difference in SIGCHLD behavior between Linux and FreeBSD breaks apt

[Matthew Macy]

 ---- 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