Set TCP_NODELAY on pydevd client connections

src/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_comm.py

@@ -480,6 +480,10 @@ def start_server(port):
         pydev_log.info(msg)
 
         new_socket, _addr = s.accept()
+        try:
+            new_socket.setsockopt(socket_module.IPPROTO_TCP, socket_module.TCP_NODELAY, 1)
+        except (AttributeError, OSError):
+            pass  # May not be available everywhere.
         pydev_log.info("Connection accepted")
         # closing server socket is not necessary but we don't need it
         s.close()
@@ -525,6 +529,10 @@ def start_client(host, port):
         s.setsockopt(socket_module.IPPROTO_TCP, socket_module.TCP_KEEPCNT, 5)
     except (AttributeError, OSError):
         pass  # May not be available everywhere.
+    try:
+        s.setsockopt(socket_module.IPPROTO_TCP, socket_module.TCP_NODELAY, 1)
+    except (AttributeError, OSError):
+        pass  # May not be available everywhere.
 
     try:
         # 10 seconds default timeout

src/debugpy/_vendored/pydevd/tests_python/test_pydevd_comm.py

@@ -0,0 +1,68 @@
+from unittest import mock
+
+import pytest
+
+from _pydevd_bundle import pydevd_comm
+
+
+def start_client(monkeypatch, sock):
+    monkeypatch.setattr(pydevd_comm, "socket", mock.Mock(return_value=sock))
+    monkeypatch.setattr(
+        pydevd_comm.socket_module,
+        "getaddrinfo",
+        lambda *_: [(pydevd_comm.AF_INET, pydevd_comm.SOCK_STREAM, 0, "", ("127.0.0.1", 5678))],
+    )
+    assert pydevd_comm.start_client("localhost", 5678) is sock
+    sock.connect.assert_called_once_with(("localhost", 5678))
+
+
+def test_start_client_sets_tcp_nodelay(monkeypatch):
+    sock = mock.Mock()
+    monkeypatch.setattr(pydevd_comm.socket_module, "TCP_NODELAY", mock.sentinel.tcp_nodelay, raising=False)
+
+    start_client(monkeypatch, sock)
+    sock.setsockopt.assert_any_call(pydevd_comm.socket_module.IPPROTO_TCP, mock.sentinel.tcp_nodelay, 1)
+
+
+@pytest.mark.parametrize("error", [AttributeError, OSError])
+def test_start_client_ignores_tcp_nodelay_error(monkeypatch, error):
+    sock = mock.Mock()
+    sock.setsockopt.side_effect = error
+    monkeypatch.setattr(pydevd_comm.socket_module, "TCP_NODELAY", mock.sentinel.tcp_nodelay, raising=False)
+
+    start_client(monkeypatch, sock)
+    sock.setsockopt.assert_any_call(pydevd_comm.socket_module.IPPROTO_TCP, mock.sentinel.tcp_nodelay, 1)
+
+
+def test_start_server_sets_tcp_nodelay(monkeypatch):
+    server, accepted = mock.Mock(), mock.Mock()
+    address = ("127.0.0.1", 5678)
+    server.configure_mock(**{"accept.return_value": (accepted, address), "getsockname.return_value": address})
+    monkeypatch.setattr(pydevd_comm, "create_server_socket", mock.Mock(return_value=server))
+    monkeypatch.setattr(pydevd_comm.socket_module, "TCP_NODELAY", mock.sentinel.tcp_nodelay, raising=False)
+    assert pydevd_comm.start_server(0) is accepted
+    accepted.setsockopt.assert_called_once_with(pydevd_comm.socket_module.IPPROTO_TCP, mock.sentinel.tcp_nodelay, 1)
+
+
+@pytest.mark.parametrize("error", [AttributeError, OSError])
+def test_start_server_ignores_tcp_nodelay_error(monkeypatch, error):
+    server, accepted = mock.Mock(), mock.Mock()
+    address = ("127.0.0.1", 5678)
+    server.configure_mock(**{"accept.return_value": (accepted, address), "getsockname.return_value": address})
+    accepted.setsockopt.side_effect = error
+    monkeypatch.setattr(pydevd_comm, "create_server_socket", mock.Mock(return_value=server))
+    monkeypatch.setattr(pydevd_comm.socket_module, "TCP_NODELAY", mock.sentinel.tcp_nodelay, raising=False)
+
+    assert pydevd_comm.start_server(0) is accepted
+    accepted.setsockopt.assert_called_once_with(pydevd_comm.socket_module.IPPROTO_TCP, mock.sentinel.tcp_nodelay, 1)
+
+
+def test_startup_ignores_missing_tcp_nodelay(monkeypatch):
+    monkeypatch.delattr(pydevd_comm.socket_module, "TCP_NODELAY", raising=False)
+    start_client(monkeypatch, mock.Mock())
+
+    server, accepted = mock.Mock(), mock.Mock()
+    address = ("127.0.0.1", 5678)
+    server.configure_mock(**{"accept.return_value": (accepted, address), "getsockname.return_value": address})
+    monkeypatch.setattr(pydevd_comm, "create_server_socket", mock.Mock(return_value=server))
+    assert pydevd_comm.start_server(0) is accepted