[memory_checker] If containers are not created during

device is (re)booted, then exits without checking its memory usage.

Signed-off-by: Yong Zhao <yozhao@microsoft.com>

files/image_config/monit/memory_checker

@@ -94,6 +94,28 @@ def check_memory_usage(container_name, threshold_value):
         sys.exit(4)
 
 
+def get_running_container_names():
+    """Retrieves names of running containers by talking to the docker daemon.
+
+    Args:
+        None.
+
+    Returns:
+        running_container_names: A list indicates names of running containers.
+    """
+    running_container_names = []
+    docker_client = docker.DockerClient(base_url='unix://var/run/docker.sock')
+    container_obj = docker_client.containers
+    try:
+        container_list = container_obj.list(filters={"status": "running"})
+        for container in container_list:
+            running_container_names.append(container.name)
+    except docker.errors.APIError as err:
+        print("Failed to retrieve the running container list. Error: '{}'".format(err))
+
+    return running_container_names
+
+
 def main():
     parser = argparse.ArgumentParser(description="Check memory usage of a container \
             and an alerting message will be written into syslog if memory usage \
@@ -104,7 +126,12 @@ def main():
     parser.add_argument("threshold_value", type=int, help="threshold value in bytes")
     args = parser.parse_args()
 
-    check_memory_usage(args.container_name, args.threshold_value)
+    running_container_names = get_running_container_names()
+    if args.container_name in running_container_names:
+        check_memory_usage(args.container_name, args.threshold_value)
+    else:
+        syslog.syslog(syslog.LOG_INFO,
+                      "[memory_checker] Exits without checking memory usage since contain '{}' is not running!".format(args.container_name))
 
 
 if __name__ == "__main__":