sendfile for sockets neglects pseudo files on Linux

[illia-v]

Bug report

socket.socket.sendfile and asyncio.base_events.BaseEventLoop.sock_sendfile rely on st_size to determine an amount of bytes to be sent if count is not provided.

But st_size is equal to zero for pseudo files like /proc/cpuinfo on Linux. Therefore, the methods send no data for such files.

Related code

cpython/Lib/socket.py

Lines 355 to 360 in a637c09

	try:
	fsize = os.fstat(fileno).st_size
	except OSError as err:
	raise _GiveupOnSendfile(err) # not a regular file
	if not fsize:
	return 0 # empty file

cpython/Lib/asyncio/unix_events.py

Lines 361 to 367 in 13c10bf

	try:
	fsize = os.fstat(fileno).st_size
	except OSError:
	raise exceptions.SendfileNotAvailableError("not a regular file")
	blocksize = count if count else fsize
	if not blocksize:
	return 0 # empty file

Example

Assuming nc -lkU some.sock and ./python -m asyncio run in the same directory:

>>> import asyncio
>>> import socket
>>> sock = socket.socket(socket.AF_UNIX)
>>> loop = asyncio.get_running_loop()
>>> await loop.sock_connect(sock, "./some.sock")
>>> with open("/proc/cpuinfo", "rb") as cpuinfo_file:
...     os.fstat(cpuinfo_file.fileno()).st_size
...     await loop.sock_sendfile(sock, cpuinfo_file)
...     sock.sendfile(cpuinfo_file)
...     len(cpuinfo_file.read())
... 
0
0
0
18294

[kumaraditya303]

Do you have any fix for this or idea of how this could be fixed? Also are there any use-case of sending pseudo files through sendfile ?

[illia-v]

@kumaraditya303 I do not have any use case, and I have not looked for fixing options.

I was exploring peculiarities of using the copy_file_range syscall for a pseudo file, and checked if a similar issue related to pseudo files exists in Python. That is how I found this issue.

[kumaraditya303]

I don't think this is a bug and there is nothing actionable here so closing. However, this can be reopened if required.