Fallback `socketpair` authentication is unreliable on WASI

[dicej]

Bug report

Bug description:

#122133 introduced authentication for the fallback socketpair implementation in Lib/socket.py. However, that's unreliable on WASI (and possibly elsewhere), where calling connect(2) on a non-blocking socket may leave the socket in a "connecting" but not yet "connected" state. In the former case, calling getpeername(2) on it will fail, leading to an unhandled exception in Python.

More generally, I'm a bit surprised the authentication code works reliably on any platform; I don't believe POSIX guarantees getpeername(2) will succeed for a socket which may or may not have finished connecting. Perhaps other platforms reliably complete loopback connections synchronously for non-blocking sockets?

Note that WASI socket support is not yet enabled by default in CPython, but is available via componentize-py, which has been carrying a patch to CPython which I'm finally getting around to submitting upstream.

Given the unofficial state of WASI sockets in CPython, reproducing the issue is a bit awkward, but can be done by:

1. cloning the componentize-py Git repo (e.g. git clone --recurse-submodules https://github.com/bytecodealliance/componentize-py),
2. switching to the cpython subdirectory, removing the aforementioned patch (e.g. git reset --hard HEAD^),
3. following the directions in CONTRIBUTING.md to build everything,
4. installing Wasmtime,
5. and finally running cargo test --release --test componentize tcp_example

...at which point you'll see something like:

Traceback (most recent call last):
  File "/0/app.py", line 17, in run
  File "/python/asyncio/runners.py", line 203, in run
  File "/python/asyncio/runners.py", line 59, in __enter__
  File "/python/asyncio/runners.py", line 146, in _lazy_init
  File "/python/asyncio/events.py", line 844, in new_event_loop
  File "/python/asyncio/events.py", line 732, in new_event_loop
  File "/python/asyncio/unix_events.py", line 61, in __init__
  File "/python/asyncio/selector_events.py", line 66, in __init__
  File "/python/asyncio/selector_events.py", line 120, in _make_self_pipe
  File "/python/socket.py", line 645, in _fallback_socketpair
OSError: [Errno 53] Socket not connected

I plan to post a PR for this shortly.

CPython versions tested on:

3.14

Operating systems tested on:

Other

Linked PRs

• gh-146139: Disable socketpair authentication on WASI #146140

[dicej]

cc @brettcannon @sethmlarson In case you have thoughts or opinions.