Added wait_for_topic builtin command

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/platforms/apps.cpp.in

@@ -19,6 +19,7 @@ int list_files_main(int argc, char *argv[]);
 int list_devices_main(int argc, char *argv[]);
 int list_topics_main(int argc, char *argv[]);
 int sleep_main(int argc, char *argv[]);
+int wait_for_topic(int argc, char *argv[]);
 
 }
 
@@ -33,6 +34,7 @@ ${builtin_apps_string}
 	apps["list_devices"] = list_devices_main;
 	apps["list_topics"] = list_topics_main;
 	apps["sleep"] = sleep_main;
+	apps["wait_for_topic"] = wait_for_topic;
 }
 
 void list_builtins(apps_map_type &apps)
@@ -84,3 +86,32 @@ int sleep_main(int argc, char *argv[])
         usleep(usecs);
         return 0;
 }
+
+#include "uORB/uORB.h"
+
+static int wait_for_topic(int argc, char *argv[])
+{
+	if (argc < 2 || argc > 3) {
+		cout << "Usage: wait_for_topic <topic> [timeout_sec]" << endl;
+		return 1;
+	}
+
+	struct orb_metadata meta = { .o_name = argv[1],
+				     .o_size = 0,
+				     .o_size_no_padding = 0,
+				     .o_fields = nullptr };
+
+	unsigned int timeout = (argc == 3) ? (unsigned int)atoi(argv[2]) : 0;
+	unsigned int elapsed = 0;
+
+	cout << "Waiting for topic " << argv[1] << " timeout " << timeout << endl;
+
+	while (orb_exists(&meta, 0) != 0 && (timeout && (elapsed < timeout)))
+	{
+		sleep(1);
+		elapsed += 1;
+	}
+
+	return 0;
+}
+