Ignore AttributeError when shutting down server

It's possible for the _server attribute to not exist if make_server
raises an error in the constructor.

Author: tjni

pytest_localserver/http.py

@@ -31,7 +31,12 @@ def __del__(self):
         self.stop()
 
     def stop(self):
-        self._server.shutdown()
+        try:
+            server = self._server
+        except AttributeError:
+            pass
+        else:
+            server.shutdown()
 
     @property
     def url(self):

tests/test_http.py

@@ -1,4 +1,7 @@
 import itertools
+import subprocess
+import sys
+import textwrap
 
 import pytest
 import requests
@@ -266,3 +269,28 @@ def test_GET_request_chunked_no_content_length(httpserver, chunked_flag):
     assert resp.status_code == 200
     assert "Transfer-encoding" in resp.headers
     assert "Content-length" not in resp.headers
+
+
+def test_httpserver_init_failure_no_stderr_during_cleanup(tmp_path):
+    """
+    Test that, when the server encounters an error during __init__, its cleanup
+    does not raise an error (in its __del__ method), which emits a warning onto
+    stderr.
+    """
+
+    script_path = tmp_path.joinpath("script.py")
+
+    script_path.write_text(textwrap.dedent("""
+        from pytest_localserver import http
+        from unittest.mock import patch
+
+        with patch("pytest_localserver.http.make_server", side_effect=RuntimeError("init failure")):
+            server = http.ContentServer()
+    """))
+
+    result = subprocess.run([sys.executable, script_path], stderr=subprocess.PIPE)
+
+    # We can't check the exit code, because we are expecting the script to fail.
+    # Instead, we ensure that no warning about an AttributeError is printed on
+    # stderr due to an error in the server's __del__ method.
+    assert b"AttributeError" not in result.stderr