git: b864b67a0d19 - main - socket: Do not include control messages in FIONREAD return value

[Michael Tuexen]

> On 13. Sep 2021, at 18:29, John Baldwin <jhb at freebsd.org> wrote:
> 
> On 9/12/21 1:39 PM, Mark Johnston wrote:
>> The branch main has been updated by markj:
>> URL: https://cgit.FreeBSD.org/src/commit/?id=b864b67a0d197f59ecf6698940600956ceee2cae
>> commit b864b67a0d197f59ecf6698940600956ceee2cae
>> Author:     Mark Johnston <markj at FreeBSD.org>
>> AuthorDate: 2021-09-12 20:05:49 +0000
>> Commit:     Mark Johnston <markj at FreeBSD.org>
>> CommitDate: 2021-09-12 20:39:44 +0000
>>     socket: Do not include control messages in FIONREAD return value
>>          Some system software expects to be able to read at least the number of
>>     bytes returned by FIONREAD.  When control messages are counted in this
>>     return value, this assumption is violated.  Follow Linux and OpenBSD
>>     here (as well as our own kevent(EVFILT_READ)) and only return the number
>>     of data bytes available.
>>          Reported by:    avg
>>     MFC after:      2 weeks
> 
> In a somewhat similar vein, it would be nice to eventually have new ioctls
> to know how much data and control are available in the next message for
> datagram-oriented sockets.  Right now if you are working with a datagram
> socket with variable-sized messages there's no good way to know how big the
> next message is (to resize a read buffer) as FIONREAD can count multiple
> messages.  There's also no way at all to cope with control messages aside
> from retrying with some naive algorithm like doubling the size if MSG_CTRUNC
> is set, but that also requires always using MSG_PEEK so that you always
> end up reading a message at least twice.
That still requires two system calls. In SCTP, there is a CMSG you can
receive with a messages, which contains the metadata of the next message
you will receive.
So it might be an idea to define a CMSG containing the length of the next
message (if available).

Best regards
Michael
> 
> -- 
> John Baldwin