Avoid preview hangs caused by idle browser connections

serve.py

@@ -1,5 +1,5 @@
 import os
-from http.server import SimpleHTTPRequestHandler, HTTPServer
+from http.server import SimpleHTTPRequestHandler, ThreadingHTTPServer
 from urllib.parse import urlparse, unquote
 import urllib
 from http import HTTPStatus
@@ -68,7 +68,7 @@ def _join_under(root, rel_path):
 
     PORT = args.port
     handler = CustomHTTPRequestHandler
-    with HTTPServer(("", PORT), handler) as httpd:
+    with ThreadingHTTPServer(("", PORT), handler) as httpd:
         print(f"Serving at http://localhost:{PORT}/analysis/")
         print(f"/analysis: {BOOK_SITE}")
         print(f"/analysis/docs: {DOCS_SITE}")

tests/test_serve_connections.py

@@ -0,0 +1,40 @@
+import http.client
+from pathlib import Path
+import socket
+import sys
+import threading
+import unittest
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
+import serve
+
+
+class TestPreviewConnections(unittest.TestCase):
+    def test_idle_preconnection_does_not_block_another_request(self):
+        server = serve.ThreadingHTTPServer(('127.0.0.1', 0), serve.CustomHTTPRequestHandler)
+        idle = socket.create_connection(server.server_address, timeout=2)
+        # Accept the idle connection before the active one is created.
+        accepted = threading.Event()
+        original = server.get_request
+
+        def get_request():
+            result = original()
+            accepted.set()
+            return result
+
+        server.get_request = get_request
+        worker = threading.Thread(target=server.serve_forever, daemon=True)
+        worker.start()
+        conn = http.client.HTTPConnection(*server.server_address, timeout=2)
+        try:
+            self.assertTrue(accepted.wait(2))
+            conn.request('GET', '/favicon.ico')
+            response = conn.getresponse()
+            self.assertEqual(response.status, 204)
+            response.read()
+        finally:
+            conn.close()
+            idle.close()
+            server.shutdown()
+            worker.join()
+            server.server_close()