eth/downloader: implement concurrent header downloads

eth/downloader/downloader_test.go

@@ -560,8 +560,8 @@ func (dl *downloadTester) peerGetAbsHeadersFn(id string, delay time.Duration) fu
 		hashes := dl.peerHashes[id]
 		headers := dl.peerHeaders[id]
 		result := make([]*types.Header, 0, amount)
-		for i := 0; i < amount && len(hashes)-int(origin)-1-i >= 0; i++ {
-			if header, ok := headers[hashes[len(hashes)-int(origin)-1-i]]; ok {
+		for i := 0; i < amount && len(hashes)-int(origin)-1-i*(skip+1) >= 0; i++ {
+			if header, ok := headers[hashes[len(hashes)-int(origin)-1-i*(skip+1)]]; ok {
 				result = append(result, header)
 			}
 		}
@@ -1348,27 +1348,28 @@ func testBlockHeaderAttackerDropping(t *testing.T, protocol int) {
 		result error
 		drop   bool
 	}{
-		{nil, false},                   // Sync succeeded, all is well
-		{errBusy, false},               // Sync is already in progress, no problem
-		{errUnknownPeer, false},        // Peer is unknown, was already dropped, don't double drop
-		{errBadPeer, true},             // Peer was deemed bad for some reason, drop it
-		{errStallingPeer, true},        // Peer was detected to be stalling, drop it
-		{errNoPeers, false},            // No peers to download from, soft race, no issue
-		{errTimeout, true},             // No hashes received in due time, drop the peer
-		{errEmptyHashSet, true},        // No hashes were returned as a response, drop as it's a dead end
-		{errEmptyHeaderSet, true},      // No headers were returned as a response, drop as it's a dead end
-		{errPeersUnavailable, true},    // Nobody had the advertised blocks, drop the advertiser
-		{errInvalidAncestor, true},     // Agreed upon ancestor is not acceptable, drop the chain rewriter
-		{errInvalidChain, true},        // Hash chain was detected as invalid, definitely drop
-		{errInvalidBlock, false},       // A bad peer was detected, but not the sync origin
-		{errInvalidBody, false},        // A bad peer was detected, but not the sync origin
-		{errInvalidReceipt, false},     // A bad peer was detected, but not the sync origin
-		{errCancelHashFetch, false},    // Synchronisation was canceled, origin may be innocent, don't drop
-		{errCancelBlockFetch, false},   // Synchronisation was canceled, origin may be innocent, don't drop
-		{errCancelHeaderFetch, false},  // Synchronisation was canceled, origin may be innocent, don't drop
-		{errCancelBodyFetch, false},    // Synchronisation was canceled, origin may be innocent, don't drop
-		{errCancelReceiptFetch, false}, // Synchronisation was canceled, origin may be innocent, don't drop
-		{errCancelProcessing, false},   // Synchronisation was canceled, origin may be innocent, don't drop
+		{nil, false},                        // Sync succeeded, all is well
+		{errBusy, false},                    // Sync is already in progress, no problem
+		{errUnknownPeer, false},             // Peer is unknown, was already dropped, don't double drop
+		{errBadPeer, true},                  // Peer was deemed bad for some reason, drop it
+		{errStallingPeer, true},             // Peer was detected to be stalling, drop it
+		{errNoPeers, false},                 // No peers to download from, soft race, no issue
+		{errTimeout, true},                  // No hashes received in due time, drop the peer
+		{errEmptyHashSet, true},             // No hashes were returned as a response, drop as it's a dead end
+		{errEmptyHeaderSet, true},           // No headers were returned as a response, drop as it's a dead end
+		{errPeersUnavailable, true},         // Nobody had the advertised blocks, drop the advertiser
+		{errInvalidAncestor, true},          // Agreed upon ancestor is not acceptable, drop the chain rewriter
+		{errInvalidChain, true},             // Hash chain was detected as invalid, definitely drop
+		{errInvalidBlock, false},            // A bad peer was detected, but not the sync origin
+		{errInvalidBody, false},             // A bad peer was detected, but not the sync origin
+		{errInvalidReceipt, false},          // A bad peer was detected, but not the sync origin
+		{errCancelHashFetch, false},         // Synchronisation was canceled, origin may be innocent, don't drop
+		{errCancelBlockFetch, false},        // Synchronisation was canceled, origin may be innocent, don't drop
+		{errCancelHeaderFetch, false},       // Synchronisation was canceled, origin may be innocent, don't drop
+		{errCancelBodyFetch, false},         // Synchronisation was canceled, origin may be innocent, don't drop
+		{errCancelReceiptFetch, false},      // Synchronisation was canceled, origin may be innocent, don't drop
+		{errCancelHeaderProcessing, false},  // Synchronisation was canceled, origin may be innocent, don't drop
+		{errCancelContentProcessing, false}, // Synchronisation was canceled, origin may be innocent, don't drop
 	}
 	// Run the tests and check disconnection status
 	tester := newTester()

