Traits for asynchronous UDP (#73)

* gitignore: Also ignore target inside nested crate

* async: Add UDP traits

Closes: #71

* async UDP: Simplify to 2 rather than 3 traits

* async UDP: Documentation fixes; receive_into consistency

One trait's receive_into still used the old receive name.

* async UDP: Make accessible through lib.rs

* async UDP: Fix lifetimes of send and receive functions

Elided lifetimes would have asked too little of the data / buffer
references.

* async UDP: Better names for bound sockets

* UDP: use async-fn-in-trait instead of associated types that are Future

This removes the lifetimes from the Connect associated types; leaving
them would cause errors (possibly related to [12681]), but I think that
stacks whose sockets are not 'static can work around this by
implementing the stack traits on a &'short stack instead.

[12681]: rust-lang/rust#102681

* async/UDP: Remove connect_default helper

The connect method can now be provided properly.

* async/CHANGELOG: Add UDP

* async/UDP: rustfmt

* async/UDP: Tie socket's error types to stack's

.gitignore

@@ -1,3 +1,4 @@
 /target
+/embedded-nal-async/target
 **/*.rs.bk
 Cargo.lock

embedded-nal-async/CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+- Add traits for UDP
+
 ## [0.3.0] - 2022-11-25
 
 - Bump `embedded-io` dependency to `0.4`

embedded-nal-async/src/lib.rs

@@ -16,3 +16,4 @@ pub use no_std_net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr, SocketAddrV4, Socke
 pub use dns::Dns;
 pub use embedded_nal::AddrType;
 pub use stack::TcpConnect;
+pub use stack::{ConnectedUdp, UdpStack, UnconnectedUdp};

embedded-nal-async/src/stack/mod.rs

@@ -1,3 +1,5 @@
 mod tcp;
+mod udp;
 
 pub use tcp::TcpConnect;
+pub use udp::{ConnectedUdp, UdpStack, UnconnectedUdp};

