busctl: monitor - only start printing messages once we have become a monitor (#5931)

A connection becomes a monitor the moment it loses its unique name, so any
messages received before that should not be dumped to the console.

Currently, we print NameAcquired and NameLost for the unique name of the
peer that becomes the monitor, simply discard all messages until we
receive our NameLost signal.

src/busctl/busctl.c

@@ -1083,6 +1083,8 @@ static int monitor(sd_bus *bus, char *argv[], int (*dump)(sd_bus_message *m, FIL
         _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
         char **i;
         uint32_t flags = 0;
+        const char *unique_name;
+        bool is_monitor = false;
         int r;
 
         /* upgrade connection; it's not used for anything else after this call */
@@ -1140,6 +1142,10 @@ static int monitor(sd_bus *bus, char *argv[], int (*dump)(sd_bus_message *m, FIL
                 return r;
         }
 
+        r = sd_bus_get_unique_name(bus, &unique_name);
+        if (r < 0)
+                return log_error_errno(r, "Failed to get unique name: %m");
+
         log_info("Monitoring bus message stream.");
 
         for (;;) {
@@ -1149,6 +1155,23 @@ static int monitor(sd_bus *bus, char *argv[], int (*dump)(sd_bus_message *m, FIL
                 if (r < 0)
                         return log_error_errno(r, "Failed to process bus: %m");
 
+                if (!is_monitor) {
+                        const char *name;
+
+                        /* wait until we lose our unique name */
+                        if (sd_bus_message_is_signal(m, "org.freedesktop.DBus", "NameLost") <= 0)
+                                continue;
+
+                        r = sd_bus_message_read(m, "s", &name);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to read lost name: %m");
+
+                        if (!streq(name, unique_name))
+                                continue;
+
+                        is_monitor = true;
+                }
+
                 if (m) {
                         dump(m, stdout);
                         fflush(stdout);