eth/downloader/peer.go

@@ -58,15 +58,18 @@ type peer struct {
 	id   string      // Unique identifier of the peer
 	head common.Hash // Hash of the peers latest known block
 
+	headerIdle  int32 // Current header activity state of the peer (idle = 0, active = 1)
 	blockIdle   int32 // Current block activity state of the peer (idle = 0, active = 1)
 	receiptIdle int32 // Current receipt activity state of the peer (idle = 0, active = 1)
 	stateIdle   int32 // Current node data activity state of the peer (idle = 0, active = 1)
 
+	headerThroughput  float64 // Number of headers measured to be retrievable per second
 	blockThroughput   float64 // Number of blocks (bodies) measured to be retrievable per second
 	receiptThroughput float64 // Number of receipts measured to be retrievable per second
 	stateThroughput   float64 // Number of node data pieces measured to be retrievable per second
 
-	blockStarted   time.Time // Time instance when the last block (body)fetch was started
+	headerStarted  time.Time // Time instance when the last header fetch was started
+	blockStarted   time.Time // Time instance when the last block (body) fetch was started
 	receiptStarted time.Time // Time instance when the last receipt fetch was started
 	stateStarted   time.Time // Time instance when the last node data fetch was started
 
@@ -118,10 +121,12 @@ func (p *peer) Reset() {
 	p.lock.Lock()
 	defer p.lock.Unlock()
 
+	atomic.StoreInt32(&p.headerIdle, 0)
 	atomic.StoreInt32(&p.blockIdle, 0)
 	atomic.StoreInt32(&p.receiptIdle, 0)
 	atomic.StoreInt32(&p.stateIdle, 0)
 
+	p.headerThroughput = 0
 	p.blockThroughput = 0
 	p.receiptThroughput = 0
 	p.stateThroughput = 0
@@ -151,6 +156,24 @@ func (p *peer) Fetch61(request *fetchRequest) error {
 	return nil
 }
 
+// FetchHeaders sends a header retrieval request to the remote peer.
+func (p *peer) FetchHeaders(from uint64, count int) error {
+	// Sanity check the protocol version
+	if p.version < 62 {
+		panic(fmt.Sprintf("header fetch [eth/62+] requested on eth/%d", p.version))
+	}
+	// Short circuit if the peer is already fetching
+	if !atomic.CompareAndSwapInt32(&p.headerIdle, 0, 1) {
+		return errAlreadyFetching
+	}
+	p.headerStarted = time.Now()
+
+	// Issue the header retrieval request (absolut upwards without gaps)
+	go p.getAbsHeaders(from, count, 0, false)
+
+	return nil
+}
+
 // FetchBodies sends a block body retrieval request to the remote peer.
 func (p *peer) FetchBodies(request *fetchRequest) error {
 	// Sanity check the protocol version
@@ -217,6 +240,13 @@ func (p *peer) FetchNodeData(request *fetchRequest) error {
 	return nil
 }
 
+// SetHeadersIdle sets the peer to idle, allowing it to execute new header retrieval
+// requests. Its estimated header retrieval throughput is updated with that measured
+// just now.
+func (p *peer) SetHeadersIdle(delivered int) {
+	p.setIdle(p.headerStarted, delivered, &p.headerThroughput, &p.headerIdle)
+}
+
 // SetBlocksIdle sets the peer to idle, allowing it to execute new block retrieval
 // requests. Its estimated block retrieval throughput is updated with that measured
 // just now.
@@ -264,6 +294,15 @@ func (p *peer) setIdle(started time.Time, delivered int, throughput *float64, id
 	*throughput = (1-throughputImpact)*(*throughput) + throughputImpact*measured
 }
 
