Allow plugins to handle player join and leave events

This includes the ability to kick a client on join for reasons other
than banning.

Closes #156.

3 files changed, 87 insertions, 0 deletions

diff --git a/client_conn.go b/client_conn.go
index 9753147..1a16d94 100644
--- a/client_conn.go
+++ b/client_conn.go
@@ -123,6 +123,10 @@ func handleClt(cc *ClientConn) {
 		pkt, err := cc.Recv()
 		if err != nil {
 			if errors.Is(err, net.ErrClosed) {
+				if cc.state() == csActive {
+					handleLeave(cc)
+				}
+
 				if errors.Is(cc.WhyClosed(), rudp.ErrTimedOut) {
 					cc.Log("<->", "timeout")
 				} else {
diff --git a/plugin_join_leave.go b/plugin_join_leave.go
new file mode 100644
index 0000000..baa4ba3
--- /dev/null
+++ b/plugin_join_leave.go
@@ -0,0 +1,82 @@
+package proxy
+
+import "sync"
+
+var (
+	onJoin     []func(*ClientConn) string
+	onJoinMu   sync.Mutex
+	onJoinOnce sync.Once
+)
+
+var (
+	onLeave     []func(*ClientConn)
+	onLeaveMu   sync.Mutex
+	onLeaveOnce sync.Once
+)
+
+// RegisterOnJoin registers a handler that is called
+// when a client finishes connecting to the proxy (TOSERVER_CLIENT_READY packet)
+// but before it is connected to an upstream server.
+// If any handler returns a non-empty string, the client is kicked
+// with that message.
+// Handlers are run sequentially and block the client's connection
+// and packet handling procedure.
+func RegisterOnJoin(handler func(*ClientConn) string) {
+	initOnJoin()
+
+	onJoinMu.Lock()
+	defer onJoinMu.Unlock()
+
+	onJoin = append(onJoin, handler)
+}
+
+// RegisterOnLeave registers a handler that is called
+// when a client disconnects for any reason after reaching the ready stage.
+// Handlers are run sequentially.
+func RegisterOnLeave(handler func(*ClientConn)) {
+	initOnLeave()
+
+	onLeaveMu.Lock()
+	defer onLeaveMu.Unlock()
+
+	onLeave = append(onLeave, handler)
+}
+
+func handleJoin(cc *ClientConn) {
+	onJoinMu.Lock()
+	defer onJoinMu.Unlock()
+
+	for _, handler := range onJoin {
+		if msg := handler(cc); msg != "" {
+			cc.Kick(msg)
+			break
+		}
+	}
+}
+
+func handleLeave(cc *ClientConn) {
+	onLeaveMu.Lock()
+	defer onLeaveMu.Unlock()
+
+	for _, handler := range onLeave {
+		handler(cc)
+	}
+}
+
+func initOnJoin() {
+	onJoinOnce.Do(func() {
+		onJoinMu.Lock()
+		defer onJoinMu.Unlock()
+
+		onJoin = make([]func(*ClientConn) string, 0)
+	})
+}
+
+func initOnLeave() {
+	onLeaveOnce.Do(func() {
+		onLeaveMu.Lock()
+		defer onLeaveMu.Unlock()
+
+		onLeave = make([]func(*ClientConn), 0)
+	})
+}
diff --git a/process.go b/process.go
index 215e6cc..862f29d 100644
--- a/process.go
+++ b/process.go
@@ -435,6 +435,7 @@ func (cc *ClientConn) process(pkt mt.Pkt) {
 		cc.formspecVer = cmd.Formspec
 
 		cc.setState(csActive)
+		handleJoin(cc)
 		close(cc.initCh)
 
 		return