Type: Bug

Behaviour

When Python testing is active on Linux, the VS Code extension host can crash during cleanup of a python-test-results-* FIFO.

Expected: the FIFO descriptor is closed exactly once and the extension host continues running.

Actual: the FIFO is first closed by its net.Socket. The socket's close handler then schedules another asynchronous fs.close(fd). If Linux reuses that descriptor number before the asynchronous close executes, the second close closes an unrelated descriptor. libuv subsequently receives EBADF from epoll_ctl and aborts the extension host.

The failure is timing-dependent.

Steps to reproduce:

1. Open a Linux remote workspace with Python extension 2026.4.0.
2. Enable pytest and project-based testing.
3. Run tests through the VS Code Testing view.
4. Continue working while the test-result pipe is being disposed. Concurrent descriptor activity, such as Git operations, makes the race more likely.
5. Eventually the extension host disconnects and restarts.

I do not yet have a deterministic UI reproduction, but strace -f -k captured the complete descriptor lifecycle described below.

Diagnostic data

[info] [test-by-project] Activating project-based testing mode
[info] [test-by-project] Discovering projects for workspace
[info] [test-by-project] Registered 4 project(s)
[info] [test-by-project] Activated 4 project(s)

Environment

• Python extension: ms-python.python 2026.4.0
• VS Code server: 1.127.0 (4fe60c8b1cdac1c4c174f2fb180d0d758272d713)
• Node.js: 24.15.0
• libuv: 1.51.0
• OS: Ubuntu 24.04.5, x86-64
• Kernel: 7.0.0-1012-aws
• Environment: remote Dev Container

Native trace

The relevant strace -f -k -yy sequence is:

# The Python test-result FIFO owns fd 31.
openat(..., "/tmp/python-test-results-<redacted>",
       O_RDONLY|O_NONBLOCK|O_CLOEXEC) = 31

# net.Socket/libuv closes the FIFO.
epoll_ctl(14, EPOLL_CTL_DEL, 31, ...) = 0
close(31</tmp/python-test-results-<redacted>>) = 0
  uv__stream_close
  uv_close
  node::HandleWrap::Close

# Linux reuses fd 31 for an unrelated child-process stdout socket.
socketpair(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC, 0,
           [31<UNIX-STREAM>, 82<UNIX-STREAM>]) = 0
  uv_socketpair
  uv_spawn
  node::ProcessWrap::Spawn

# A libuv filesystem worker performs the second asynchronous close.
close(31<UNIX-STREAM>) = 0
  uv__close_nocancel
  uv__fs_work
  worker

# The event loop tries to register the newly created socket.
epoll_ctl(14, EPOLL_CTL_ADD, 31, {events=EPOLLIN, ...})
    = -1 EBADF (Bad file descriptor)
  uv__io_poll
  uv_run
  node::SpinEventLoopInternal

The first and second closes were ~ 177 ms apart. The new socket was created between them.

Suspected source

The current POSIX reader appears to assign ownership of the same descriptor to both net.Socket and the explicit close handler:

https://github.com/microsoft/vscode-python/blob/main/src/client/common/pipes/namedPipes.ts#L191-L197

const fd = await fs.open(pipeName, fs.constants.O_RDONLY | fs.constants.O_NONBLOCK);
const socket = new net.Socket({ fd });
const reader = new rpc.SocketMessageReader(socket, "utf-8");

socket.on("close", () => {
    fs.close(fd).catch(noop);
    reader.dispose();
});

The trace shows that net.Socket has already closed the descriptor through uv__stream_close before the handler's fs.close(fd) runs.

Draft PR #26082 replaces this implementation with a directly owned FIFO reader and guarded disposal, so it may also fix this crash.

However, those currently describe the five-second FIFO drain delay rather than this extension-host crash.

Extension version: 2026.4.0
VS Code version: Code 1.139.1 (04c0d99f4fb0d8afe6ce4f0c58e31e183ac3e4b1, 2026-09-25T03:40:57Z)
OS version: Darwin arm64 27.0.0
Modes:

• Python version (& distribution if applicable, e.g. Anaconda): 3.14.2
• Type of virtual environment used (e.g. conda, venv, virtualenv, etc.): Pyenv
• Value of the python.languageServer setting: Default

languageServer: "Pylance"

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ccr_pr_nudge_control:1319470
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jc553158:1416789