Implement UTP_GET_READ_BUFFER_SIZE callback

This means the read buffer size will impact the read window. See bittorrent/libutp#82

callbacks.go

@@ -115,3 +115,17 @@ func acceptCallback(a *C.utp_callback_arguments) C.uint64 {
 	s.pushBacklog(s.newConn(a.socket))
 	return 0
 }
+
+//export getReadBufferSizeCallback
+func getReadBufferSizeCallback(a *C.utp_callback_arguments) (ret C.uint64) {
+	s := libContextToSocket[a.context]
+	c := s.conns[a.socket]
+	if c == nil {
+		// socket hasn't been added to the Socket.conns yet. The read buffer
+		// starts out empty, and the default implementation for this callback
+		// returns 0, so we'll return that.
+		return 0
+	}
+	ret = C.uint64(c.readBuf.Len())
+	return
+}

utp.go

@@ -18,6 +18,7 @@ uint64_t acceptCallback(utp_callback_arguments *);
 uint64_t sendtoCallback(utp_callback_arguments *);
 uint64_t stateChangeCallback(utp_callback_arguments *);
 uint64_t readCallback(utp_callback_arguments *);
+uint64_t getReadBufferSizeCallback(utp_callback_arguments *);
 */
 import "C"
 import "unsafe"
@@ -31,6 +32,7 @@ func (ctx *C.utp_context) setCallbacks() {
 	C.utp_set_callback(ctx, C.UTP_ON_STATE_CHANGE, (*C.utp_callback_t)(C.stateChangeCallback))
 	C.utp_set_callback(ctx, C.UTP_ON_READ, (*C.utp_callback_t)(C.readCallback))
 	C.utp_set_callback(ctx, C.UTP_ON_ERROR, (*C.utp_callback_t)(C.errorCallback))
+	C.utp_set_callback(ctx, C.UTP_GET_READ_BUFFER_SIZE, (*C.utp_callback_t)(C.getReadBufferSizeCallback))
 }
 
 func (ctx *C.utp_context) setOption(opt, val int) int {