The unreliability of AF_UNIX datagram sockets

[Ken Brown]

On 5/17/2021 6:26 AM, Corinna Vinschen wrote:
> On May 13 10:30, Ken Brown wrote:
>> While working on sendmsg, I thought of another useful thing that the mqueue
>> code could provide: an internal _mq_send function that returns EPIPE if no
>> one has the mqueue open for reading.  This plays the role of the
>> STATUS_PIPE_IS_CLOSED macro in the pipe implementation.
> 
> I don't see how to do that.  The code is written in a way not requiring
> to keep track of stuff at runtime.  All shared info is part of the mmap
> so far.  We don't know if any other mqd_t descriptor is open at the time,
> it's not even a descriptor in the usual sense, just a pointer to a private
> malloc'ed area.  Keeping track of open descriptors requires another
> shared mem region at runtime.

OK, then I think I can handle this in the AF_UNIX shared memory.  Each socket 
can keep track of its own number of open (socket) descriptors, and then it can 
send a shutdown message to its peer when the last one is about to close.  The 
sendmsg code will have to be tweaked to repeatedly check for shutdown messages.

Ken