fix: failing DNS integration tests by using long-running functions (#8889)

* fix: fix failing DNS integration tests by using long-running functions

The DNS container inspection tests were failing because Lambda containers
exit immediately after the handler completes, making inspection impossible.

Changes:
- test_invoke_dns.py: Use WriteToStdoutFunction instead of HelloWorldServerlessFunction
  to avoid KeyError when no_event=True is passed
- test_start_api_dns.py: Use threading with /sleepfortenseconds endpoint to keep
  container alive during inspection
- test_start_lambda_dns.py: Use threading with HelloWorldSleepFunction to keep
  container alive during inspection

All tests now use long-running functions (10+ seconds) with async threading
pattern, allowing enough time to inspect running containers before they exit.

Fixes tests that were failing after #8878 was merged.

* fix: black formatting

---------

Co-authored-by: Kandarp Ajvalia <ajvalia@amazon.com>

Author: KandarpAjvalia

tests/integration/local/invoke/test_invoke_dns.py

@@ -90,7 +90,7 @@ def test_invoke_accepts_multiple_dns_servers(self):
         and executes successfully
         """
         command_list = self.get_command_list(
-            function_to_invoke="HelloWorldServerlessFunction",
+            function_to_invoke="WriteToStdoutFunction",
             template_path=self.template_path,
             no_event=True,
         )
@@ -104,4 +104,4 @@ def test_invoke_accepts_multiple_dns_servers(self):
         self.assertEqual(return_code, 0, f"Command failed with stderr: {stderr}")
 
         output = stdout.decode("utf-8")
-        self.assertIn("Hello World", output)
+        self.assertIn("wrote to stdout", output)

tests/integration/local/start_api/test_start_api_dns.py

@@ -2,6 +2,8 @@
 Integration tests for DNS option in sam local start-api
 """
 
+import time
+import threading
 import requests
 import pytest
 from tests.integration.local.start_api.start_api_integ_base import StartApiIntegBaseClass
@@ -44,8 +46,19 @@ def test_dns_configured_in_container(self):
         Test that DNS servers are actually configured in the Docker container.
         This inspects the container configuration to verify DNS was set correctly.
         """
-        response = requests.get(f"http://127.0.0.1:{self.port}/anyandall", timeout=300)
-        self.assertEqual(response.status_code, 200)
+        # Start async request to keep container alive during inspection
+        response_holder = {}
+
+        def make_request():
+            response_holder["response"] = requests.get(
+                f"http://127.0.0.1:{self.port}/sleepfortenseconds/function0", timeout=300
+            )
+
+        request_thread = threading.Thread(target=make_request)
+        request_thread.start()
+
+        # Wait for container to start and be in sleep phase
+        time.sleep(7)
 
         sam_containers = self.docker_client.containers.list(
             all=False, filters={"label": "sam.cli.container.type=lambda"}
@@ -76,5 +89,9 @@ def test_dns_configured_in_container(self):
 
         self.assertTrue(
             dns_verified,
-            f"Could not verify DNS configuration in any container. " f"Checked {len(sam_containers)} containers",
+            f"Could not verify DNS configuration in any container. Checked {len(sam_containers)} containers",
         )
+
+        # Wait for request to complete
+        request_thread.join()
+        self.assertEqual(response_holder["response"].status_code, 200)

tests/integration/local/start_lambda/test_start_lambda_dns.py

@@ -2,6 +2,8 @@
 Integration tests for DNS option in sam local start-lambda
 """
 
+import time
+import threading
 import pytest
 from tests.integration.local.start_lambda.start_lambda_api_integ_base import StartLambdaIntegBaseClass
 
@@ -78,8 +80,19 @@ def test_dns_configured_in_container(self):
         Test that DNS servers are actually configured in the Docker container.
         This inspects the container configuration to verify DNS was set correctly.
         """
-        response = self.lambda_client.invoke(FunctionName="EchoEventFunction", Payload='{"key": "value"}')
-        self.assertEqual(response.get("StatusCode"), 200)
+        # Start async invocation to keep container alive during inspection
+        response_holder = {}
+
+        def invoke_function():
+            response_holder["response"] = self.lambda_client.invoke(
+                FunctionName="HelloWorldSleepFunction", Payload="{}"
+            )
+
+        invoke_thread = threading.Thread(target=invoke_function)
+        invoke_thread.start()
+
+        # Wait for container to start and be in sleep phase
+        time.sleep(5)
 
         sam_containers = self.docker_client.containers.list(
             all=False, filters={"label": "sam.cli.container.type=lambda"}
@@ -113,5 +126,9 @@ def test_dns_configured_in_container(self):
 
         self.assertTrue(
             dns_verified,
-            f"Could not verify DNS configuration in any container. " f"Checked {len(sam_containers)} containers",
+            f"Could not verify DNS configuration in any container. Checked {len(sam_containers)} containers",
         )
+
+        # Wait for invocation to complete
+        invoke_thread.join()
+        self.assertEqual(response_holder["response"].get("StatusCode"), 200)