Filter duplicate getblocks requests.

This commit adds detection and filtering for back-to-back duplicate
getblocks requests.  This is needed because the trigger for requesting
more blocks is receiving an orphan.  When the peer is further behind than
the number of blocks advertised via a single inventory message, the same
orphan block will be sent multiple times.  When the peer receives the
final inventory message, it too contains the orphan that was previously
sent.  This leads to a duplicate getblocks request that must be filtered
to prevent requesting the final series of blocks again.

peer.go

@@ -91,31 +91,34 @@ func newNetAddress(addr net.Addr, services btcwire.ServiceFlag) (*btcwire.NetAdd
 
 // peer provides a bitcoin peer for handling bitcoin communications.
 type peer struct {
-	server          *server
-	protocolVersion uint32
-	btcnet          btcwire.BitcoinNet
-	services        btcwire.ServiceFlag
-	started         bool
-	conn            net.Conn
-	addr            string
-	na              *btcwire.NetAddress
-	timeConnected   time.Time
-	inbound         bool
-	disconnect      bool
-	persistent      bool
-	versionKnown    bool
-	knownAddresses  map[string]bool
-	knownInventory  *MruInventoryMap
-	knownInvMutex   sync.Mutex
-	lastBlock       int32
-	requestQueue    *list.List
-	invSendQueue    *list.List
-	continueHash    *btcwire.ShaHash
-	wg              sync.WaitGroup
-	outputQueue     chan btcwire.Message
-	outputInvChan   chan *btcwire.InvVect
-	blockProcessed  chan bool
-	quit            chan bool
+	server             *server
+	protocolVersion    uint32
+	btcnet             btcwire.BitcoinNet
+	services           btcwire.ServiceFlag
+	started            bool
+	conn               net.Conn
+	addr               string
+	na                 *btcwire.NetAddress
+	timeConnected      time.Time
+	inbound            bool
+	disconnect         bool
+	persistent         bool
+	versionKnown       bool
+	knownAddresses     map[string]bool
+	knownInventory     *MruInventoryMap
+	knownInvMutex      sync.Mutex
+	lastBlock          int32
+	prevGetBlocksBegin *btcwire.ShaHash
+	prevGetBlocksStop  *btcwire.ShaHash
+	prevGetBlockMutex  sync.Mutex
+	requestQueue       *list.List
+	invSendQueue       *list.List
+	continueHash       *btcwire.ShaHash
+	wg                 sync.WaitGroup
+	outputQueue        chan btcwire.Message
+	outputInvChan      chan *btcwire.InvVect
+	blockProcessed     chan bool
+	quit               chan bool
 }
 
 // isKnownInventory returns whether or not the peer is known to have the passed
@@ -340,8 +343,30 @@ func (p *peer) pushBlockMsg(sha btcwire.ShaHash) error {
 }
 
 // pushGetBlocksMsg sends a getblocks message for the provided block locator
-// and stop hash.
+// and stop hash.  It will ignore back-to-back duplicate requests.
 func (p *peer) pushGetBlocksMsg(locator btcchain.BlockLocator, stopHash *btcwire.ShaHash) error {
+	p.prevGetBlockMutex.Lock()
+	defer p.prevGetBlockMutex.Unlock()
+
+	// Extract the begin hash from the block locator, if one was specified,
+	// to use for filtering duplicate getblocks requests.
+	// request.
+	var beginHash *btcwire.ShaHash
+	if len(locator) > 0 {
+		beginHash = locator[0]
+	}
+
+	// Filter duplicate getblocks requests.
+	if p.prevGetBlocksStop != nil && p.prevGetBlocksBegin != nil &&
+		beginHash != nil && stopHash.IsEqual(p.prevGetBlocksStop) &&
+		beginHash.IsEqual(p.prevGetBlocksBegin) {
+
+		log.Tracef("[PEER] Filtering duplicate [getblocks] with begin "+
+			"hash %v, stop hash %v", beginHash, stopHash)
+		return nil
+	}
+
+	// Construct the getblocks request and queue it to be sent.
 	msg := btcwire.NewMsgGetBlocks(stopHash)
 	for _, hash := range locator {
 		err := msg.AddBlockLocatorHash(hash)
@@ -350,6 +375,11 @@ func (p *peer) pushGetBlocksMsg(locator btcchain.BlockLocator, stopHash *btcwire
 		}
 	}
 	p.QueueMessage(msg)
+
+	// Update the previous getblocks request information for filtering
+	// duplicates.
+	p.prevGetBlocksBegin = beginHash
+	p.prevGetBlocksStop = stopHash
 	return nil
 }