hostname: Fallback to transient hostname

After pretty hostname, and static hostname, also support transient
hostname as a last resort before 'BlueZ X.XX'.

This happens on Fedora's Workstation installation as it calls
"hostnamectl set-hostname" on startup. In Fedora Silverblue, the default
hostname is set as fedora in /etc/os-release.

In both cases, we should fall back to that transient hostname, as bad as
it could be.

Note that the transient hostname needs to be monitored through the
kernel directly, as explained in:
https://www.freedesktop.org/software/systemd/man/org.freedesktop.hostname1.html

plugins/hostname.c

@@ -16,6 +16,8 @@
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
+#include <fcntl.h>
+#include <sys/utsname.h>
 
 #include "lib/bluetooth.h"
 #include "lib/sdp.h"
@@ -44,8 +46,10 @@
 static uint8_t major_class = MAJOR_CLASS_MISCELLANEOUS;
 static uint8_t minor_class = MINOR_CLASS_UNCATEGORIZED;
 
-static char *pretty_hostname = NULL;
-static char *static_hostname = NULL;
+static char *pretty_hostname    = NULL;
+static char *static_hostname    = NULL;
+static char *transient_hostname = NULL;
+static guint hostname_id = 0;
 
 /*
  * Fallback to static hostname only if empty pretty hostname was already
@@ -60,6 +64,10 @@ static const char *get_hostname(void)
 		if (static_hostname &&
 				g_str_equal(static_hostname, "") == FALSE)
 			return static_hostname;
+
+		if (transient_hostname &&
+				g_str_equal(transient_hostname, "") == FALSE)
+			return transient_hostname;
 	}
 
 	return NULL;
@@ -181,6 +189,32 @@ static void property_changed(GDBusProxy *proxy, const char *name,
 	}
 }
 
+static void read_transient_hostname(void)
+{
+	struct utsname u;
+
+	if (uname(&u) != 0) {
+		g_free(transient_hostname);
+		transient_hostname = NULL;
+		DBG("failed to read transient hostname");
+		return;
+	}
+
+	g_free(transient_hostname);
+	transient_hostname = g_strdup(u.nodename);
+
+	DBG("read transient hostname: '%s'", transient_hostname);
+}
+
+static gboolean hostname_cb(GIOChannel *io, GIOCondition cond,
+							gpointer user_data)
+{
+	DBG("transient hostname changed");
+	read_transient_hostname();
+	adapter_foreach(update_class, NULL);
+	return TRUE;
+}
+
 static int hostname_probe(struct btd_adapter *adapter)
 {
 	DBG("");
@@ -261,9 +295,11 @@ static GDBusProxy *hostname_proxy = NULL;
 static int hostname_init(void)
 {
 	DBusConnection *conn = btd_get_dbus_connection();
+	int fd;
 	int err;
 
 	read_dmi_fallback();
+	read_transient_hostname();
 
 	hostname_client = g_dbus_client_new(conn, "org.freedesktop.hostname1",
 						"/org/freedesktop/hostname1");
@@ -289,6 +325,17 @@ static int hostname_init(void)
 		hostname_client = NULL;
 	}
 
+	fd = open("/proc/sys/kernel/hostname", O_RDONLY);
+	if (fd < 0) {
+		error("open(/proc/sys/kernel/hostname): %s (%d)",
+					strerror(errno), errno);
+	} else {
+		GIOChannel *io = g_io_channel_unix_new(fd);
+
+		hostname_id = g_io_add_watch(io, G_IO_ERR, hostname_cb, NULL);
+		g_io_channel_unref(io);
+	}
+
 	return err;
 }
 
@@ -306,8 +353,14 @@ static void hostname_exit(void)
 		hostname_client = NULL;
 	}
 
+	if (hostname_id != 0) {
+		g_source_remove(hostname_id);
+		hostname_id = 0;
+	}
+
 	g_free(pretty_hostname);
 	g_free(static_hostname);
+	g_free(transient_hostname);
 }
 
 BLUETOOTH_PLUGIN_DEFINE(hostname, VERSION, BLUETOOTH_PLUGIN_PRIORITY_DEFAULT,