+// HeaderCapacity retrieves the peers header download allowance based on its
+// previously discovered throughput.
+func (p *peer) HeaderCapacity() int {
+	p.lock.RLock()
+	defer p.lock.RUnlock()
+
+	return int(math.Max(1, math.Min(p.headerThroughput*float64(headerTargetRTT)/float64(time.Second), float64(MaxHeaderFetch))))
+}
+
 // BlockCapacity retrieves the peers block download allowance based on its
 // previously discovered throughput.
 func (p *peer) BlockCapacity() int {
@@ -323,14 +362,15 @@ func (p *peer) String() string {
 	defer p.lock.RUnlock()
 
 	return fmt.Sprintf("Peer %s [%s]", p.id,
-		fmt.Sprintf("blocks %3.2f/s, ", p.blockThroughput)+
+		fmt.Sprintf("headers %3.2f/s, ", p.headerThroughput)+
+			fmt.Sprintf("blocks %3.2f/s, ", p.blockThroughput)+
 			fmt.Sprintf("receipts %3.2f/s, ", p.receiptThroughput)+
 			fmt.Sprintf("states %3.2f/s, ", p.stateThroughput)+
 			fmt.Sprintf("lacking %4d", len(p.lacking)),
 	)
 }
 
-// peerSet represents the collection of active peer participating in the block
+// peerSet represents the collection of active peer participating in the chain
 // download procedure.
 type peerSet struct {
 	peers map[string]*peer
@@ -359,7 +399,7 @@ func (ps *peerSet) Reset() {
 // peer is already known.
 //
 // The method also sets the starting throughput values of the new peer to the
-// average of all existing peers, to give it a realistic change of being used
+// average of all existing peers, to give it a realistic chance of being used
 // for data retrievals.
 func (ps *peerSet) Register(p *peer) error {
 	ps.lock.Lock()
@@ -369,15 +409,17 @@ func (ps *peerSet) Register(p *peer) error {
 		return errAlreadyRegistered
 	}
 	if len(ps.peers) > 0 {
-		p.blockThroughput, p.receiptThroughput, p.stateThroughput = 0, 0, 0
+		p.headerThroughput, p.blockThroughput, p.receiptThroughput, p.stateThroughput = 0, 0, 0, 0
 
 		for _, peer := range ps.peers {
 			peer.lock.RLock()
+			p.headerThroughput += peer.headerThroughput
 			p.blockThroughput += peer.blockThroughput
 			p.receiptThroughput += peer.receiptThroughput
 			p.stateThroughput += peer.stateThroughput
 			peer.lock.RUnlock()
 		}
+		p.headerThroughput /= float64(len(ps.peers))
 		p.blockThroughput /= float64(len(ps.peers))
 		p.receiptThroughput /= float64(len(ps.peers))
 		p.stateThroughput /= float64(len(ps.peers))
@@ -441,6 +483,20 @@ func (ps *peerSet) BlockIdlePeers() ([]*peer, int) {
 	return ps.idlePeers(61, 61, idle, throughput)
 }
 
+// HeaderIdlePeers retrieves a flat list of all the currently header-idle peers
+// within the active peer set, ordered by their reputation.
+func (ps *peerSet) HeaderIdlePeers() ([]*peer, int) {
+	idle := func(p *peer) bool {
+		return atomic.LoadInt32(&p.headerIdle) == 0
+	}
+	throughput := func(p *peer) float64 {
+		p.lock.RLock()
+		defer p.lock.RUnlock()
+		return p.headerThroughput
+	}
+	return ps.idlePeers(62, 64, idle, throughput)
+}
+
 // BodyIdlePeers retrieves a flat list of all the currently body-idle peers within
 // the active peer set, ordered by their reputation.
 func (ps *peerSet) BodyIdlePeers() ([]*peer, int) {

eth/downloader/queue.go

@@ -39,8 +39,8 @@ import (
 )
 
 var (
-	blockCacheLimit   = 8192 // Maximum number of blocks to cache before throttling the download
-	maxInFlightStates = 4096 // Maximum number of state downloads to allow concurrently
+	blockCacheLimit   = 16384 // Maximum number of blocks to cache before throttling the download
+	maxInFlightStates = 8192  // Maximum number of state downloads to allow concurrently
 )
 
 var (
@@ -52,6 +52,7 @@ var (
 // fetchRequest is a currently running data retrieval operation.
 type fetchRequest struct {
 	Peer    *peer               // Peer to which the request was sent
+	From    uint64              // [eth/62] Requested chain element index (used for skeleton fills only)
 	Hashes  map[common.Hash]int // [eth/61] Requested hashes with their insertion index (priority)
 	Headers []*types.Header     // [eth/62] Requested headers, sorted by request order
 	Time    time.Time           // Time when the request was made
@@ -79,6 +80,17 @@ type queue struct {
 
 	headerHead common.Hash // [eth/62] Hash of the last queued header to verify order
 
