client: Don't wait forever for a ping reply

In the event that plymouthd is not responding the plymouth client will
wait forever when sending a ping to the daemon.

This commit introduces a timeout of 30 seconds to better cope with a
hung plymouthd process.

(Some small changes to original patch by Ray Strode)

https://gitlab.freedesktop.org/plymouth/plymouth/-/issues/130

src/client/plymouth.c

@@ -42,6 +42,12 @@
 #define PLY_MAX_COMMAND_LINE_SIZE 4097
 #endif
 
+
+#ifndef PLY_PING_TIMEOUT
+#define PLY_PING_TIMEOUT 30.0
+#endif
+
+
 #define KEY_CTRL_C ('\100' ^ 'C')
 
 typedef struct
@@ -75,6 +81,13 @@ typedef struct
         char    *keys;
 } key_answer_state_t;
 
+static void
+on_ping_timeout (state_t *state)
+{
+        ply_trace ("ping: timed out waiting for plymouthd");
+        ply_event_loop_exit (state->loop, 1);
+}
+
 static char **
 split_string (const char *command,
               const char  delimiter)
@@ -1145,6 +1158,9 @@ main (int    argc,
                                              on_success,
                                              (ply_boot_client_response_handler_t)
                                              on_failure, &state);
+                ply_event_loop_watch_for_timeout (state.loop, PLY_PING_TIMEOUT,
+                                                  (ply_event_loop_timeout_handler_t)
+                                                  on_ping_timeout, &state);
         } else if (should_check_for_active_vt) {
                 ply_boot_client_ask_daemon_has_active_vt (state.client,
                                                           (ply_boot_client_response_handler_t)