[releases/1.1] ni_measurementlink_service: Launch discovery service just-in-time before registering a measurement service (#317)

ni_measurementlink_service: Launch discovery service just-in-time before registering a measurement service. (#290)

* discovery_client: Launch discovery service just-in-time

* discovery_client: add comments to functions

* discovery_client: Fix lint errors.

* discovery_client: Fix function docstring and type hint

* discovery_client: Wait for key file existence and time out when 30 seconds elapse

* discovery_client: Replace polling2 module with time module

* discovery_client: Added service already running functionality

* discovery_client: Refactor open method with write permission.

Co-authored-by: Brad Keryan <brad.keryan@ni.com>

---------

Co-authored-by: Brad Keryan <brad.keryan@ni.com>
(cherry picked from commit 5af9b2f)

Co-authored-by: Vikram S A <vikram.avudaiappan@ni.com>

Author: bkeryan

ni_measurementlink_service/_internal/discovery_client.py

@@ -4,7 +4,9 @@
 import logging
 import os
 import pathlib
+import subprocess
 import sys
+import time
 import typing
 from typing import Optional
 
@@ -26,6 +28,9 @@
 
 _logger = logging.getLogger(__name__)
 
+_START_SERVICE_TIMEOUT = 30.0
+_START_SERVICE_POLLING_INTERVAL = 100e-3
+
 
 class ServiceLocation(typing.NamedTuple):
     """Represents the location of a service."""
@@ -204,12 +209,78 @@ def resolve_service(self, provided_interface: str, service_class: str = "") -> S
 
 def _get_discovery_service_address() -> str:
     key_file_path = _get_key_file_path()
+    _ensure_discovery_service_started(key_file_path)
     _logger.debug("Discovery service key file path: %s", key_file_path)
     with _open_key_file(str(key_file_path)) as key_file:
         key_json = json.load(key_file)
         return "localhost:" + key_json["InsecurePort"]
 
 
+def _ensure_discovery_service_started(key_file_path: pathlib.Path) -> None:
+    """Check whether discovery service already running, if not start the discovery service."""
+    if _service_already_running(key_file_path):
+        return
+
+    exe_file_path = _get_discovery_service_location()
+    _start_service(exe_file_path, key_file_path)
+
+
+def _get_discovery_service_location() -> pathlib.PurePath:
+    """Gets the location of the discovery service process executable."""
+    registration_json_path = _get_registration_json_file_path()
+    registration_json_obj = json.loads(registration_json_path.read_text())
+    return registration_json_path.parent / registration_json_obj["discovery"]["path"]
+
+
+def _get_registration_json_file_path() -> pathlib.Path:
+    if sys.platform == "win32":
+        return (
+            pathlib.Path(os.environ["ProgramW6432"])
+            / "National Instruments"
+            / "Shared"
+            / "MeasurementLink"
+            / "MeasurementLinkServices.json"
+        )
+    else:
+        raise NotImplementedError("Platform not supported")
+
+
+def _key_file_exists(key_file_path: pathlib.Path) -> bool:
+    return key_file_path.is_file() and key_file_path.stat().st_size > 0
+
+
+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([exe_file_path], cwd=exe_file_path.parent)
+    # After the execution of process, check for key file existence in the path
+    # stop checking after 30 seconds have elapsed and throw error
+    timeout_time = time.time() + _START_SERVICE_TIMEOUT
+    while True:
+        try:
+            with _open_key_file(str(key_file_path)) as _:
+                return
+        except IOError:
+            pass
+        if time.time() >= timeout_time:
+            raise TimeoutError("Timed out waiting for discovery service to start")
+        time.sleep(_START_SERVICE_POLLING_INTERVAL)
+
+
+def _service_already_running(key_file_path: pathlib.Path) -> bool:
+    try:
+        _delete_existing_key_file(key_file_path)
+    except IOError:
+        return True
+    return False
+
+
+def _delete_existing_key_file(key_file_path: pathlib.Path) -> None:
+    if _key_file_exists(key_file_path):
+        with key_file_path.open("w") as _:
+            pass
+        key_file_path.unlink()
+
+
 def _get_key_file_path(cluster_id: Optional[str] = None) -> pathlib.Path:
     if cluster_id is not None:
         return _get_key_file_directory() / f"DiscoveryService_{cluster_id}.json"