[memory_checker] Do not check memory usage of containers if docker da…

…emon is not running (sonic-net#11476)

Fix in Monit memory_checker plugin. Skip fetching running containers if docker engine is down (can happen in deinit).
This PR fixes issue sonic-net#11472.

Signed-off-by: liora liora@nvidia.com

Why I did it
In the case where Monit runs during deinit flow, memory_checker plugin is fetching the running containers without checking if Docker service is still running. I added this check.

How I did it
Use systemctl is-active to check if Docker engine is still running.

How to verify it
Use systemctl to stop docker engine and reload Monit, no errors in log and relevant print appears in log.

Which release branch to backport (provide reason below if selected)
The fix is required in 202205 and 202012 since the PR that introduced the issue was cherry picked to those branches (sonic-net#11129).

files/image_config/monit/memory_checker

@@ -96,6 +96,19 @@ def check_memory_usage(container_name, threshold_value):
         sys.exit(4)
 
 
+def is_service_active(service_name):
+    """Test if service is running.
+
+    Args:
+        service_name: A string contains the service name
+
+    Returns:
+        True if service is running, False otherwise
+    """
+    status = subprocess.run("systemctl is-active --quiet {}".format(service_name), shell=True, check=False)
+    return status.returncode == 0
+
+
 def get_running_container_names():
     """Retrieves names of running containers by talking to the docker daemon.
 
@@ -128,6 +141,12 @@ def main():
     parser.add_argument("threshold_value", type=int, help="threshold value in bytes")
     args = parser.parse_args()
 
+    if not is_service_active("docker"):
+        syslog.syslog(syslog.LOG_INFO,
+                      "[memory_checker] Exits without checking memory usage of container '{}' since docker daemon is not running!"
+                      .format(args.container_name))
+        sys.exit(0)
+
     running_container_names = get_running_container_names()
     if args.container_name in running_container_names:
         check_memory_usage(args.container_name, args.threshold_value)