path: root/les/distributor.go

// Copyright 2017 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

// Package light implements on-demand retrieval capable state and chain objects
// for the Ethereum Light Client.
package les

import (
    "container/list"
    "errors"
    "sync"
    "time"
)

// ErrNoPeers is returned if no peers capable of serving a queued request are available
var ErrNoPeers = errors.New("no suitable peers available")

// requestDistributor implements a mechanism that distributes requests to
// suitable peers, obeying flow control rules and prioritizing them in creation
// order (even when a resend is necessary).
type requestDistributor struct {
    reqQueue         *list.List
    lastReqOrder     uint64
    peers            map[distPeer]struct{}
    peerLock         sync.RWMutex
    stopChn, loopChn chan struct{}
    loopNextSent     bool
    lock             sync.Mutex
}

// distPeer is an LES server peer interface for the request distributor.
// waitBefore returns either the necessary waiting time before sending a request
// with the given upper estimated cost or the estimated remaining relative buffer
// value after sending such a request (in which case the request can be sent
// immediately). At least one of these values is always zero.
type distPeer interface {
    waitBefore(uint64) (time.Duration, float64)
    canQueue() bool
    queueSend(f func())
}

// distReq is the request abstraction used by the distributor. It is based on
// three callback functions:
// - getCost returns the upper estimate of the cost of sending the request to a given peer
// - canSend tells if the server peer is suitable to serve the request
// - request prepares sending the request to the given peer and returns a function that
// does the actual sending. Request order should be preserved but the callback itself should not
// block until it is sent because other peers might still be able to receive requests while
// one of them is blocking. Instead, the returned function is put in the peer's send queue.
type distReq struct {
    getCost func(distPeer) uint64
    canSend func(distPeer) bool
    request func(distPeer) func()

    reqOrder uint64
    sentChn  chan distPeer
    element  *list.Element
}

// newRequestDistributor creates a new request distributor
func newRequestDistributor(peers *peerSet, stopChn chan struct{}) *requestDistributor {
    d := &requestDistributor{
        reqQueue: list.New(),
        loopChn:  make(chan struct{}, 2),
        stopChn:  stopChn,
        peers:    make(map[distPeer]struct{}),
    }
    if peers != nil {
        peers.notify(d)
    }
    go d.loop()
    return d
}

// registerPeer implements peerSetNotify
func (d *requestDistributor) registerPeer(p *peer) {
    d.peerLock.Lock()
    d.peers[p] = struct{}{}
    d.peerLock.Unlock()
}

// unregisterPeer implements peerSetNotify
func (d *requestDistributor) unregisterPeer(p *peer) {
    d.peerLock.Lock()
    delete(d.peers, p)
    d.peerLock.Unlock()
}

// registerTestPeer adds a new test peer
func (d *requestDistributor) registerTestPeer(p distPeer) {
    d.peerLock.Lock()
    d.peers[p] = struct{}{}
    d.peerLock.Unlock()
}

// distMaxWait is the maximum waiting time after which further necessary waiting
// times are recalculated based on new feedback from the servers
const distMaxWait = time.Millisecond * 10

// main event loop
func (d *requestDistributor) loop() {
    for {
        select {
        case <-d.stopChn:
            d.lock.Lock()
            elem := d.reqQueue.Front()
            for elem != nil {
                close(elem.Value.(*distReq).sentChn)
                elem = elem.Next()
            }
            d.lock.Unlock()
            return
        case <-d.loopChn:
            d.lock.Lock()
            d.loopNextSent = false
        loop:
            for {
                peer, req, wait := d.nextRequest()
                if req != nil && wait == 0 {
                    chn := req.sentChn // save sentChn because remove sets it to nil
                    d.remove(req)
                    send := req.request(peer)
                    if send != nil {
                        peer.queueSend(send)
                    }
                    chn <- peer
                    close(chn)
                } else {
                    if wait == 0 {
                        // no request to send and nothing to wait for; the next
                        // queued request will wake up the loop
                        break loop
                    }
                    d.loopNextSent = true // a "next" signal has been sent, do not send another one until this one has been received
                    if wait > distMaxWait {
                        // waiting times may be reduced by incoming request replies, if it is too long, recalculate it periodically
                        wait = distMaxWait
                    }
                    go func() {
                        time.Sleep(wait)
                        d.loopChn <- struct{}{}
                    }()
                    break loop
                }
            }
            d.lock.Unlock()
        }
    }
}

