This way we don't expose more of a bufferevent than we need to. One
motivation is to make it easier to automatically get deferred callbacks
with a bufferevent without exposing the deferred_cb structure.
svn:r1169
The new bufferevent_pair abstraction works like a set of buferevent_sockets
connected by a socketpair, except that it doesn't require a socketpair,
and therefore doesn't need to get the kernel involved.
It's also a good way to make sure that deferred callbacks work. It's a good
use case for deferred callbacks: before I implemented them, the recursive
relationship between the evbuffer callback and the read callback would
make the unit tests overflow the stack.
svn:r1152
In particular, we don't allow adding any data to end front of inbuf
(we do that when we read), or removing it from the front of outbuf (we
drain data only when we write).
svn:r1144
