data-control: Ignore our own offers via custom MIME type

Offer a custom MIME type (unique per wayvnc_client) when setting the
selections of the Wayland clipboard which we can detect when processing
incoming Wayland selections, and avoid unnecessarily echoing an
incoming VNC client clipboard update back to the same VNC client.

It would be useful if when a wlr_data_control_device sends
set_selection, the selection event received could have an enum
argument value describing that the wlr_data_control_offer in the id
argument is associated with a wlr_data_control_source passed to
set_selection by the same wlr_data_control_device.

https://gitlab.freedesktop.org/wlroots/wlr-protocols/-/issues/111
swaywm/wlroots#2406

include/data-control.h

@@ -28,7 +28,10 @@ struct data_control {
 	struct zwlr_data_control_source_v1* selection;
 	struct zwlr_data_control_source_v1* primary_selection;
 	struct zwlr_data_control_offer_v1* offer;
+	bool is_own_offer;
 	const char* mime_type;
+	/* x-wayvnc-client-(8 hexadecimal digits) + \0 */
+	char custom_mime_type_name[32];
 	char* cb_data;
 	size_t cb_len;
 };

src/data-control.c

@@ -26,6 +26,8 @@
 
 #include "data-control.h"
 
+static const char custom_mime_type_data[] = "wayvnc";
+
 struct receive_context {
 	struct data_control* data_control;
 	struct zwlr_data_control_offer_v1* offer;
@@ -124,11 +126,15 @@ static void data_control_offer(void* data,
 {
 	struct data_control* self = data;
 
+	if (strcmp(mime_type, self->custom_mime_type_name) == 0) {
+		self->is_own_offer = true;
+		return;
+	}
+
 	if (self->offer)
 		return;
-	if (strcmp(mime_type, self->mime_type) != 0) {
+	if (strcmp(mime_type, self->mime_type) != 0)
 		return;
-	}
 
 	self->offer = zwlr_data_control_offer_v1;
 }
@@ -153,9 +159,13 @@ static void data_control_device_selection(void* data,
 {
 	struct data_control* self = data;
 	if (id && self->offer == id) {
-		receive_data(data, id);
+		if (!self->is_own_offer)
+			receive_data(data, id);
+		else
+			zwlr_data_control_offer_v1_destroy(self->offer);
 		self->offer = NULL;
 	}
+	self->is_own_offer = false;
 }
 
 static void data_control_device_finished(void* data,
@@ -170,10 +180,13 @@ static void data_control_device_primary_selection(void* data,
 {
 	struct data_control* self = data;
 	if (id && self->offer == id) {
-		receive_data(data, id);
+		if (!self->is_own_offer)
+			receive_data(data, id);
+		else
+			zwlr_data_control_offer_v1_destroy(self->offer);
 		self->offer = NULL;
-		return;
 	}
+	self->is_own_offer = false;
 }
 
 static struct zwlr_data_control_device_v1_listener data_control_device_listener = {
@@ -196,10 +209,15 @@ data_control_source_send(void* data,
 
 	assert(d);
 
-	ret = write(fd, d, len);
-
-	if (ret < (int)len)
-		nvnc_log(NVNC_LOG_ERROR, "write from clipboard incomplete");
+	if (strcmp(mime_type, self->custom_mime_type_name) == 0) {
+		ret = write(fd, custom_mime_type_data, strlen(custom_mime_type_data));
+		if (ret < (int)strlen(custom_mime_type_data))
+			nvnc_log(NVNC_LOG_ERROR, "custom mime type data write incomplete");
+	} else {
+		ret = write(fd, d, len);
+		if (ret < (int)len)
+			nvnc_log(NVNC_LOG_ERROR, "write from clipboard incomplete");
+	}
 
 	close(fd);
 }
@@ -235,6 +253,7 @@ static struct zwlr_data_control_source_v1* set_selection(struct data_control* se
 
 	zwlr_data_control_source_v1_add_listener(selection, &data_control_source_listener, self);
 	zwlr_data_control_source_v1_offer(selection, self->mime_type);
+	zwlr_data_control_source_v1_offer(selection, self->custom_mime_type_name);
 
 	if (primary)
 		zwlr_data_control_device_v1_set_primary_selection(self->device, selection);
@@ -252,9 +271,14 @@ void data_control_init(struct data_control* self, struct wl_display* wl_display,
 	zwlr_data_control_device_v1_add_listener(self->device, &data_control_device_listener, self);
 	self->selection = NULL;
 	self->primary_selection = NULL;
+	self->offer = NULL;
+	self->is_own_offer = false;
 	self->cb_data = NULL;
 	self->cb_len = 0;
 	self->mime_type = "text/plain;charset=utf-8";
+	snprintf(self->custom_mime_type_name,
+			sizeof(self->custom_mime_type_name),
+			"x-wayvnc-client-%08x", (unsigned int)rand());
 }
 
 void data_control_destroy(struct data_control* self)

src/main.c

@@ -19,6 +19,7 @@
 #include <string.h>
 #include <stdint.h>
 #include <stdbool.h>
+#include <time.h>
 #include <unistd.h>
 #include <assert.h>
 #include <inttypes.h>
@@ -2005,6 +2006,8 @@ int main(int argc, char* argv[])
 	self.disable_input = disable_input;
 	self.use_transient_seat = use_transient_seat;
 
+	srand(time(NULL));
+
 	struct aml* aml = aml_new();
 	if (!aml)
 		goto failure;