+	// Headers are "special", they download in batches, supported by a skeleton chain
+	headerTaskPool  map[uint64]*types.Header       // [eth/62] Pending header retrieval tasks, mapping starting indexes to skeleton headers
+	headerTaskQueue *prque.Prque                   // [eth/62] Priority queue of the skeleton indexes to fetch the filling headers for
+	headerPeerMiss  map[string]map[uint64]struct{} // [eth/62] Set of per-peer header batches known to be unavailable
+	headerPendPool  map[string]*fetchRequest       // [eth/62] Currently pending header retrieval operations
+	headerDonePool  map[uint64]struct{}            // [eth/62] Set of the completed header fetches
+	headerResults   []*types.Header                // [eth/62] Result cache accumulating the completed headers
+	headerOffset    uint64                         // [eth/62] Number of the first header in the result cache
+	headerContCh    chan bool                      // [eth/62] Channel to notify when header download finishes
+
+	// All data retrievals below are based on an already assembles header chain
 	blockTaskPool  map[common.Hash]*types.Header // [eth/62] Pending block (body) retrieval tasks, mapping hashes to headers
 	blockTaskQueue *prque.Prque                  // [eth/62] Priority queue of the headers to fetch the blocks (bodies) for
 	blockPendPool  map[string]*fetchRequest      // [eth/62] Currently pending block (body) retrieval operations
@@ -113,6 +125,8 @@ func newQueue(stateDb ethdb.Database) *queue {
 	return &queue{
 		hashPool:         make(map[common.Hash]int),
 		hashQueue:        prque.New(),
+		headerPendPool:   make(map[string]*fetchRequest),
+		headerContCh:     make(chan bool),
 		blockTaskPool:    make(map[common.Hash]*types.Header),
 		blockTaskQueue:   prque.New(),
 		blockPendPool:    make(map[string]*fetchRequest),
@@ -149,6 +163,8 @@ func (q *queue) Reset() {
 
 	q.headerHead = common.Hash{}
 
+	q.headerPendPool = make(map[string]*fetchRequest)
+
 	q.blockTaskPool = make(map[common.Hash]*types.Header)
 	q.blockTaskQueue.Reset()
 	q.blockPendPool = make(map[string]*fetchRequest)
@@ -178,6 +194,14 @@ func (q *queue) Close() {
 	q.active.Broadcast()
 }
 
+// PendingHeaders retrieves the number of header requests pending for retrieval.
+func (q *queue) PendingHeaders() int {
+	q.lock.Lock()
+	defer q.lock.Unlock()
+
+	return q.headerTaskQueue.Size()
+}
+
 // PendingBlocks retrieves the number of block (body) requests pending for retrieval.
 func (q *queue) PendingBlocks() int {
 	q.lock.Lock()
@@ -205,6 +229,15 @@ func (q *queue) PendingNodeData() int {
 	return 0
 }
 
+// InFlightHeaders retrieves whether there are header fetch requests currently
+// in flight.
+func (q *queue) InFlightHeaders() bool {
+	q.lock.Lock()
+	defer q.lock.Unlock()
+
+	return len(q.headerPendPool) > 0
+}
+
 // InFlightBlocks retrieves whether there are block fetch requests currently in
 // flight.
 func (q *queue) InFlightBlocks() bool {
@@ -317,6 +350,44 @@ func (q *queue) Schedule61(hashes []common.Hash, fifo bool) []common.Hash {
 	return inserts
 }
 
