The unreliability of AF_UNIX datagram sockets

[Ken Brown]

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.

Ken