== Sending and receiving file descriptors in Python

On some but not all modern Unix systems, file descriptors (the
underlying operating system level thing behind open files, sockets, and
so on) can be passed between cooperating processes using Unix domain
sockets and special options to _sendmsg()_ and _recvmsg()_. There are a
number of uses for this under various circumstances; the one that I'm
interested in is selectively offloading incoming network connections
from one process to another one that is better suited to handle some
particular connections.

In Python 3.3 and later, doing this is simple because
it is directly supported by [[the socket module
http://docs.python.org/3/library/socket.html]]. The documentation
even includes code examples for both _sendmsg()_ and _recvmsg()_,
which is handy because they don't exactly have the most Pythonic
of interfaces; instead it's basically a thin cover over the system
call data structures.  If you are receiving file descriptors that
are sockets you're still left with [[the socket _.fromfd()_ problem
SocketFromFdMistake]].

([[I was encouraged https://twitter.com/glyph/status/353379788624363520]]
to report the socket fd problem as [[an actual Python bug
http://bugs.python.org/issue18391]], where it has quietly been
neglected just as I expected.)

Unfortunately Python 2 does not have any support for this in [[the
socket module http://docs.python.org/2/library/socket.html]], thereby
creating yet another gratuitous Python 2 to Python 3 difference.
Fortunately a number of people have written add on modules to support
this; the ones I found in a casual Internet search are [[python-passfd
http://code.google.com/p/python-passfd/]], [[python-fdsend
https://gitorious.org/python-fdsend/]], and [[sendmsg
https://pypi.python.org/pypi/sendmsg]] (which is notably lacking
in documentation). Of these, [[python-fdsend]] seems to have the
best API (and is packaged for Debian and Ubuntu); I expect that
it's what I'll use if (or when) I need this feature in my Python 2
code. Note that it doesn't solve [[the socket _.fromfd()_ problem]].

If you're sending sockets to another process, remember that it is safe
to call _.close()_ on them afterwards but it is *not* safe to call
_.shutdown()_ on them; [[as I discovered, _shutdown()_ is a global
operation on a socket ../programming/ReadingManpages]] and applies to
all file descriptors for it, including ones now held by other processes.