Re: difference in SIGCHLD behavior between Linux and FreeBSD breaks apt

On 7/18/16 4:15 PM, Matthew Macy wrote:
>
>
>   ---- On Mon, 18 Jul 2016 16:11:53 -0700 Alfred Perlstein <alfred_at_freebsd.org> wrote ----
>   > I believe the
>   >
>   >
>   > On 7/6/16 9:34 PM, 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.
>   > >
>   > > Thanks in advance.
>   >
>   > Are you sure you need a hack in pipe_read and not one of the following
>   > possibilities:
>   > 1) a setting for the default signal disposition for linux processes
>   > needs to be fixed.
>   > 2) a flag set in p_flag2 that says set this behavior properly in a
>   > generic manner.
>   >
>   > Again not sure why you need to hack pipe_read and not just make sure
>   > that SIGCHLD is generated...
>   >
>   > Finally that sure is oddball behavior, dpkg probably has a bug where the
>   > parent is keeping the write side of the pipe open, you might be able to
>   > get them to take a patch upstream to fix that.
>   >
>
> If you read my final mail it turns out I was holding a reference to the pipe in question in linprocfs. Maintaining the reference kept apt from getting the EOF on the pipe. I've since fixed this.
>
> -M
>
Ah that makes sense. :)

-Alfred