This optimization makes sure that enqueueing to an empty ring buffer
would never wrap around, in turn ensuring that enqueueing to an empty
packet buffer would never use more than one metadata entry.
Right now, pushing buffer-sized packets into a packet buffer requires
at least two metadata entries, which is surprising and wasteful.
Some of them already use Result, but the ones that can return
an empty slice (or a slice shorter than requested) also must have
their return value (at least) checked.
set_len() with a positive length change is better represented
with enqueue(), and set_len() with a negative length change
is not really a valid operation on its own.
This also makes TcpSocket::{send,recv}_slice slightly more efficient
in case when the slice wraps around the corresponding buffer,
halving the necessary amount of calls.
Before this commit, anything that touched RawSocket or TapInterface
worked partly by accident and partly because of a horrible crutch
that resulted in massive latencies as well as inevitable packet loss
every time an ARP request had to be issued. Also, there was no way
to use poll() other than by continuously calling it in a busy loop.
After this commit, poll() indicates when the earliest timer expires,
and so the caller can sleep until that moment (or until packets
arrive).
Note that there is a subtle problem remaining: every time poll()
is called, every socket with a pending outbound packet whose
IP address doesn't correspond to a MAC address will send a new
ARP request, resulting in potentially a whole lot of such requests.
ARP rate limiting is a separate topic though.
The use of this type has several drawbacks:
* It does not allow distinguishing between different error
conditions. In fact, we wrongly conflated some of them
before this commit.
* It does not allow propagation via ? and requires manual use
of map_err, which is especially tiresome for downstream code.
* It prevents us from expanding the set of error conditions
even if right now we have only one.
* It prevents us from blanket using Result<T> everywhere
(a nitpick at most).
Instead, use Result<T, Error> everywhere, and differentiate error
conditions where applicable.
Dario Nieuwenhuis
Astro
Philipp Oppermann
whitequark
Egor Karavaev