+// ScheduleSkeleton adds a batch of header retrieval tasks to the queue to fill
+// up an already retrieved header skeleton.
+func (q *queue) ScheduleSkeleton(from uint64, skeleton []*types.Header) {
+	q.lock.Lock()
+	defer q.lock.Unlock()
+
+	// No skeleton retrieval can be in progress, fail hard if so (huge implementation bug)
+	if q.headerResults != nil {
+		panic("skeleton assembly already in progress")
+	}
+	// Shedule all the header retrieval tasks for the skeleton assembly
+	q.headerTaskPool = make(map[uint64]*types.Header)
+	q.headerTaskQueue = prque.New()
+	q.headerPeerMiss = make(map[string]map[uint64]struct{}) // Reset availability to correct invalid chains
+	q.headerResults = make([]*types.Header, len(skeleton)*MaxHeaderFetch)
+	q.headerOffset = from
+	q.headerContCh = make(chan bool, 1)
+
+	for i, header := range skeleton {
+		index := from + uint64(i*MaxHeaderFetch)
+
+		q.headerTaskPool[index] = header
+		q.headerTaskQueue.Push(index, -float32(index))
+	}
+}
+
+// RetrieveHeaders retrieves the header chain assemble based on the scheduled
+// skeleton.
+func (q *queue) RetrieveHeaders() []*types.Header {
+	q.lock.Lock()
+	defer q.lock.Unlock()
+
+	headers := q.headerResults
+	q.headerResults = nil
+
+	return headers
+}
+
 // Schedule adds a set of headers for the download queue for scheduling, returning
 // the new headers encountered.
 func (q *queue) Schedule(headers []*types.Header, from uint64) []*types.Header {
@@ -437,6 +508,46 @@ func (q *queue) countProcessableItems() int {
 	return len(q.resultCache)
 }
 
+// ReserveHeaders reserves a set of headers for the given peer, skipping any
+// previously failed batches.
+func (q *queue) ReserveHeaders(p *peer, count int) *fetchRequest {
+	q.lock.Lock()
+	defer q.lock.Unlock()
+
+	// Short circuit if the peer's already downloading something (sanity check to
+	// not corrupt state)
+	if _, ok := q.headerPendPool[p.id]; ok {
+		return nil
+	}
+	// Retrieve a batch of hashes, skipping previously failed ones
+	send, skip := uint64(0), []uint64{}
+	for send == 0 && !q.headerTaskQueue.Empty() {
+		from, _ := q.headerTaskQueue.Pop()
+		if q.headerPeerMiss[p.id] != nil {
+			if _, ok := q.headerPeerMiss[p.id][from.(uint64)]; ok {
+				skip = append(skip, from.(uint64))
+				continue
+			}
+		}
+		send = from.(uint64)
+	}
+	// Merge all the skipped batches back
+	for _, from := range skip {
+		q.headerTaskQueue.Push(from, -float32(from))
+	}
+	// Assemble and return the block download request
+	if send == 0 {
+		return nil
+	}
+	request := &fetchRequest{
+		Peer: p,
+		From: send,
+		Time: time.Now(),
+	}
+	q.headerPendPool[p.id] = request
+	return request
+}
+
 // ReserveBlocks reserves a set of block hashes for the given peer, skipping any
 // previously failed download.
 func (q *queue) ReserveBlocks(p *peer, count int) *fetchRequest {
@@ -635,6 +746,11 @@ func (q *queue) reserveHeaders(p *peer, count int, taskPool map[common.Hash]*typ
 	return request, progress, nil
 }
 
+// CancelHeaders aborts a fetch request, returning all pending skeleton indexes to the queue.
+func (q *queue) CancelHeaders(request *fetchRequest) {
+	q.cancel(request, q.headerTaskQueue, q.headerPendPool)
+}
+
 // CancelBlocks aborts a fetch request, returning all pending hashes to the queue.
 func (q *queue) CancelBlocks(request *fetchRequest) {
 	q.cancel(request, q.hashQueue, q.blockPendPool)
@@ -663,6 +779,9 @@ func (q *queue) cancel(request *fetchRequest, taskQueue *prque.Prque, pendPool m
 	q.lock.Lock()
 	defer q.lock.Unlock()
 
+	if request.From > 0 {
+		taskQueue.Push(request.From, -float32(request.From))
+	}
 	for hash, index := range request.Hashes {
 		taskQueue.Push(hash, float32(index))
 	}
@@ -702,6 +821,15 @@ func (q *queue) Revoke(peerId string) {
 	}
 }
 