embedded-nal-async/src/stack/udp.rs

@@ -0,0 +1,199 @@
+//! Traits for using UDP on embedded devices
+//!
+//! ## Notes for implementers
+//!
+//! * At several places, the APIs expect to provide a local address. Backends that can not obtain
+//!   it, such as some AT-command based stacks, <!-- should question whether they may really call
+//!   themselves UDP and --> may pretend to have performed some form of network address
+//!   translation, and present invalid addresses as the local address.
+//!
+//! * Implementing [`UdpStack::UniquelyBound`] and [`UdpStack::MultiplyBound`] unconnected sockets
+//!   separately allows discarding the local addresses in the bound case. With LTO enabled, all the
+//!   overhead compared with a third trait variant between [ConnectedUdp] and [UnconnectedUdp] (in
+//!   which the local address is static but the remote address is flexible) should optimized out.
+//!   Implementing `UniquelyBound` and `MultiplyBound` with the same type is expected to be a
+//!   common choice.
+
+use no_std_net::SocketAddr;
+
+/// This trait is implemented by UDP sockets.
+///
+/// The socket it represents is both bound (has a local IP address, port and interface) and
+/// connected (has a remote IP address and port).
+///
+/// The term "connected" here refers to the semantics of POSIX datagram sockets, through which datagrams
+/// are sent and received without having a remote address per call. It does not imply any process
+/// of establishing a connection (which is absent in UDP). While there is typically no POSIX
+/// `bind()` call in the creation of such sockets, these are implicitly bound to a suitable local
+/// address at connect time.
+pub trait ConnectedUdp {
+	/// Error type returned by send and receive operations.
+	type Error: embedded_io::Error;
+
+	/// Send the provided data to the connected peer
+	async fn send(&mut self, data: &[u8]) -> Result<(), Self::Error>;
+
+	/// Receive a datagram into the provided buffer.
+	///
+	/// If the received datagram exceeds the buffer's length, it is received regardless, and the
+	/// remaining bytes are discarded. The full datagram size is still indicated in the result,
+	/// allowing the recipient to detect that truncation.
+	///
+	/// ## Compatibility note
+	///
+	/// This deviates from the sync/nb equivalent trait in that it describes the overflow behavior
+	/// (a possibility not considered there). The name deviates from the original `receive()` to
+	/// make room for a version that is more zero-copy friendly.
+	async fn receive_into(&mut self, buffer: &mut [u8]) -> Result<usize, Self::Error>;
+
+	// WIP to allow zero-copy operation
+	// The plain receive is simple and can be provided -- implementations that don't populate
+	// receive calls from scatter-gather can just return a slice of the raw data instead, and rely
+	// on the socket still being exclusively owned. receive_oned is harder as providing it requires
+	// alloc.
+	//
+	// async fn receive(&mut self, buffer: &mut [u8]) -> utput = Result<impl AsRef<u8> + '_, Self::Error>;
+	// async fn receive_owned(&mut self) -> Result<impl AsRef<u8> + 'static, Self::Error>;
+}
+
+/// This trait is implemented by UDP sockets.
+///
+/// The socket it represents is not necessarily bound (may not have a single local IP address, port
+/// and interface), and is typically not connected (has no remote IP address and port). Both are
+/// addresses are explicitly given in every call.
+///
+/// If there were constraints in place at socket creation time (typically on the local side), the
+/// caller MUST pass in the same (or compatible) values, MAY and pass in unspecified values where
+/// applicable. The implementer MAY check them for compatibility, and SHOULD do that in debug mode.
+pub trait UnconnectedUdp {
+	/// Error type returned by send and receive operations.
+	type Error: embedded_io::Error;
+
+	/// Send the provided data to a peer
+	///
+	/// ## Sending initial messages
+	///
+	/// The local address can be left unspecified by leaving any of its component zero -- that
+	/// gives the "any" address (`[::]` / `0.0.0.0`), the uncspecified port (0) or the unspecified
+	/// zone identifier (0). Unless the operating system provides facilities exceeding this crate's traits for
+	/// enumerating local interfaces and addresses, this is the only way to initiate outbound
+	/// traffic.
+	///
+	/// ## Responding to messages
+	///
+	/// Users who have previously received data from a peer and want to respond have a choice of
+	/// sending from the address to which the original datagram was addressed, or from an unbound
+	/// address. Both are valid choices in some situations, and the right choice depends on the
+	/// protocol used.
+	///
+	/// Note that users of sockets created through [`UdpStack::bind_single()`] should always pass
+	/// in that single address -- even though they've made their intention clear at construction.
+	/// They can pass either the one obtained at socket creation time, or the one obtained at
+	/// receive time; these should be equal. This allows implementations of the trait to use a
+	/// single kind of socket for both sockets bound to a single and sockets bound to multiple
+	/// addresses.
+	async fn send(
+		&mut self,
+		local: SocketAddr,
+		remote: SocketAddr,
+		data: &[u8],
+	) -> Result<(), Self::Error>;
+
+	/// Receive a datagram into the provided buffer.
+	///
+	/// If the received datagram exceeds the buffer's length, it is received regardless, and the
+	/// remaining bytes are discarded. The full datagram size is still indicated in the result,
+	/// allowing the recipient to detect that truncation.
+	///
+	/// The local and remote address are given, in that order, in the result along with the number
+	/// of bytes.
+	async fn receive_into(
+		&mut self,
+		buffer: &mut [u8],
+	) -> Result<(usize, SocketAddr, SocketAddr), Self::Error>;
+}
+
+/// This trait is implemented by UDP/IP stacks. The trait allows the underlying driver to
+/// construct multiple connections that implement the I/O traits from embedded-io.
+///
+/// Note that stacks with exotic connection creation methods may still not implement this, yet have
+/// objects that implement [`ConnectedUdp`] or similar.
+pub trait UdpStack {
+	/// Error type returned on socket creation failure.
+	type Error: embedded_io::Error;
+
+	/// Eventual socket return type of the [`.connect()`] method
+	type Connected: ConnectedUdp<Error = Self::Error>;
+	/// Eventual socket return type of the [`.bind_single()`] method
+	type UniquelyBound: UnconnectedUdp<Error = Self::Error>;
+	/// Eventual return type of the [`.bind_multiple()`] method
+	type MultiplyBound: UnconnectedUdp<Error = Self::Error>;
+
+	/// Create a socket that has a fixed remote address.
+	///
+	/// The local address is chosen automatically.
+	///
+	/// There is a provided implementation that implements this from the maximally unspecified
+	/// local address and [`.connect_from()`], but may be provided more efficiently by
+	/// implementers.
+	async fn connect(
+		&self,
+		remote: SocketAddr,
+	) -> Result<(SocketAddr, Self::Connected), Self::Error> {
+		use no_std_net::{Ipv4Addr, Ipv6Addr, SocketAddr::*, SocketAddrV4, SocketAddrV6};
+
+		let local = match remote {
+			V4(_) => V4(SocketAddrV4::new(Ipv4Addr::UNSPECIFIED, 0)),
+			V6(_) => V6(SocketAddrV6::new(Ipv6Addr::UNSPECIFIED, 0, 0, 0)),
+		};
+		self.connect_from(local, remote).await
+	}
+
+	/// Create a socket that has a fixed remote address.
+	///
+	/// The local address is given explicitly, but may be partially unspecified; it is fixed by the
+	/// network stack at connection time. The full local address is returned along with the
+	/// connected socket, primarily for debugging purposes.
+	async fn connect_from(
+		&self,
+		local: SocketAddr,
+		remote: SocketAddr,
+	) -> Result<(SocketAddr, Self::Connected), Self::Error>;
+
+	/// Create a socket that has a fixed local address.
+	///
+	/// Note that the giving an unspecified address here is *not* the same as a POSIX `bind()` --
+	/// if the underlying stack supports multiple local addresses, it will pick *one* of the
+	/// applicable addresses, rather than binding to all of them.
+	///
+	/// The full local address is returned along with the bound socket; it may then be passed on to
+	/// other protocols for advertising purposes.
+	async fn bind_single(
+		&self,
+		local: SocketAddr,
+	) -> Result<(SocketAddr, Self::UniquelyBound), Self::Error>;
+
+	/// Create a socket that has no single fixed local address.
+	///
+	/// The IP address part of the local address is typically left unspecified, and the port is
+	/// given. There are use cases for other constellations, and this interface does not rule out
+	/// that they can be used, but they are rare (e.g. using the same IP address on different
+	/// network interfaces, and listening to datagrams arriving at any of them) or not well
+	/// supported by operating systems (e.g., binding to all ports at the same is not possible on
+	/// POSIX systems, where giving port 0 to a bind makes the OS pick *some* suitable port).
+	///
+	/// Caveats:
+	///
+	/// * There is currently no way to pass in a local address that has an unspecified address
+	///   family (which would effectively create a single socket that servers both IPv4 and IPv6);
+	///   it is not specified whether stacks that use V6MAPPED IPv4 addresses could simply used
+	///   that mechanism.
+	///
+	/// * It is currently not specified whether this mechanism can be used to join multicast
+	///   groups.
+	///
+	/// * There is currently no hybrid binding that allows emulating what POSIX systems do when
+	///   binding to `[::]:0`, that is, picking some available port but then still leaving the
+	///   interface and IP address unspecified.
+	async fn bind_multiple(&self, local: SocketAddr) -> Result<Self::MultiplyBound, Self::Error>;
+}