.SenderConnection;
-            using (IncomingMessage message = new IncomingMessage())
-            using (Stream readStream = (Stream)NetworkingPatch.NetBufferReadStreamConstructor.Invoke(new object[] { dataMsg }))
-            using (BinaryReader reader = new BinaryReader(readStream))
-            {
-                while (dataMsg.LengthBits - dataMsg.Position >= 8)
-                {
-                    message.Read(reader);
-                    if (___peers.ContainsLeft(message.FarmerID) && ___peers[message.FarmerID] == peer)
-                        ___gameServer.processIncomingMessage(message);
-                    else if (message.MessageType == Multiplayer.playerIntroduction)
-                    {
-                        NetFarmerRoot farmer = multiplayer.readFarmer(message.Reader);
-                        ___gameServer.checkFarmhandRequest("", farmer, msg => server.SendMessage(peer, msg), () => ___peers[farmer.Value.UniqueMultiplayerID] = peer);
-                    }
-                    else
-                        multiplayer.ProcessMessageFromUnknownFarmhand(__instance, dataMsg, message); // added hook
-                }
-            }
-
-            return false;
-        }
-
-        /// <summary>Get the constructor for the internal <c>NetBufferReadStream</c> type.</summary>
-        private static ConstructorInfo GetNetBufferReadStreamConstructor()
-        {
-            // get type
-            string typeName = $"StardewValley.Network.NetBufferReadStream, {Constants.GameAssemblyName}";
-            Type type = Type.GetType(typeName);
-            if (type == null)
-                throw new InvalidOperationException($"Can't find type: {typeName}");
-
-            // get constructor
-            ConstructorInfo constructor = type.GetConstructor(new[] { typeof(NetBuffer) });
-            if (constructor == null)
-                throw new InvalidOperationException($"Can't find constructor for type: {typeName}");
-
-            return constructor;
-        }
-    }
-}
diff --git a/src/SMAPI/StardewModdingAPI.csproj b/src/SMAPI/StardewModdingAPI.csproj
index 11fa5d35..dfe55eb9 100644
--- a/src/SMAPI/StardewModdingAPI.csproj
+++ b/src/SMAPI/StardewModdingAPI.csproj
@@ -174,6 +174,7 @@
     <Compile Include="Framework\Events\ModPlayerEvents.cs" />
     <Compile Include="Framework\Events\ModSpecialisedEvents.cs" />
     <Compile Include="Framework\Events\ModWorldEvents.cs" />
+    <Compile Include="Framework\Networking\MessageType.cs" />
     <Compile Include="Framework\ModHelpers\DataHelper.cs" />
     <Compile Include="Framework\Networking\ModMessageModel.cs" />
     <Compile Include="Framework\Networking\MultiplayerPeer.cs" />
@@ -322,7 +323,7 @@
     <Compile Include="Metadata\InstructionMetadata.cs" />
     <Compile Include="Mod.cs" />
     <Compile Include="Patches\DialogueErrorPatch.cs" />
-    <Compile Include="Patches\NetworkingPatch.cs" />
+    <Compile Include="Patches\LidgrenServerPatch.cs" />
     <Compile Include="PatchMode.cs" />
     <Compile Include="GamePlatform.cs" />
     <Compile Include="Program.cs" />