discovery_client: Fix auto-launching discovery service producing a Re…

…sourceWarning (#344)

* discovery_client: Get rid of resource warning

* Ran black

* discovery_client: Fix comments and private variable for subprocess

ni_measurementlink_service/_internal/discovery_client.py

@@ -27,6 +27,8 @@
     import winerror
 
 _logger = logging.getLogger(__name__)
+# Save Popen object to avoid "ResourceWarning: subprocess N is still running"
+_discovery_service_subprocess: Optional[subprocess.Popen] = None
 
 _START_SERVICE_TIMEOUT = 30.0
 _START_SERVICE_POLLING_INTERVAL = 100e-3
@@ -257,7 +259,8 @@ def _key_file_exists(key_file_path: pathlib.Path) -> bool:
 
 def _start_service(exe_file_path: pathlib.PurePath, key_file_path: pathlib.Path) -> None:
     """Starts the service at the specified path and wait for the service to get up and running."""
-    subprocess.Popen(
+    global _discovery_service_subprocess  # save Popen object to avoid ResourceWarning
+    _discovery_service_subprocess = subprocess.Popen(
         [exe_file_path],
         cwd=exe_file_path.parent,
         stdout=subprocess.DEVNULL,