The unreliability of AF_UNIX datagram sockets

[Corinna Vinschen]

On May  3 16:50, Ken Brown wrote:
> On 5/3/2021 3:48 PM, Ken Brown wrote:
> > On 5/3/2021 2:40 PM, Corinna Vinschen wrote:
> > > On May  3 12:56, Ken Brown wrote:
> > > > On 5/3/2021 11:45 AM, Corinna Vinschen wrote:
> > > > > 7. The idea of _mq_recv partial reads is entirely broken.  Given that
> > > > >      the information in the queue consists of header info plus payload,
> > > > >      the entire block has to be read, and then a new block with fixed
> > > > >      header and shortened payload has to be rewritten with bumped priority.
> > > > >      This in turn can only be performed by the AF_UNIX code, unless we
> > > > >      expect knowledge of the AF_UNIX packet layout in the mqueue code.
> > > > 
> > > > The partial read is actually OK as is, since it's comparable to what happens
> > > > on a partial read from a pipe.  I already have AF_UNIX code (on the
> > > > topic/af_unix branch) that deals with that.  A boolean variable _unread
> > > > keeps track of whether there's unread data from a previous partial read.  If
> > > > so, the next read just reads data without expecting a header.
> > > 
> > > Ok, never mind.
> > > 
> > > One advantage of the mqueue when utilized as above would be that this
> > > kind of state info is not required.  The content of a packet would
> > > always be self-contained and bumping the priority would automagically
> > > move the packet content to the top of the queue.  But that's just
> > > idle musing at this point.
> > 
> > I thought about that but rejected it for the following reason: Suppose
> > the receiver reads a message and tries to rewrite it with modified
> > header, shortened payload, and bumped priority.  The sender might have
> > already written more messages between the read and the write, and the
> > queue could be full.
> > 
> > Now that I'm rethinking this, however, maybe we could get around that
> > problem with an internal _mq_lock function that would block senders
> > while the receiver decides whether it needs to do a partial read.
> 
> Alternatively, _mq_recv could accept an _MQ_LOCK flag, which means "don't
> release the mutex", and then there could be an _mq_unlock function, which
> simply releases the mutex.

That's an idea.  However, I think this is something we can push back for
now, and ultimately we can use any of the above solutions which makes most
sense.  Implemanting a defered unlock if required is not much of a problem.


Corinna