Re: [BProc] bproc_vfork and IO forwarding

[<er...@he...>]

On Thu, Jun 26, 2003 at 12:50:43PM -0400, Nicholas Henke wrote:
> When running vfork, it appears that only the first node in the nodelist
> has the stdio redirected correctly. The rest of the nodes output appears
> to go to /dev/null. Is this the expected behavior ?
> 
> BTW -- I am running 3.2.0

Umm... Yeah.  It's certainly a quirk and it should probably be fixed
but that's normal.  Here's what's going on:

Normally, when you move to a node, bproc will setup a socket
connection between the two processes to move the process information.
As a nice hack to provide basic support for printf, the socket is kept
around and attached to the process's STDOUT, STDERR.  This works out
because the socket connection is usually back to the front end where
bproc can do some mostly sane IO forwarding.  In the vrfork case, only
the first process gets the process image from the front end.  The rest
of the processes get their process image from one of the previous
proceses.  This adds parallelism which makes it go faster and blah
blah blah...  The upshot is that the sockets which were used for the
built-in forwarding don't go back to the front end anymore so it
doesn't work.

If you look at the bpsh source, you'll see that it provides explicit
instructions to vexecmove on how to wire up STDIN, STDOUT, STDERR.
bpsh itself becomes the IO forwarder in that case.  This makes things
like bpsh much more complicated than they might otherwise be.  On the
bright side, bpsh is a MUCH better IO forwarder than what's built into
BProc at this point.

I've been wanting to get rid fo the IO forwarding daemon in BProc
since the very first version.  It's one of those thing that's lingered
because it's a nice crutch which does an ok job simple prints.

- Erik