hbbft/src/broadcast/mod.rs

//! Reliable broadcast algorithm.
use std::fmt::Debug;
use std::hash::Hash;
use std::sync::{Arc, Mutex};
use std::sync::mpsc;
use spmc;
use crossbeam;
use proto::*;
use std::marker::{Send, Sync};
use merkle::MerkleTree;
use merkle::proof::{Proof, Lemma, Positioned};
use reed_solomon_erasure::ReedSolomon;

/// Temporary placeholders for the number of participants and the maximum
/// envisaged number of faulty nodes. Only one is required since N >= 3f +
/// 1. There are at least two options for where should N and f come from:
///
/// - start-up parameters
///
/// - initial socket setup phase in node.rs
///
const PLACEHOLDER_N: usize = 8;
const PLACEHOLDER_F: usize = 2;

/// Broadcast stage. See the TODO note below!
///
/// TODO: The ACS algorithm will require multiple broadcast instances running
/// asynchronously, see Figure 4 in the HBBFT paper. So, it's likely that the
/// broadcast *stage* has to be replaced with N asynchronous threads, each
/// responding to values from one particular remote node. The paper doesn't make
/// it clear though how other messages - Echo and Ready - are distributed over
/// the instances. Also it appears that the sender of a message has to become
/// part of the message for this to work.
pub struct Stage<T: Send + Sync> {
    /// The transmit side of the multiple consumer channel to comms threads.
    pub tx: Arc<Mutex<spmc::Sender<Message<T>>>>,
    /// The receive side of the multiple producer channel from comms threads.
    pub rx: Arc<Mutex<mpsc::Receiver<Message<T>>>>,
    /// Value to be broadcast.
    pub broadcast_value: Option<T>,
}

impl<T: Clone + Debug + Eq + Hash + Send + Sync + Into<Vec<u8>>
     + From<Vec<u8>> + AsRef<[u8]>>
    Stage<T>
where Vec<u8>: From<T>
{
    pub fn new(tx: Arc<Mutex<spmc::Sender<Message<T>>>>,
               rx: Arc<Mutex<mpsc::Receiver<Message<T>>>>,
               broadcast_value: Option<T>) -> Self
    {
        Stage {
            tx: tx,
            rx: rx,
            broadcast_value: broadcast_value,
        }
    }

    /// Broadcast stage task returning the computed values in case of success,
    /// and an error in case of failure.
    ///
    /// TODO: Detailed error status.
    pub fn run(&mut self) -> Result<T, BroadcastError> {
        // Broadcast state machine thread.
        //
        // rx cannot be cloned due to its type constraint but can be used inside
        // a thread with the help of an `Arc` (`Rc` wouldn't work for the same
        // reason). A `Mutex` is used to grant write access.
        let rx = self.rx.to_owned();
        let tx = self.tx.to_owned();
        let bvalue = self.broadcast_value.to_owned();
        let result: Result<T, BroadcastError>;
        let result_r = Arc::new(Mutex::new(None));
        let result_r_scoped = result_r.clone();

        crossbeam::scope(|scope| {
            scope.spawn(move || {
                *result_r_scoped.lock().unwrap() =
                    Some(inner_run(tx, rx, bvalue));
            });
        });
        if let Some(ref r) = *result_r.lock().unwrap() {
            result = r.to_owned();
        }
        else {
            result = Err(BroadcastError::Threading);
        }
        result
    }
}

/// Errors returned by the broadcast instance.
#[derive(Debug, Clone, PartialEq)]
pub enum BroadcastError {
    RootHashMismatch,
    Threading
}

/// Breaks the input value into shards of equal length and encodes them -- and
/// some extra parity shards -- with a Reed-Solomon erasure coding scheme.
fn send_shards<T>(value: T,
                  tx: Arc<Mutex<spmc::Sender<Message<T>>>>,
                  coding: &ReedSolomon,
                  data_shard_num: usize,
                  parity_shard_num: usize)
where T: Clone + Debug + Send + Sync + Into<Vec<u8>>
    + From<Vec<u8>> + AsRef<[u8]>
    , Vec<u8>: From<T>
{
    let mut v: Vec<u8> = Vec::from(value).to_owned();

    // Pad the value vector with zeros to allow for shards of equal sizes.
    let shard_pad_len = v.len() % data_shard_num;
    for _i in 0..shard_pad_len {
        v.push(0);
    }
    // Size of a Merkle tree leaf value, in bytes.
    // Now the vector length is evenly divisible by the number of shards.
    let shard_len = v.len() / data_shard_num;
    // Pad the parity shards with zeros.
    for _i in 0 .. shard_len * parity_shard_num {
        v.push(0);
    }

    // Divide the vector into chunks/shards.
    let shards_iter = v.chunks_mut(shard_len);
    // Convert the iterator over slices into a vector of slices.
    let mut shards: Vec<&mut [u8]> = Vec::new();
    for s in shards_iter {
        shards.push(s);
    }

    // Construct the parity chunks/shards
    coding.encode(shards.as_mut_slice()).unwrap();

