[Python-Dev] PEP 446: Add new parameters to configure the inherance of files and for non-blocking sockets

[Victor Stinner]

2013/7/5 Cameron Simpson <cs at zip.com.au>:
> You might want to make clear that the "blocking" parameter refers
> only to the file creation calls (eg socket.socket) and not to the
> file descriptor itself, and is not to be confused with the UNIX
> O_NONBLOCK file descriptor flag (and whatever equivalent flag may
> apply on Windows).

The two following codes are the same:

s = socket.socket(..., blocking=False)

and

s = socket.socket(...)
s.setblocking(False)

Both set O_NONBLOCK flag (UNIX) or clear HANDLE_FLAG_INHERIT (Windows)
on the socket (which is a file descriptor).

socket.socket(..., blocking=False) cannot fail with EAGAIN or
EWOULDBLOCK (at least on Linux according to the manual page).

> This is deducable from your PEP, but I was at first confused, and
> initially expected get_blocking/set_blocking functions in New
> Functions.

socket objects already have get_blocking() and set_blocking() methods.
If we supported the blocking flag on any file descriptor, it would
make sense to add such function to the os module. But I explained in
the "Add blocking parameter for file descriptors and Windows
overlapped I/O" section why I only want to set/clear the blocking file
on sockets.

> I would expect Popen and friends to need to both clear the flag to
> get the descriptors across the fork() call, and _possibly_ to set
> the flag again after the fork. Naively, I would expect the the flag
> to be as it was before the Popen call, after the call.

As explained in the "Inheritance of file descriptors" section, the
close-on-exec flag has no effect on fork:

"The flag has no effect on fork(), all file descriptors are inherited
by the child process."

The close-on-exec flag is cleared after the fork and before the
exec(). Pseudo-code for Popen:

pid = os.fork()
if pid:
  return pid
# child process
for fd in pass_fds: os.set_cloexec(fd, False)
os.execv(...)

Victor