adopt(pid_t pid) syscall proposal [patch included]

[vcaputo]

 Hello all,

I'd like to propose a new system call, I know I'll probably be flamed,
but here goes.

Today there is no way to get a process which has been orphaned, and thus
become a child of init, back as a child of another process.  This is
annoying me, please allow me to explain why.

The CONFIG_CHECKPOINT_RESTORE feature enables a useful little thing in
proc which I've been taking advantage of to make some interesting
omnipresent desktop monitoring features built into my own composited
window manager.  That thing is the /proc/$pid/task/$pid/children file.

Using this file, I've created scoped per-window process monitors which
can efficiently limit their monitoring to only the client pid of the X
window and its decendants.  These monitors are always running, but
visibility is toggled when desired by a keypress, efficiency is
important and the children file helps tremendously in this regard.

So I'm thrilled with all this and everything is fantastically efficient
and simple, until the first time I detach from my screen session and
reattach.

Initially, I'll have a scenario like:

 xterm
  bash
   screen -S stuff
    screen -S stuff
     bash
     bash
      top
     bash
      vi foo.c
     bash
      make
       sh -c cc -Wall ...
        cc1 -quiet -I ...
     bash

Behind these rows of text I have user/cpu graphs for each process
sliding by.

After detaching from screen, then reattaching, this is what the above
turns into, as you can probably predict:

 xterm
  bash
   screen -dr stuff

All subsequent processes created through my interactions with this
screen are lost to the monitoring, since the "screen -S stuff" backend
is now stuck being a child of init.

With the attached adopt() syscall patch, and a 1 line change to screen's
attacher function invoking adopt(), I get the following on reattach:

 xterm
  bash
   screen -dr stuff
    screen -S stuff
     bash
     bash
      top
     bash
      vi foo.c
     bash
      make
       sh -c cc -Wall ...
        cc1 -quiet -I ...


Which makes me very happy, dance around the room happy, and I'm not a
dancer.

Here is a screenshot of the results enabled by the patch:
http://pengaru.com/~swivel/vwm/screen_sys_adopt.png

Thanks for reading, and hopefuly considering this addition, I'm sure
there are other uses as well.  Please CC me in any replies as I'm not
currently subscribed.

Regards,
Vito Caputo