How can I initiate IPC with a child process, without letting it inherit all handles? To make it more interesting, this shoud work on windows as well as unix.
The background: I am writing a library that interfaces with a 3rparty shared library (let's just call it IT) which in turn contains global data (that really should be objects!). I want to have multiple instances of this global data. As far as I understand, I have two options to solve this:
create a cython module that links against a static variant of IT, then copy and import the module whenever I want a new instance. Analogously, I could copy IT but that's even more work to create a ctypes interface.
spawn a subprocess that loads IT and establish an IPC connection to it.
There are a few reasons to use (2):
I am not sure, if (1) is reliable in any way and it feels like a bad idea (what happens with all the extra modules, when the application exits in an uncontrolled way?).
boxing IT into a separate process might actually be a good idea anyway for security considerations: IT deals with potentially unsafe input and IT's code quality isn't overly good. So, I'd rather not have any secure resources open when running it.
there is probably lot's of need for this kind of IPC in future applications
So what are my options? I have already looked into:
multiprocessing.Process
at first looked nice, until I realized that the new process gets a copy of all my handles. Needless to say that this is quite problematic, since now resources cannot be reliably freed by closing them in the parent process + the security issues mentioned earlier.
Use os.closerange
within a multiprocessing.Process
to close to all handles manually - except for the Pipe I'm interested in. Does os.closerange
close only files or does it take care of other types of resources as well? If so: how can I determine the range, given the Pipe
object?
subprocess.Popen(.., close_fds=True, stdin=PIPE, stdout=PIPE)
works fine on unix but isn't possible on win32.
Named pipes are very different on win32 and unix. Are their any libraries that their usage?
Sockets. Promising, especially since their are handy RPC libraries that can work with sockets. On the other hand, I fear that this may cause a whole bunch of security issues. Are sockets that I have determined to be of local origin (sock.getpeername()[0] == '127.0.0.1'
) secure against tempering?
Are there any possibilities that I have overlooked?
To round up: the main question is how to establish a secure IPC with a child process on windows+unix? But please don't hesitate to answer if you know any answers to only partial problems.
Thanks for taking the time to read it!
It seems on python>=3.4 subprocess.Popen(..., stdin=PIPE, stdout=PIPE, close_fds=False)
is a possible option. This is due to a patch that makes all opened file descriptors non-inheritable by default. To be more precise, they will be automatically closed on execv
(so still can't use multiprocessing.Process
), see PEP 446.
This is also a valid option for other python versions:
for a corresponding example see:
https://github.com/coldfix/python-ipc-test
The most useful combinations are:
stdio:pickle
inherit_unidir:pickle
socket:sockpipe
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