path: root/eth/downloader/peer.go

// Contains the active peer-set of the downloader, maintaining both failures
// as well as reputation metrics to prioritize the block retrievals.

package downloader

import (
    "errors"
    "fmt"
    "math"
    "sync"
    "sync/atomic"
    "time"

    "github.com/ethereum/go-ethereum/common"
    "gopkg.in/fatih/set.v0"
)

type hashFetcherFn func(common.Hash) error
type blockFetcherFn func([]common.Hash) error

var (
    errAlreadyFetching   = errors.New("already fetching blocks from peer")
    errAlreadyRegistered = errors.New("peer is already registered")
    errNotRegistered     = errors.New("peer is not registered")
)

// peer represents an active peer from which hashes and blocks are retrieved.
type peer struct {
    id   string      // Unique identifier of the peer
    head common.Hash // Hash of the peers latest known block

    idle int32 // Current activity state of the peer (idle = 0, active = 1)
    rep  int32 // Simple peer reputation

    capacity int32     // Number of blocks allowed to fetch per request
    started  time.Time // Time instance when the last fetch was started

    ignored *set.Set // Set of hashes not to request (didn't have previously)

    getHashes hashFetcherFn  // Method to retrieve a batch of hashes (mockable for testing)
    getBlocks blockFetcherFn // Method to retrieve a batch of blocks (mockable for testing)
}

// newPeer create a new downloader peer, with specific hash and block retrieval
// mechanisms.
func newPeer(id string, head common.Hash, getHashes hashFetcherFn, getBlocks blockFetcherFn) *peer {
    return &peer{
        id:        id,
        head:      head,
        capacity:  1,
        getHashes: getHashes,
        getBlocks: getBlocks,
        ignored:   set.New(),
    }
}

// Reset clears the internal state of a peer entity.
func (p *peer) Reset() {
    atomic.StoreInt32(&p.idle, 0)
    atomic.StoreInt32(&p.capacity, 1)
    p.ignored.Clear()
}

// Fetch sends a block retrieval request to the remote peer.
func (p *peer) Fetch(request *fetchRequest) error {
    // Short circuit if the peer is already fetching
    if !atomic.CompareAndSwapInt32(&p.idle, 0, 1) {
        return errAlreadyFetching
    }
    p.started = time.Now()

    // Convert the hash set to a retrievable slice
    hashes := make([]common.Hash, 0, len(request.Hashes))
    for hash, _ := range request.Hashes {
        hashes = append(hashes, hash)
    }
    p.getBlocks(hashes)

    return nil
}

// SetIdle sets the peer to idle, allowing it to execute new retrieval requests.
// Its block retrieval allowance will also be updated either up- or downwards,
// depending on whether the previous fetch completed in time or not.
func (p *peer) SetIdle() {
    // Update the peer's download allowance based on previous performance
    scale := 2.0
    if time.Since(p.started) > blockSoftTTL {
        scale = 0.5
        if time.Since(p.started) > blockHardTTL {
            scale = 1 / float64(MaxBlockFetch) // reduces capacity to 1
        }
    }
    for {
        // Calculate the new download bandwidth allowance
        prev := atomic.LoadInt32(&p.capacity)
        next := int32(math.Max(1, math.Min(float64(MaxBlockFetch), float64(prev)*scale)))

        // Try to update the old value
        if atomic.CompareAndSwapInt32(&p.capacity, prev, next) {
            // If we're having problems at 1 capacity, try to find better peers
            if next == 1 {
                p.Demote()
            }
            break
        }
    }
    // Set the peer to idle to allow further block requests
    atomic.StoreInt32(&p.idle, 0)
}

// Capacity retrieves the peers block download allowance based on its previously
// discovered bandwidth capacity.
func (p *peer) Capacity() int {
    return int(atomic.LoadInt32(&p.capacity))
}

// Promote increases the peer's reputation.
func (p *peer) Promote() {
    atomic.AddInt32(&p.rep, 1)
}

// Demote decreases the peer's reputation or leaves it at 0.
func (p *peer) Demote() {
    for {
        // Calculate the new reputation value
        prev := atomic.LoadInt32(&p.rep)
        next := prev / 2

        // Try to update the old value
        if atomic.CompareAndSwapInt32(&p.rep, prev, next) {
            return
        }
    }
}

// String implements fmt.Stringer.
func (p *peer) String() string {
    return fmt.Sprintf("Peer %s [%s]", p.id,
        fmt.Sprintf("reputation %3d, ", atomic.LoadInt32(&p.rep))+
            fmt.Sprintf("capacity %3d, ", atomic.LoadInt32(&p.capacity))+
            fmt.Sprintf("ignored %4d", p.ignored.Size()),
    )
}

// peerSet represents the collection of active peer participating in the block
// download procedure.
type peerSet struct {
    peers map[string]*peer
    lock  sync.RWMutex
}

// newPeerSet creates a new peer set top track the active download sources.
func newPeerSet() *peerSet {
    return &peerSet{
        peers: make(map[string]*peer),
    }
}

// Reset iterates over the current peer set, and resets each of the known peers
// to prepare for a next batch of block retrieval.
func (ps *peerSet) Reset() {
    ps.lock.RLock()
    defer ps.lock.RUnlock()

    for _, peer := range ps.peers {
        peer.Reset()
    }
}

// Register injects a new peer into the working set, or returns an error if the
// peer is already known.
func (ps *peerSet) Register(p *peer) error {
    ps.lock.Lock()
    defer ps.lock.Unlock()

    if _, ok := ps.peers[p.id]; ok {
        return errAlreadyRegistered
    }
    ps.peers[p.id] = p
    return nil
}

// Unregister removes a remote peer from the active set, disabling any further
// actions to/from that particular entity.
func (ps *peerSet) Unregister(id string) error {
    ps.lock.Lock()
    defer ps.lock.Unlock()

    if _, ok := ps.peers[id]; !ok {
        return errNotRegistered
    }
    delete(ps.peers, id)
    return nil
}

// Peer retrieves the registered peer with the given id.
func (ps *peerSet) Peer(id string) *peer {
    ps.lock.RLock()
    defer ps.lock.RUnlock()

    return ps.peers[id]
}

// Len returns if the current number of peers in the set.
func (ps *peerSet) Len() int {
    ps.lock.RLock()
    defer ps.lock.RUnlock()

    return len(ps.peers)
}

// AllPeers retrieves a flat list of all the peers within the set.
func (ps *peerSet) AllPeers() []*peer {
    ps.lock.RLock()
    defer ps.lock.RUnlock()

    list := make([]*peer, 0, len(ps.peers))
    for _, p := range ps.peers {
        list = append(list, p)
    }
    return list
}

// IdlePeers retrieves a flat list of all the currently idle peers within the
// active peer set, ordered by their reputation.
func (ps *peerSet) IdlePeers() []*peer {
    ps.lock.RLock()
    defer ps.lock.RUnlock()

    list := make([]*peer, 0, len(ps.peers))
    for _, p := range ps.peers {
        if atomic.LoadInt32(&p.idle) == 0 {
            list = append(list, p)
        }
    }
    for i := 0; i < len(list); i++ {
        for j := i + 1; j < len(list); j++ {
            if atomic.LoadInt32(&list[i].rep) < atomic.LoadInt32(&list[j].rep) {
                list[i], list[j] = list[j], list[i]
            }
        }
    }
    return list
}