bpo-43332: Buffer proxy connection setup packets before sending. (GH-24780)

We now buffer the CONNECT request + tunnel HTTP headers into a single
send call.  This prevents the OS from generating multiple network
packets for connection setup when not necessary, improving efficiency.
(cherry picked from commit c25910a135)

Co-authored-by: Gregory P. Smith <greg@krypto.org>

Lib/http/client.py

@@ -886,23 +886,24 @@ class HTTPConnection:
         self.debuglevel = level
 
     def _tunnel(self):
-        connect_str = "CONNECT %s:%d HTTP/1.0\r\n" % (self._tunnel_host,
-            self._tunnel_port)
-        connect_bytes = connect_str.encode("ascii")
-        self.send(connect_bytes)
+        connect = b"CONNECT %s:%d HTTP/1.0\r\n" % (
+            self._tunnel_host.encode("ascii"), self._tunnel_port)
+        headers = [connect]
         for header, value in self._tunnel_headers.items():
-            header_str = "%s: %s\r\n" % (header, value)
-            header_bytes = header_str.encode("latin-1")
-            self.send(header_bytes)
-        self.send(b'\r\n')
+            headers.append(f"{header}: {value}\r\n".encode("latin-1"))
+        headers.append(b"\r\n")
+        # Making a single send() call instead of one per line encourages
+        # the host OS to use a more optimal packet size instead of
+        # potentially emitting a series of small packets.
+        self.send(b"".join(headers))
+        del headers
 
         response = self.response_class(self.sock, method=self._method)
         (version, code, message) = response._read_status()
 
         if code != http.HTTPStatus.OK:
             self.close()
-            raise OSError("Tunnel connection failed: %d %s" % (code,
-                                                               message.strip()))
+            raise OSError(f"Tunnel connection failed: {code} {message.strip()}")
         while True:
             line = response.fp.readline(_MAXLINE + 1)
             if len(line) > _MAXLINE:

Lib/test/test_httplib.py

@@ -10,6 +10,7 @@ import threading
 import warnings
 
 import unittest
+from unittest import mock
 TestCase = unittest.TestCase
 
 from test import support
@@ -2021,6 +2022,23 @@ class TunnelTests(TestCase):
         # This test should be removed when CONNECT gets the HTTP/1.1 blessing
         self.assertNotIn(b'Host: proxy.com', self.conn.sock.data)
 
+    def test_tunnel_connect_single_send_connection_setup(self):
+        """Regresstion test for https://bugs.python.org/issue43332."""
+        with mock.patch.object(self.conn, 'send') as mock_send:
+            self.conn.set_tunnel('destination.com')
+            self.conn.connect()
+            self.conn.request('GET', '/')
+        mock_send.assert_called()
+        # Likely 2, but this test only cares about the first.
+        self.assertGreater(
+                len(mock_send.mock_calls), 1,
+                msg=f'unexpected number of send calls: {mock_send.mock_calls}')
+        proxy_setup_data_sent = mock_send.mock_calls[0][1][0]
+        self.assertIn(b'CONNECT destination.com', proxy_setup_data_sent)
+        self.assertTrue(
+                proxy_setup_data_sent.endswith(b'\r\n\r\n'),
+                msg=f'unexpected proxy data sent {proxy_setup_data_sent!r}')
+
     def test_connect_put_request(self):
         self.conn.set_tunnel('destination.com')
         self.conn.request('PUT', '/', '')

Misc/NEWS.d/next/Library/2021-03-07-11-23-20.bpo-43332.weatsh.rst

@@ -0,0 +1,3 @@
+Improves the networking efficiency of :mod:`http.client` when using a proxy
+via :meth:`~HTTPConnection.set_tunnel`.  Fewer small send calls are made
+during connection setup.