Adding read_exact and write_all OPs?

[Artyom Pavlov <newpavlov@gmail.com>]

Greetings!

In application code it's quite common to write/read a whole buffer and 
only then continue task execution. The traditional approach is to wrap 
read/write sycall/OP in loop, which is often done as part of a language 
std. In synchronous context it makes sense because it allows to process 
things like EINTR. But in asynchronous (event-driven) context I think it 
makes a bit less sense.

What do you think about potential addition of OPs like read_exact and 
write_all, i.e. OPs which on successful CQE guarantee that an input 
buffer was processed completely? They would allow to simplify user code 
and in some cases to significantly reduce ring traffic.

Best regards,
Artyom Pavlov.