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

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

On closer reading of the man pages it looks like linux's clone is supposed to change the disposition for the exit signal. I'll have to write a test program to reproduce this behavior in isolation.

-M


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