Use SDP to auto-detect the rfcomm port number

When the port number is set to zero (which is an invalid value), detect
the port number automatically. On Windows, we can simply supply the UUID
of the serial port service, and the Windows api will take care of the
discovery. On Linux (bluez), the SDP discovery needs to be performed
manually to retrieve the port number.

src/bluetooth.c

@@ -30,6 +30,7 @@
 #ifdef _WIN32
 #ifdef HAVE_WS2BTH_H
 #define BLUETOOTH
+#include <initguid.h>
 #include <ws2bth.h>
 #endif
 #else
@@ -39,6 +40,8 @@
 #include <bluetooth/rfcomm.h>
 #include <bluetooth/hci.h>
 #include <bluetooth/hci_lib.h>
+#include <bluetooth/sdp.h>
+#include <bluetooth/sdp_lib.h>
 #endif
 #endif
 
@@ -105,6 +108,89 @@ dc_address_set (bdaddr_t *ba, dc_bluetooth_address_t address)
 		shift += 8;
 	}
 }
+
+static dc_status_t
+dc_bluetooth_sdp (uint8_t *port, dc_context_t *context, const bdaddr_t *ba)
+{
+	dc_status_t status = DC_STATUS_SUCCESS;
+	sdp_session_t *session = NULL;
+	sdp_list_t *search = NULL, *attrid = NULL;
+	sdp_list_t *records = NULL;
+	uint8_t channel = 0;
+
+	// Connect to the SDP server on the remote device.
+	session = sdp_connect (BDADDR_ANY, ba, SDP_RETRY_IF_BUSY);
+	if (session == NULL) {
+		s_errcode_t errcode = S_ERRNO;
+		SYSERROR (context, errcode);
+		status = dc_socket_syserror(errcode);
+		goto error;
+	}
+
+	// Specify the UUID of the serial port service with all attributes.
+	uuid_t uuid = {0};
+	uint32_t range = 0x0000FFFF;
+	sdp_uuid16_create (&uuid, SERIAL_PORT_SVCLASS_ID);
+	search = sdp_list_append (NULL, &uuid);
+	attrid = sdp_list_append (NULL, &range);
+	if (search == NULL || attrid == NULL) {
+		s_errcode_t errcode = S_ERRNO;
+		SYSERROR (context, errcode);
+		status = dc_socket_syserror(errcode);
+		goto error;
+	}
+
+	// Get a list of the service records with their attributes.
+	if (sdp_service_search_attr_req (session, search, SDP_ATTR_REQ_RANGE, attrid, &records) != 0) {
+		s_errcode_t errcode = S_ERRNO;
+		SYSERROR (context, errcode);
+		status = dc_socket_syserror(errcode);
+		goto error;
+	}
+
+	// Go through each of the service records.
+	for (sdp_list_t *r = records; r; r = r->next ) {
+		sdp_record_t *record = (sdp_record_t *) r->data;
+
+		// Get a list of the protocol sequences.
+		sdp_list_t *protos = NULL;
+		if (sdp_get_access_protos (record, &protos) != 0 ) {
+			s_errcode_t errcode = S_ERRNO;
+			SYSERROR (context, errcode);
+			status = dc_socket_syserror(errcode);
+			goto error;
+		}
+
+		// Get the rfcomm port number.
+		int ch = sdp_get_proto_port (protos, RFCOMM_UUID);
+
+		sdp_list_foreach (protos, (sdp_list_func_t) sdp_list_free, NULL);
+		sdp_list_free (protos, NULL);
+
+		if (ch > 0) {
+			channel = ch;
+			break;
+		}
+	}
+
+	if (channel == 0) {
+		ERROR (context, "No serial port service found!");
+		status = DC_STATUS_IO;
+		goto error;
+	}
+
+	INFO (context, "SDP: channel=%u", channel);
+
+	*port = channel;
+
+error:
+	sdp_list_free (records, (sdp_free_func_t) sdp_record_free);
+	sdp_list_free (attrid, NULL);
+	sdp_list_free (search, NULL);
+	sdp_close (session);
+
+	return status;
+}
 #endif
 #endif
 
@@ -298,12 +384,23 @@ dc_bluetooth_connect (dc_iostream_t *abstract, dc_bluetooth_address_t address, u
 	sa.addressFamily = AF_BTH;
 	sa.btAddr = address;
 	sa.port = port;
-	memset(&sa.serviceClassId, 0, sizeof(sa.serviceClassId));
+	if (port == 0) {
+		sa.serviceClassId = SerialPortServiceClass_UUID;
+	} else {
+		memset(&sa.serviceClassId, 0, sizeof(sa.serviceClassId));
+	}
 #else
 	struct sockaddr_rc sa;
 	sa.rc_family = AF_BLUETOOTH;
-	sa.rc_channel = port;
 	dc_address_set (&sa.rc_bdaddr, address);
+	if (port == 0) {
+		dc_status_t rc = dc_bluetooth_sdp (&sa.rc_channel, abstract->context, &sa.rc_bdaddr);
+		if (rc != DC_STATUS_SUCCESS) {
+			return rc;
+		}
+	} else {
+		sa.rc_channel = port;
+	}
 #endif
 
 	return dc_socket_connect (&device->base, (struct sockaddr *) &sa, sizeof (sa));