Implement xattr attribute removal

As I wrote on GNOME Discourse, currently it is possible to set a xattr attribute via GIO, but it is impossible to unset it.

For instance, PART 1 of the following code works and the attribute is created, but PART 2 fails and the attribute is not removed.

#include <glib.h>
#include <gio/gio.h>
#include <stdio.h>

#define G_FILE_ATTRIBUTE_XATTR_USER_CUSTOM "xattr::my_custom_attribute"

int main (int argc, char ** argv) {

	GFile * location;
	GError * err = NULL;
	location = g_file_new_for_path("testfile.txt");

	/* PART 1: Write "My custom content" into "user.my_custom_attribute" */

	if (
		!g_file_set_attribute_string(
			location,
			G_FILE_ATTRIBUTE_XATTR_USER_CUSTOM,
			"My custom content",
			G_FILE_QUERY_INFO_NONE,
			NULL,
			&err
		)
	) {

		g_message(
			"Sorry, something went wrong :-( // %s",
			err->message
		);

		g_error_free(err);
		g_object_unref(location);
		return 1;

	}

	/* PART 2: Remove "user.my_custom_attribute" */

	if (
		!g_file_set_attribute(
			location,
			G_FILE_ATTRIBUTE_XATTR_USER_CUSTOM,
			G_FILE_ATTRIBUTE_TYPE_INVALID,
			NULL,
			G_FILE_QUERY_INFO_NONE,
			NULL,
			&err
		)
	) {

		g_message(
			"Sorry, something went wrong :-( // %s",
			err->message
		);

		g_error_free(err);
		g_object_unref(location);
		return 1;

	}

	g_object_unref(location);
	return 0;

}

After searching a bit in the forum I found a similar question. Then I found out that a patch was proposed five years ago (#1187 (closed)), which idiomatically tried to implement G_FILE_ATTRIBUTE_TYPE_INVALID for removing xattr attributes. However after five years the patch cannot be merged anymore. So I tried to update it to the current version of GIO.

--madmurphy

Closes #1187 (closed)