pydevd_comm.py start_client() does not set TCP_NODELAY on the connections it opens

Start in 3rdparty/debugpy/DEBUGPY/src/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_comm.py at start_client(), and inspect how the socket is configured before the connection is opened. Confirm the connection used by debugpy --connect enables TCP_NODELAY and preserves the existing fallback behavior when the option is unavailable. Done means the source and an established connection reflect the expected setting.

Environment data

• debugpy version: 1.8.20
• OS and version: Linux and MacOS
• Python version (& distribution if applicable, e.g. Anaconda): 3.11 and later
• Using VS Code or Visual Studio: No

Actual behavior

TCP_NODELAY is not set on the connections debugpy opens

Expected behavior

TCP_NODELAY is set on the connections debugpy opens

Steps to reproduce:

A connection established with debugpy --connect does not set TCP_NODELAY on the socket connection, which can cause latency when sending responses and events. This is can be verified simply by inspecting the pydevd_comm.py source code, or by using external tools to inspect the connection once it has been established.

We patched our copy of debugpy 1.8.20 as follows:

% diff --git a/3rdparty/debugpy/DEBUGPY/src/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_comm.py b/3rdparty/debugpy/DEBUGPY/src/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_comm.py
index 64ac454f29..812bf59eb0 100644
--- a/3rdparty/debugpy/DEBUGPY/src/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_comm.py
+++ b/3rdparty/debugpy/DEBUGPY/src/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_comm.py
@@ -524,6 +524,15 @@ def start_client(host, port):
     except (AttributeError, OSError):
         pass  # May not be available everywhere.
 
+    try:
+        # TVT-46072: "Missing TCP_NODELAY on debugpy's DAP tracer connection
+        # causes Nagle-induced latency in Python debugging". Disable Nagle's
+        # algorithm so small DAP messages are sent immediately instead of
+        # being buffered, reducing round-trip latency.
+        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
         timeout = int(os.environ.get("PYDEVD_CONNECT_TIMEOUT", 10))
%