Add test for LoggingHandler deduplication in app creation

Author: Harmanpreet-Microsoft

src/tests/backend/app_test.py

@@ -1,13 +1,18 @@
 # pylint: disable=redefined-outer-name
 """Tests for the FastAPI application."""
 
+import logging
+import os
+
 from backend.app import create_app
 
 from fastapi import FastAPI
 
 from httpx import ASGITransport
 from httpx import AsyncClient
 
+from opentelemetry.sdk._logs import LoggingHandler
+
 import pytest
 
 
@@ -34,3 +39,46 @@ async def test_backend_routes_exist(app: FastAPI):
     routes = [route.path for route in app.router.routes]
     backend_routes = [r for r in routes if r.startswith("/api")]
     assert backend_routes, "No backend routes found under /api prefix"
+
+
+def test_logging_handler_deduplication():
+    """Test that creating multiple apps doesn't accumulate LoggingHandler instances."""
+    # Set up Application Insights connection string to trigger telemetry setup
+    connection_string = "InstrumentationKey=test-key;IngestionEndpoint=https://test.com"
+    original_env = os.environ.get("APPLICATIONINSIGHTS_CONNECTION_STRING")
+
+    try:
+        os.environ["APPLICATIONINSIGHTS_CONNECTION_STRING"] = connection_string
+
+        # Get root logger and count existing LoggingHandlers
+        root_logger = logging.getLogger()
+        initial_handler_count = sum(1 for h in root_logger.handlers if isinstance(h, LoggingHandler))
+
+        # Create first app
+        app1 = create_app()
+        handler_count_after_first = sum(1 for h in root_logger.handlers if isinstance(h, LoggingHandler))
+
+        # Create second app
+        app2 = create_app()
+        handler_count_after_second = sum(1 for h in root_logger.handlers if isinstance(h, LoggingHandler))
+
+        # Assert only one LoggingHandler exists after multiple create_app() calls
+        assert handler_count_after_first == initial_handler_count + 1, \
+            "First create_app() should add one LoggingHandler"
+        assert handler_count_after_second == handler_count_after_first, \
+            "Second create_app() should not add another LoggingHandler (de-duplication should work)"
+
+        # Clean up - remove the handler we added
+        for handler in list(root_logger.handlers):
+            if isinstance(handler, LoggingHandler):
+                root_logger.removeHandler(handler)
+                try:
+                    handler.close()
+                except Exception:
+                    pass
+    finally:
+        # Restore original environment
+        if original_env is not None:
+            os.environ["APPLICATIONINSIGHTS_CONNECTION_STRING"] = original_env
+        else:
+            os.environ.pop("APPLICATIONINSIGHTS_CONNECTION_STRING", None)