    // Convert shards back to type `T` for proof generation.
    let mut shards_t: Vec<T> = Vec::new();
    for s in shards.iter() {
        let s = Vec::into(s.to_vec());
        shards_t.push(s);
    }

    // Convert the Merkle tree into a partial binary tree for later
    // deconstruction into compound branches.
    let mtree = MerkleTree::from_vec(&::ring::digest::SHA256, shards_t);

    // Send each proof to a node.
    //
    // FIXME: use a single consumer TX channel.
    for leaf_value in mtree.iter().cloned() {
        let proof = mtree.gen_proof(leaf_value);
        if let Some(proof) = proof {
            tx.lock().unwrap().send(Message::Broadcast(
                BroadcastMessage::Value(proof))).unwrap();
        }
    }
}

/// The main loop of the broadcast task.
fn inner_run<T>(tx: Arc<Mutex<spmc::Sender<Message<T>>>>,
                rx: Arc<Mutex<mpsc::Receiver<Message<T>>>>,
                broadcast_value: Option<T>) -> Result<T, BroadcastError>
where T: Clone + Debug + Eq + Hash + Send + Sync + Into<Vec<u8>>
    + From<Vec<u8>> + AsRef<[u8]>
    , Vec<u8>: From<T>
{
    // Erasure coding scheme: N - 2f value shards and 2f parity shards
    let parity_shard_num = 2 * PLACEHOLDER_F;
    let data_shard_num = PLACEHOLDER_N - parity_shard_num;
    let coding = ReedSolomon::new(data_shard_num, parity_shard_num).unwrap();

    // Split the value into chunks/shards, encode them with erasure codes.
    // Assemble a Merkle tree from data and parity shards. Take all proofs from
    // this tree and send them, each to its own node.
    //
    // FIXME: Does the node send a proof to itself?
    if let Some(v) = broadcast_value {
        send_shards(v, tx.clone(), &coding, data_shard_num, parity_shard_num);
    }

    // currently known leaf values
    let mut leaf_values: Vec<Option<Box<[u8]>>> = vec![None; PLACEHOLDER_N];
    // number of non-None leaf values
    let mut leaf_values_num = 0;
    // return value
    let mut result: Option<Result<T, BroadcastError>> = None;
    // Write-once root hash of a tree broadcast from the sender associated with
    // this instance.
    let mut root_hash: Option<Vec<u8>> = None;
    // Number of times Echo was received with the same root hash.
    let mut echo_num = 0;
    // Number of times Ready was received with the same root hash.
    let mut ready_num = 0;
    let mut ready_sent = false;
    let mut ready_to_decode = false;

    // TODO: handle exit conditions
    while result == None {
        // Receive a message from the socket IO task.
        let message = rx.lock().unwrap().recv().unwrap();
        if let Message::Broadcast(message) = message {
            match message {
                // A value received. Record the value and multicast an echo.
                //
                // TODO: determine if the paper treats multicast as reflexive and
                // add an echo to this node if it does.
                BroadcastMessage::Value(p) => {
                    if let None = root_hash {
                        root_hash = Some(p.root_hash.clone());
                    }

                    if let &Some(ref h) = &root_hash {
                        if p.validate(h.as_slice()) {
                            // Save the leaf value for reconstructing the tree
                            // later.
                            leaf_values[index_of_proof(&p)] =
                                Some(Vec::from(p.value.clone())
                                     .into_boxed_slice());
                            leaf_values_num = leaf_values_num + 1;
                        }
                    }
                    // Broadcast an echo of this proof.
                    tx.lock().unwrap()
                        .send(Message::Broadcast(
                            BroadcastMessage::Echo(p)))
                        .unwrap()
                },

                // An echo received. Verify the proof it contains.
                BroadcastMessage::Echo(p) => {
                    if let None = root_hash {
                        root_hash = Some(p.root_hash.clone());
                    }

                    // call validate with the root hash as argument
                    if let &Some(ref h) = &root_hash {
                        if p.validate(h.as_slice()) {
                            echo_num += 1;
                            // Save the leaf value for reconstructing the tree
                            // later.
                            leaf_values[index_of_proof(&p)] =
                                Some(Vec::from(p.value.clone())
                                     .into_boxed_slice());
                            leaf_values_num = leaf_values_num + 1;

                            // upon receiving 2f + 1 matching READY(h)
                            // messages, wait for N − 2 f ECHO messages, then
                            // decode v
                            if ready_to_decode &&
                                leaf_values_num >=
                                PLACEHOLDER_N - 2 * PLACEHOLDER_F
                            {
                                result = Some(
                                    decode_from_shards(&mut leaf_values,
                                                       &coding,
                                                       data_shard_num, h));
                            }
                            else if leaf_values_num >=
                                PLACEHOLDER_N - PLACEHOLDER_F
                            {
                                result = Some(
                                    decode_from_shards(&mut leaf_values,
                                                       &coding,
                                                       data_shard_num, h));
                                // if Ready has not yet been sent, multicast
                                // Ready
                                if !ready_sent {
                                    ready_sent = true;
                                    tx.lock().unwrap().send(Message::Broadcast(
                                        BroadcastMessage::Ready(h.to_owned())))
                                        .unwrap();
                                }
                            }
                        }
                    }
                },

