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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.
Footnotes
The error mapping depends on context. When writing to a pipe, if the read end is closed, this also fails with ERROR_NO_DATA, which we need to map to EPIPE and raise BrokenPipeError. These cases are distinct at the lower level of the NT API. When NtReadFile() is called to read from an empty pipe in non-blocking mode, the call fails with STATUS_PIPE_EMPTY. When NtWriteFile() is called to Write to a pipe for which the read end is closed, the call fails with STATUS_PIPE_CLOSING. Unfortunately the Windows API maps both of these status codes to ERROR_NO_DATA. ↩
It's documented that as much as possible will be written if a byte-type pipe (as opposed to a message-type pipe) is in non-blocking mode. However, in my testing back to Windows NT 4.0 (1996), no data is written to a pipe in non-blocking mode if the write size exceeds the available space, even if the pipe is empty. It's all or nothing. ↩
This is bizarre because nothing nowadays implements special behavior for writing Ctrl+Z to a character device. Reading from console input via ReadFile() is special cased to return 0 bytes read if the line read from the console begins with Ctrl+Z. ↩
@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.DWORDmode, pipe_size;
HANDLEhfile=_Py_get_osfhandle(fd);
if (hfile==INVALID_HANDLE_VALUE) {
return-1;
}
if (gil_held) {
Py_BEGIN_ALLOW_THREADSif (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
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eryksun commentedon Feb 13, 2023
When WinAPI
ReadFile()is called to read from an empty pipe that's in non-blocking mode, the call fails withERROR_NO_DATA(232). The C runtime'sread()function maps this error code to theerrnovalueEINVAL(invalid argument). We would have to update all code that callsread()to clarify this case by checking_doserrno. This is the Windows error code that was mapped to the Cerrnovalue. We can manually mapERROR_NO_DATAtoEAGAIN(resource temporarily unavailable) in theread()context1 and possibly raiseBlockingIOError, unlessEAGAINshould 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'swrite()function usually handles a 'successful' write with nothing written by failing withENOSPC(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, theerrnovalue that we want in this case isEAGAIN.Footnotes
The error mapping depends on context. When writing to a pipe, if the read end is closed, this also fails with
ERROR_NO_DATA, which we need to map toEPIPEand raiseBrokenPipeError. These cases are distinct at the lower level of the NT API. WhenNtReadFile()is called to read from an empty pipe in non-blocking mode, the call fails withSTATUS_PIPE_EMPTY. WhenNtWriteFile()is called to Write to a pipe for which the read end is closed, the call fails withSTATUS_PIPE_CLOSING. Unfortunately the Windows API maps both of these status codes toERROR_NO_DATA. ↩It's documented that as much as possible will be written if a byte-type pipe (as opposed to a message-type pipe) is in non-blocking mode. However, in my testing back to Windows NT 4.0 (1996), no data is written to a pipe in non-blocking mode if the write size exceeds the available space, even if the pipe is empty. It's all or nothing. ↩
This is bizarre because nothing nowadays implements special behavior for writing Ctrl+Z to a character device. Reading from console input via
ReadFile()is special cased to return 0 bytes read if the line read from the console begins with Ctrl+Z. ↩RayyanAnsari commentedon Feb 14, 2023
@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.
eryksun commentedon Feb 14, 2023
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.
eryksun commentedon Feb 14, 2023
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, fromGetNamedPipeInfo(). Eventually the reader will empty the pipe, and a write of the full pipe size will succeed. This allowstest_nonblock_pipe_write_bigbuf()to succeed.For example, append the following code to the first
MS_WINDOWSsection in_Py_write_impl().eryksun commentedon Feb 15, 2023
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:
pythongh-101881: os: support blocking functions on Windows
RayyanAnsari commentedon Feb 15, 2023
done, anything else needed?
pythongh-101881: os: support blocking functions on Windows
pythongh-101881: os: support blocking functions on Windows
gh-101881: Support (non-)blocking read/write functions on Windows pip…
win32: special-case `ENOSPC` when writing to a pipe
win32: special-case `ENOSPC` when writing to a pipe
win32: special-case `ENOSPC` when writing to a pipe
win32: special-case `ENOSPC` when writing to a pipe
win32: special-case `ENOSPC` when writing to a pipe