difference in SIGCHLD behavior between Linux and FreeBSD breaks apt

[Don Lewis]

On  6 Jul, Matthew Macy wrote:
> As a first step towards managing linux user space in a chrooted
> /compat/linux, initially for i915 testing with intel gpu tools, later
> on to get widevine and steam to work I'm trying to get apt to work.
> I've fixed a number of issues to date in pseudofs/linprocfs but now
> I'm running in to a bug caused by differences in SIGCHLD handling
> between Linux and FreeBSD. The situation is that apt will spawn dpkg
> and wait on a pipe read. On Linux when dpkg exits the  SIGCHLD to apt
> causes a short read on the pipe which lets apt then continue. On
> FreeBSD a SIGCHLD is silently ignored. I've even experimented with
> doing a kill -20 <apt pid> to no effect.
>  
> It would be easy enough to check sysvec against linux in pipe_read and
> break out of the loop when it's awakened from msleep (assuming there
> aren't deeper issues with signal propagation for anything other than
> SIGINT/SIGKILL) and then do a short read. However, I'm assuming that
> anyone who has worked in this area probably has a cleaner solution.

It shoulds like SA_RESTART is set in sa_flags for SIGCHLD but shouldn't
be in this case.