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//! Definitions for circuit padding.
//!
//! This module defines the API for a circuit padder, which is divided into two parts:
//! a [`PaddingController`] and [`PaddingEventStream`].
//!
//! When the `circ-padding` feature is enabled, the [`PaddingController`] is used
//! to tell a set of padding machines about incoming and outgoing
//! traffic to a circuit hop,
//! and the [`PaddingEventStream`] is used to decide when to send padding to that hop.
//!
//! When `circ-padding` is not enabled, both types are empty, and do nothing.
//!
//! # Padding event semantics
//!
//! Our events here are fairly tightly coupled
//! to the semantics provided by [`maybenot`].
//!
//! In brief, `maybenot` assumes:
//! * That incoming traffic arrives on a queue,
//! then is decrypted and sorted into "normal" and "padding".
//! * That outgoing traffic (normal or padding) is placed onto a queue,
//! and then eventually sent.
//!
//! For each of these cases, the circuit/tunnnel reactor
//! needs to call an appropriate method on [`PaddingController`]
//! to inform each hop's padding machines about the event.
//!
//! See the [`maybenot`] documentation for more information about when,
//! exactly, each method needs to be invoked.
//!
//! # Design considerations
//!
//! We expect that a substantial fraction of all circuit hops
//! will require padding in some form.
//! Because of this, we've taken some effort to optimize the
//! storage overhead of our padding state.
//!
//! [`maybenot`]: https://docs.rs/maybenot/latest/maybenot/index.html
cfg_if::cfg_if! {
if #[cfg(feature = "circ-padding")] {
mod maybenot_padding;
use maybenot_padding as padding_impl;
pub(crate) use maybenot_padding::{Replace, Bypass};
} else {
mod no_padding;
use no_padding as padding_impl;
}
}
// These types are made public using the super-experimental circ-padding-manual feature.
//
// TODO circpad: When we stabilize padding support, we should probably rename these,
// since they are not exclusively for circuits: They can also be used to send padding to
// padding on channels.
#[cfg(feature = "circ-padding-manual")]
pub use maybenot_padding::{CircuitPadder, CircuitPadderConfig, CircuitPadderConfigError};
pub(crate) use padding_impl::{
PaddingController, PaddingEvent, PaddingEventStream, QueuedCellPaddingInfo, SendPadding,
StartBlocking, new_padding,
};
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