Issue #26586: Merge excessive HTTP header handling from 3.5

Lib/http/server.py

@@ -338,6 +338,13 @@ class BaseHTTPRequestHandler(socketserver.StreamRequestHandler):
                 HTTPStatus.BAD_REQUEST,
                 "Line too long")
             return False
+        except http.client.HTTPException as err:
+            self.send_error(
+                HTTPStatus.REQUEST_HEADER_FIELDS_TOO_LARGE,
+                "Too many headers",
+                str(err)
+            )
+            return False
 
         conntype = self.headers.get('Connection', "")
         if conntype.lower() == 'close':

Lib/test/test_httpservers.py

@@ -859,6 +859,13 @@ class BaseHTTPRequestHandlerTestCase(unittest.TestCase):
         self.assertFalse(self.handler.get_called)
         self.assertEqual(self.handler.requestline, 'GET / HTTP/1.1')
 
+    def test_too_many_headers(self):
+        result = self.send_typical_request(
+            b'GET / HTTP/1.1\r\n' + b'X-Foo: bar\r\n' * 101 + b'\r\n')
+        self.assertEqual(result[0], b'HTTP/1.1 431 Too many headers\r\n')
+        self.assertFalse(self.handler.get_called)
+        self.assertEqual(self.handler.requestline, 'GET / HTTP/1.1')
+
     def test_close_connection(self):
         # handle_one_request() should be repeatedly called until
         # it sets close_connection

Misc/NEWS

@@ -237,6 +237,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #26586: In http.server, respond with "413 Request header fields too
+  large" if there are too many header fields to parse, rather than killing
+  the connection and raising an unhandled exception.  Patch by Xiang Zhang.
+
 - Issue #26676: Added missing XMLPullParser to ElementTree.__all__.
 
 - Issue #22854: Change BufferedReader.writable() and