// selectPeerItem represents a peer to be selected for a request by weightedRandomSelect
type selectPeerItem struct {
    peer   distPeer
    req    *distReq
    weight int64
}

// Weight implements wrsItem interface
func (sp selectPeerItem) Weight() int64 {
    return sp.weight
}

// nextRequest returns the next possible request from any peer, along with the
// associated peer and necessary waiting time
func (d *requestDistributor) nextRequest() (distPeer, *distReq, time.Duration) {
    checkedPeers := make(map[distPeer]struct{})
    elem := d.reqQueue.Front()
    var (
        bestPeer distPeer
        bestReq  *distReq
        bestWait time.Duration
        sel      *weightedRandomSelect
    )

    d.peerLock.RLock()
    defer d.peerLock.RUnlock()

    for (len(d.peers) > 0 || elem == d.reqQueue.Front()) && elem != nil {
        req := elem.Value.(*distReq)
        canSend := false
        for peer := range d.peers {
            if _, ok := checkedPeers[peer]; !ok && peer.canQueue() && req.canSend(peer) {
                canSend = true
                cost := req.getCost(peer)
                wait, bufRemain := peer.waitBefore(cost)
                if wait == 0 {
                    if sel == nil {
                        sel = newWeightedRandomSelect()
                    }
                    sel.update(selectPeerItem{peer: peer, req: req, weight: int64(bufRemain*1000000) + 1})
                } else {
                    if bestReq == nil || wait < bestWait {
                        bestPeer = peer
                        bestReq = req
                        bestWait = wait
                    }
                }
                checkedPeers[peer] = struct{}{}
            }
        }
        next := elem.Next()
        if !canSend && elem == d.reqQueue.Front() {
            close(req.sentChn)
            d.remove(req)
        }
        elem = next
    }

    if sel != nil {
        c := sel.choose().(selectPeerItem)
        return c.peer, c.req, 0
    }
    return bestPeer, bestReq, bestWait
}

// queue adds a request to the distribution queue, returns a channel where the
// receiving peer is sent once the request has been sent (request callback returned).
// If the request is cancelled or timed out without suitable peers, the channel is
// closed without sending any peer references to it.
func (d *requestDistributor) queue(r *distReq) chan distPeer {
    d.lock.Lock()
    defer d.lock.Unlock()

    if r.reqOrder == 0 {
        d.lastReqOrder++
        r.reqOrder = d.lastReqOrder
    }

    back := d.reqQueue.Back()
    if back == nil || r.reqOrder > back.Value.(*distReq).reqOrder {
        r.element = d.reqQueue.PushBack(r)
    } else {
        before := d.reqQueue.Front()
        for before.Value.(*distReq).reqOrder < r.reqOrder {
            before = before.Next()
        }
        r.element = d.reqQueue.InsertBefore(r, before)
    }

    if !d.loopNextSent {
        d.loopNextSent = true
        d.loopChn <- struct{}{}
    }

    r.sentChn = make(chan distPeer, 1)
    return r.sentChn
}

// cancel removes a request from the queue if it has not been sent yet (returns
// false if it has been sent already). It is guaranteed that the callback functions
// will not be called after cancel returns.
func (d *requestDistributor) cancel(r *distReq) bool {
    d.lock.Lock()
    defer d.lock.Unlock()

    if r.sentChn == nil {
        return false
    }

    close(r.sentChn)
    d.remove(r)
    return true
}

// remove removes a request from the queue
func (d *requestDistributor) remove(r *distReq) {
    r.sentChn = nil
    if r.element != nil {
        d.reqQueue.Remove(r.element)
        r.element = nil
    }
}