When WinAPI ReadFile() is called to read from an empty pipe that's in non-blocking mode, the call fails with ERROR_NO_DATA (232). The C runtime's read() function maps this error code to the errno value EINVAL (invalid argument). We would have to update all code that calls read() to clarify this case by checking _doserrno. This is the Windows error code that was mapped to the C errno value. We can manually map ERROR_NO_DATA to EAGAIN (resource temporarily unavailable) in the read() context¹ and possibly raise BlockingIOError, unless EAGAIN should be ignored.

When WinAPI WriteFile() is called to write to a pipe that's in non-blocking mode, if the size of the write exceeds the available space, then the call succeeds with 0 bytes written². The C runtime's write() function usually handles a 'successful' write with nothing written by failing with ENOSPC (no space left on device), except it's not considered an error when writing a string that starts with Ctrl+Z to a character device³. For POSIX compatibility, the errno value that we want in this case is EAGAIN.

@eryksun thanks for the detailed information about the Win32 API, I've implemented your suggestions in #101882. Though I'm not sure why test_nonblock_pipe_write_bigbuf is failing when test_nonblock_pipe_write_smallbuf is not.

Usage in the io module and test cases will have to be examined for assumptions that partial writes are supported up to the available space in the pipe. On Windows, it's all or nothing in non-blocking mode. If the pipe has 1024 bytes available, then trying to write 2048 bytes will write nothing at all and fail.

Here's an idea to get around the annoying behavior in non-blocking mode. Modify _Py_write_impl() to check for this case. If so, cap the write to the size of the pipe, from GetNamedPipeInfo(). Eventually the reader will empty the pipe, and a write of the full pipe size will succeed. This allows test_nonblock_pipe_write_bigbuf() to succeed.

For example, append the following code to the first MS_WINDOWS section in _Py_write_impl().

    // A write that exceeds the available size of a pipe never succeeds in
    // non-blocking mode. Limiting writes to the pipe size in this case allows
    // a buffered write to succeed eventually, as the pipe is read.
    DWORD mode, pipe_size;
    HANDLE hfile = _Py_get_osfhandle(fd);
    if (hfile == INVALID_HANDLE_VALUE) {
        return -1;
    }
    if (gil_held) {
        Py_BEGIN_ALLOW_THREADS
        if (GetFileType(hfile) == FILE_TYPE_PIPE &&
            GetNamedPipeHandleStateW(hfile, &mode,
              NULL, NULL, NULL, NULL, 0) &&
            mode & PIPE_NOWAIT)
        {
            // GetNamedPipeInfo() requires FILE_READ_ATTRIBUTES access.
            // CreatePipe() includes this access for the write handle.
            if (!GetNamedPipeInfo(hfile, NULL, NULL, &pipe_size, NULL)) {
                pipe_size = 4096;
            }
            if (count > pipe_size) {
                count = pipe_size;
            }
        }
        Py_END_ALLOW_THREADS
    }
    else {
        if (GetFileType(hfile) == FILE_TYPE_PIPE &&
            GetNamedPipeHandleStateW(hfile, &mode,
              NULL, NULL, NULL, NULL, 0) &&
            mode & PIPE_NOWAIT)
        {
            if (!GetNamedPipeInfo(hfile, NULL, NULL, &pipe_size, NULL)) {
                pipe_size = 4096;
            }
            if (count > pipe_size) {
                count = pipe_size;
            }
        }
    }

The documentation in "Doc/library/os.rst" needs to be updated to indicate the new Windows support in 3.12, i.e. .. availability:: and .. versionchanged::, plus a brief description that Windows support is limited to pipes.

For example:

.. function:: get_blocking(fd, /)

   Get the blocking mode of the file descriptor: ``False`` if the
   :data:`O_NONBLOCK` flag is set, ``True`` if the flag is cleared.

   See also :func:`set_blocking` and :meth:`socket.socket.setblocking`.

   .. availability:: Unix, Windows.

      The function is limited on Emscripten and WASI, see
      :ref:`wasm-availability` for more information.

      On Windows, this function is limited to pipe files.

   .. versionadded:: 3.5
   
   .. versionchanged:: 3.12
      Added support for Windows

.. function:: set_blocking(fd, blocking, /)

   Set the blocking mode of the specified file descriptor. Set the
   :data:`O_NONBLOCK` flag if blocking is ``False``, clear the flag otherwise.

   See also :func:`get_blocking` and :meth:`socket.socket.setblocking`.

   .. availability:: Unix, Windows.

      The function is limited on Emscripten and WASI, see
      :ref:`wasm-availability` for more information.

      On Windows, this function is limited to pipe files.

   .. versionadded:: 3.5

   .. versionchanged:: 3.12
      Added support for Windows

done, anything else needed?