moving Connection/PipeConnection between processes

Mike Kazantsev mk.fraggod at gmail.com
Sat Jun 13 10:11:47 CEST 2009


On Sat, 13 Jun 2009 02:23:37 -0500
Randall Smith <randall at tnr.cc> wrote:

> I've got a situation in which I'd like to hand one end of a pipe to 
> another process.  First, in case you ask why, a spawner process is 
> created early before many modules are imported.  That spawner process is 
> responsible for creating new processes and giving a proxy to the parent 
> process.
>
...
> 
> Looking at the pickle docs, I wonder if this could be resolved by adding 
> a __getnewargs__ method to _multiprocessing.Connection.  But even if 
> that would work I couldn't do it now since it's an extension module. 
> I've thought about trying to recreate the Connection.  Looks like it 
> should be possible with Connection.fileno().  The Unix portion looks 
> easy, but the win32 portion does not.
> 
> So if it's possible, what's the best way to pass a Connection to another 
> process?

Pickle has nothing to do with the problem since it lay much deeper: in
the OS.

From kernel point of view, every process has it's own "descriptor
table" and the integer id of the descriptor is all the process gets, so
when you say "os.pipe()" kernel actually gives you a number which is
completely meaningless for any other process - it either doesn't exists
in it's descriptor table or points to something else.

So, what you actually need is to tell the kernel to duplicate
underlying object in another process' table (with it's own numbering),
which is usually done via special flag for sendmsg(2) in C, so you
should probably look out for py implementation of this call, which I
haven't stumbled upon, but, admittely, never looked for.


-- 
Mike Kazantsev // fraggod.net
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