                BroadcastMessage::Ready(ref h) => {
                    // TODO: Prioritise the Value root hash, possibly. Prevent
                    // an incorrect node from blocking progress which it could
                    // achieve by sending an incorrect hash.
                    if let None = root_hash {
                        root_hash = Some(h.clone());
                    }
                    // Check that the root hash matches.
                    if let &Some(ref h) = &root_hash {
                        ready_num += 1;

                        // Upon receiving f + 1 matching Ready(h) messages, if
                        // Ready has not yet been sent, multicast Ready(h).
                        if (ready_num == PLACEHOLDER_F + 1) &&
                            !ready_sent
                        {
                            tx.lock().unwrap().send(Message::Broadcast(
                                BroadcastMessage::Ready(h.to_vec()))).unwrap();
                        }

                        // Upon receiving 2f + 1 matching Ready(h) messages,
                        // wait for N − 2f Echo messages, then decode v.
                        if ready_num > 2 * PLACEHOLDER_F {
                            // Wait for N - 2f Echo messages, then decode v.
                            if echo_num >= PLACEHOLDER_N - 2 * PLACEHOLDER_F {
                                result = Some(
                                    decode_from_shards(&mut leaf_values,
                                                       &coding,
                                                       data_shard_num, h));
                            }
                            else {
                                ready_to_decode = true;
                            }
                        }
                    }
                }
            }
        }
        else {
            error!("Incorrect message from the socket: {:?}",
                   message);
        }
    }
    result.unwrap()
}

fn decode_from_shards<T>(leaf_values: &mut Vec<Option<Box<[u8]>>>,
                         coding: &ReedSolomon,
                         data_shard_num: usize,
                         root_hash: &Vec<u8>) ->
    Result<T, BroadcastError>
where T: AsRef<[u8]> + From<Vec<u8>>, Vec<u8>: From<T>
{
    // Try to interpolate the Merkle tree using the Reed-Solomon erasure coding
    // scheme.
    coding.reconstruct_shards(leaf_values.as_mut_slice()).unwrap();

    // Recompute the Merkle tree root.
    //
    // Convert shards back to type `T` for tree construction.
    let mut shards_t: Vec<T> = Vec::new();
    for l in leaf_values.iter() {
        if let Some(ref v) = *l {
            let s = Vec::into(v.to_vec());
            shards_t.push(s);
        }
    }
    // Construct the Merkle tree.
    let mtree = MerkleTree::from_vec(&::ring::digest::SHA256, shards_t);
    // If the root hash of the reconstructed tree does not match the one
    // received with proofs then abort.
    if *mtree.root_hash() != *root_hash {
        // NOTE: The paper does not define the meaning of *abort*. But it is
        // sensible not to continue trying to reconstruct the tree after this
        // point. This instance must have received incorrect shards.
        Err(BroadcastError::RootHashMismatch)
    }
    else {
        // Reconstruct the value from the data shards.
        Ok(glue_shards(mtree, data_shard_num))
    }
}


/// Concatenates the first `n` leaf values of a Merkle tree `m` in one value of
/// type `T`. This is useful for reconstructing the data value held in the tree
/// and forgetting the leaves that contain parity information.
fn glue_shards<T>(m: MerkleTree<T>, n: usize) -> T
where T: From<Vec<u8>>, Vec<u8>: From<T>
{
    let mut t: Vec<u8> = Vec::new();
    let mut i = 0;

    for s in m.into_iter() {
        i += 1;
        if i > n {
            break;
        }
        for b in Vec::from(s).into_iter() {
            t.push(b);
        }
    }
    Vec::into(t)
}

/// An additional path conversion operation on `Lemma` to allow reconstruction
/// of erasure-coded `Proof` from `Lemma`s. The output path, when read from left
/// to right, goes from leaf to root (LSB order).
fn path_of_lemma(lemma: &Lemma) -> Vec<bool> {
    match lemma.sub_lemma {
        None => {
            match lemma.sibling_hash {
                // lemma terminates with no leaf
                None => vec![],
                // the leaf is on the right
                Some(Positioned::Left(_)) => vec![true],
                // the leaf is on the left
                Some(Positioned::Right(_)) => vec![false],
            }
        }
        Some(ref l) => {
            let mut p = path_of_lemma(l.as_ref());

            match lemma.sibling_hash {
                // lemma terminates
                None => (),
                // lemma branches out to the right
                Some(Positioned::Left(_)) => p.push(true),
                // lemma branches out to the left
                Some(Positioned::Right(_)) => p.push(false),
            }
            p
        }
    }
}

/// Further conversion of a binary tree path into an array index.
fn index_of_path(mut path: Vec<bool>) -> usize {
    let mut idx = 0;
    // Convert to the MSB order.
    path.reverse();

    for &dir in path.iter() {
        if dir == false {
            idx = idx << 1;
        }
        else {
            idx = (idx << 1) | 1;
        }
    }
    idx
}

/// Computes the Merkle tree leaf index of a value in a given proof.
fn index_of_proof<T>(p: &Proof<T>) -> usize {
    index_of_path(path_of_lemma(&p.lemma))
}