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//! # Multistream-select Protocol Negotiation
//!
//! This crate implements the `multistream-select` protocol, which is the protocol
//! used by libp2p to negotiate which application-layer protocol to use with the
//! remote on a connection or substream.
//!
//! > **Note**: This crate is used primarily by core components of *libp2p* and it
//! > is usually not used directly on its own.
//!
//! ## Roles
//!
//! Two peers using the multistream-select negotiation protocol on an I/O stream
//! are distinguished by their role as a _dialer_ (or _initiator_) or as a _listener_
//! (or _responder_). Thereby the dialer plays the active part, driving the protocol,
//! whereas the listener reacts to the messages received.
//!
//! The dialer has two options: it can either pick a protocol from the complete list
//! of protocols that the listener supports, or it can directly suggest a protocol.
//! Either way, a selected protocol is sent to the listener who can either accept (by
//! echoing the same protocol) or reject (by responding with a message stating
//! "not available"). If a suggested protocol is not available, the dialer may
//! suggest another protocol. This process continues until a protocol is agreed upon,
//! yielding a [`Negotiated`](self::Negotiated) stream, or the dialer has run out of
//! alternatives.
//!
//! See [`dialer_select_proto`](self::dialer_select_proto) and
//! [`listener_select_proto`](self::listener_select_proto).
//!
//! ## [`Negotiated`](self::Negotiated)
//!
//! A `Negotiated` represents an I/O stream that has settled on a protocol
//! to use. By default, with [`Version::V1`], protocol negotiation is always
//! at least one dedicated round-trip message exchange, before application
//! data for the negotiated protocol can be sent by the dialer. There is
//! a variant [`Version::V1Lazy`] that permits 0-RTT negotiation if the
//! dialer only supports a single protocol. In that case, when a dialer
//! settles on a protocol to use, the [`DialerSelectFuture`] yields a
//! [`Negotiated`](self::Negotiated) I/O stream before the negotiation
//! data has been flushed. It is then expecting confirmation for that protocol
//! as the first messages read from the stream. This behaviour allows the dialer
//! to immediately send data relating to the negotiated protocol together with the
//! remaining negotiation message(s). Note, however, that a dialer that performs
//! multiple 0-RTT negotiations in sequence for different protocols layered on
//! top of each other may trigger undesirable behaviour for a listener not
//! supporting one of the intermediate protocols. See
//! [`dialer_select_proto`](self::dialer_select_proto) and the documentation
//! of [`Version::V1Lazy`] for further details.
//!
//! ## Examples
//!
//! For a dialer:
//!
//! ```no_run
//! use async_std::net::TcpStream;
//! use multistream_select::{dialer_select_proto, Version};
//! use futures::prelude::*;
//!
//! async_std::task::block_on(async move {
//!     let socket = TcpStream::connect("127.0.0.1:10333").await.unwrap();
//!
//!     let protos = vec!["/echo/1.0.0", "/echo/2.5.0"];
//!     let (protocol, _io) = dialer_select_proto(socket, protos, Version::V1).await.unwrap();
//!
//!     println!("Negotiated protocol: {:?}", protocol);
//!     // You can now use `_io` to communicate with the remote.
//! });
//! ```
//!

#![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]

mod dialer_select;
mod length_delimited;
mod listener_select;
mod negotiated;
mod protocol;

pub use self::dialer_select::{dialer_select_proto, DialerSelectFuture};
pub use self::listener_select::{listener_select_proto, ListenerSelectFuture};
pub use self::negotiated::{Negotiated, NegotiatedComplete, NegotiationError};
pub use self::protocol::ProtocolError;

/// Supported multistream-select versions.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Default)]
pub enum Version {
    /// Version 1 of the multistream-select protocol. See [1] and [2].
    ///
    /// [1]: https://github.com/libp2p/specs/blob/master/connections/README.md#protocol-negotiation
    /// [2]: https://github.com/multiformats/multistream-select
    #[default]
    V1,
    /// A "lazy" variant of version 1 that is identical on the wire but whereby
    /// the dialer delays flushing protocol negotiation data in order to combine
    /// it with initial application data, thus performing 0-RTT negotiation.
    ///
    /// This strategy is only applicable for the node with the role of "dialer"
    /// in the negotiation and only if the dialer supports just a single
    /// application protocol. In that case the dialer immedidately "settles"
    /// on that protocol, buffering the negotiation messages to be sent
    /// with the first round of application protocol data (or an attempt
    /// is made to read from the `Negotiated` I/O stream).
    ///
    /// A listener will behave identically to `V1`. This ensures interoperability with `V1`.
    /// Notably, it will immediately send the multistream header as well as the protocol
    /// confirmation, resulting in multiple frames being sent on the underlying transport.
    /// Nevertheless, if the listener supports the protocol that the dialer optimistically
    /// settled on, it can be a 0-RTT negotiation.
    ///
    /// > **Note**: `V1Lazy` is specific to `rust-libp2p`. The wire protocol is identical to `V1`
    /// > and generally interoperable with peers only supporting `V1`. Nevertheless, there is a
    /// > pitfall that is rarely encountered: When nesting multiple protocol negotiations, the
    /// > listener should either be known to support all of the dialer's optimistically chosen
    /// > protocols or there is must be no intermediate protocol without a payload and none of
    /// > the protocol payloads must have the potential for being mistaken for a multistream-select
    /// > protocol message. This avoids rare edge-cases whereby the listener may not recognize
    /// > upgrade boundaries and erroneously process a request despite not supporting one of
    /// > the intermediate protocols that the dialer committed to. See [1] and [2].
    ///
    /// [1]: https://github.com/multiformats/go-multistream/issues/20
    /// [2]: https://github.com/libp2p/rust-libp2p/pull/1212
    V1Lazy,
    // Draft: https://github.com/libp2p/specs/pull/95
    // V2,
}