+// ExpireHeaders checks for in flight requests that exceeded a timeout allowance,
+// canceling them and returning the responsible peers for penalisation.
+func (q *queue) ExpireHeaders(timeout time.Duration) map[string]int {
+	q.lock.Lock()
+	defer q.lock.Unlock()
+
+	return q.expire(timeout, q.headerPendPool, q.headerTaskQueue, headerTimeoutMeter)
+}
+
 // ExpireBlocks checks for in flight requests that exceeded a timeout allowance,
 // canceling them and returning the responsible peers for penalisation.
 func (q *queue) ExpireBlocks(timeout time.Duration) map[string]int {
@@ -753,6 +881,9 @@ func (q *queue) expire(timeout time.Duration, pendPool map[string]*fetchRequest,
 			timeoutMeter.Mark(1)
 
 			// Return any non satisfied requests to the pool
+			if request.From > 0 {
+				taskQueue.Push(request.From, -float32(request.From))
+			}
 			for hash, index := range request.Hashes {
 				taskQueue.Push(hash, float32(index))
 			}
@@ -842,6 +973,73 @@ func (q *queue) DeliverBlocks(id string, blocks []*types.Block) (int, error) {
 	}
 }
 
+// DeliverHeaders injects a header retrieval response into the header results
+// cache. This method either accepts all headers it received, or none of them
+// if they do not map correctly to the skeleton.
+func (q *queue) DeliverHeaders(id string, headers []*types.Header) (int, error) {
+	q.lock.Lock()
+	defer q.lock.Unlock()
+
+	// Short circuit if the data was never requested
+	request := q.headerPendPool[id]
+	if request == nil {
+		return 0, errNoFetchesPending
+	}
+	headerReqTimer.UpdateSince(request.Time)
+	delete(q.headerPendPool, id)
+
+	// Ensure headers can be mapped onto the skeleton chain
+	target := q.headerTaskPool[request.From].Hash()
+
+	accepted := len(headers) == MaxHeaderFetch
+	if accepted {
+		if headers[0].Number.Uint64() != request.From {
+			glog.V(logger.Detail).Infof("Peer %s: first header #%v [%x] broke chain ordering, expected %d", id, headers[0].Number, headers[0].Hash().Bytes()[:4], request.From)
+			accepted = false
+		} else if headers[len(headers)-1].Hash() != target {
+			glog.V(logger.Detail).Infof("Peer %s: last header #%v [%x] broke skeleton structure, expected %x", id, headers[len(headers)-1].Number, headers[len(headers)-1].Hash().Bytes()[:4], target[:4])
+			accepted = false
+		}
+	}
+	if accepted {
+		for i, header := range headers[1:] {
+			hash := header.Hash()
+			if want := request.From + 1 + uint64(i); header.Number.Uint64() != want {
+				glog.V(logger.Warn).Infof("Peer %s: header #%v [%x] broke chain ordering, expected %d", id, header.Number, hash[:4], want)
+				accepted = false
+				break
+			}
+			if headers[i].Hash() != header.ParentHash {
+				glog.V(logger.Warn).Infof("Peer %s: header #%v [%x] broke chain ancestry", id, header.Number, hash[:4])
+				accepted = false
+				break
+			}
+		}
+	}
+	// If the batch of headers wasn't accepted, mark as unavailable
+	if !accepted {
+		glog.V(logger.Detail).Infof("Peer %s: skeleton filling from header #%d not accepted", id, request.From)
+
+		miss := q.headerPeerMiss[id]
+		if miss == nil {
+			q.headerPeerMiss[id] = make(map[uint64]struct{})
+			miss = q.headerPeerMiss[id]
+		}
+		miss[request.From] = struct{}{}
+
+		q.headerTaskQueue.Push(request.From, -float32(request.From))
+		return 0, errors.New("delivery not accepted")
+	}
+	// Clean up a successful fetch, check for termination and return
+	copy(q.headerResults[request.From-q.headerOffset:], headers)
+	delete(q.headerTaskPool, request.From)
+
+	if len(q.headerTaskPool) == 0 {
+		q.headerContCh <- false
+	}
+	return len(headers), nil
+}
+
 // DeliverBodies injects a block body retrieval response into the results queue.
 // The method returns the number of blocks bodies accepted from the delivery and
 // also wakes any threads waiting for data delivery.