difference in SIGCHLD behavior between Linux and FreeBSD breaks apt

[Andrey Chernov]

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