signalling upstream

From: bill pursell (bill.pursell_at_gmail.com)
Date: 11/11/05 

Date: 11 Nov 2005 04:02:49 -0800

I would like to be able to do the following:
% a | b | c | d
and have the process d send a sigusr1 to process a. The only
reasonable way I can see of doing this is to have everyone in the pipe
pass along meta-information about the pipe. (ie, a writes its pid, b
reads it and writes out a's pid as well as its own, etc.)
Unfortunately, this prohibits the use of standard apps in the pipe, so
I want to avoid that if possible.

The second possible solution I see is to have d use its gid information
and look in /proc (or a registry that each process registers with, eg
in a file in /var/run) to determine a's pid. This leads to
difficulties when the pipe looks like this:
% a | b | c | a | b | d

I'm trying to avoid having a controlling process, since I really want
the flexibility of setting up the streams on the command line. Does
anyone know of implementations that have done something like this?
Feed-back in a data flow is pretty standard, but I'm having a hard time
fitting it in to the *-nix pipe philosophy. Can I pull this off
without writing meta-information into the pipe?

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