difference in SIGCHLD behavior between Linux and FreeBSD breaks apt

[Andrey Chernov]

On 07.07.2016 9:40, Matthew Macy wrote:
> 
> 
> 
>  ---- On Wed, 06 Jul 2016 23:28:40 -0700 Andrey Chernov <ache at freebsd.org> wrote ---- 
>  > On 07.07.2016 7:52, K. Macy wrote: 
>  > > On Wednesday, July 6, 2016, Don Lewis <truckman at freebsd.org> wrote: 
>  > >  
>  > >> 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. 
>  > >  
>  > >  
>  > >  
>  > > Good point. 
>  > >  
>  > > Thinking more about it, this seems like a bug in FreeBSD. Not a valid 
>  > > behavioral difference. 
>  >  
>  > You better need consult with POSIX before fixing things toward any 
>  > Linuxisms blindly in our native code. I don't have a time now to see, is 
>  > it really a bug according to POSIX, but please read or just find all 
>  > SIGCHLD there: 
>  > http://pubs.opengroup.org/onlinepubs/9699919799/functions/wait.html 
>  > it explain SIGCHLD actions in deep details. 
>  > And that one too: 
>  > http://pubs.opengroup.org/onlinepubs/009695399/functions/sigaction.html 
> 
> 
> 
> I was pretty clear in my initial email that I'm only interested in changing behavior for Linux programs.

Of course, but in case it is FreeBSD bug, it should be fixed in our
native code first before making any changes in Linuxator.

> And I was asking for help with that, not a link to SUSv3 or POSIX. 

In case I was not helpful, sorry for that. Before you try to change
something in Linuxator you need to be sure that FreeBSD does it right
(or wrong, then fix FreeBSD native code first). I am just insisting on
proper steps of fixing it.