diff --git a/.travis.yml b/.travis.yml
index ff84c75c..8d52cda4 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,10 +12,11 @@ go_import_path: github.com/vulcanize/vulcanizedb
 before_install:
   # ginkgo golint dep migrate
   - make installtools
+  - bash ./scripts/install-postgres-10.sh
 
 before_script:
   - sudo -u postgres createdb vulcanize_private
-  - make migrate HOST_NAME=localhost NAME=vulcanize_private PORT=5432
+  - make migrate NAME=vulcanize_private
 
 script:
   - make test
diff --git a/Gopkg.lock b/Gopkg.lock
index 121d7124..bde55c82 100644
--- a/Gopkg.lock
+++ b/Gopkg.lock
@@ -274,6 +274,12 @@
   ]
   revision = "faacc1b5e36e3ff02cbec9661c69ac63dd5a83ad"
 
+[[projects]]
+  branch = "master"
+  name = "golang.org/x/sync"
+  packages = ["errgroup"]
+  revision = "1d60e4601c6fd243af51cc01ddf169918a5407ca"
+
 [[projects]]
   branch = "master"
   name = "golang.org/x/sys"
@@ -334,6 +340,6 @@
 [solve-meta]
   analyzer-name = "dep"
   analyzer-version = 1
-  inputs-digest = "b35f7b80a7695fb1a7d247be178212e16a0647363affc0071c5a2280817dd906"
+  inputs-digest = "4dcddfb7fa2db8ba7e3d8e70ec8ba23a7e9b6a1d99c62933ed63624412967aeb"
   solver-name = "gps-cdcl"
   solver-version = 1
diff --git a/Makefile b/Makefile
index 4de14e51..886dbe7b 100644
--- a/Makefile
+++ b/Makefile
@@ -18,7 +18,7 @@ $(BIN)/migrate:
 
 LINT = $(BIN)/golint
 $(BIN)/golint:
-	go get github.com/golang/lint/golint
+	go get -u golang.org/x/lint/golint
 
 METALINT = $(BIN)/gometalinter.v2
 $(BIN)/gometalinter.v2:
diff --git a/integration_test/block_rewards_test.go b/integration_test/block_rewards_test.go
index 02ca5e72..0fd7b65b 100644
--- a/integration_test/block_rewards_test.go
+++ b/integration_test/block_rewards_test.go
@@ -11,13 +11,15 @@ var _ = Describe("Rewards calculations", func() {
 
 	It("calculates a block reward for a real block", func() {
 		blockchain := geth.NewBlockchain(test_config.InfuraClient.IPCPath)
-		block := blockchain.GetBlockByNumber(1071819)
+		block, err := blockchain.GetBlockByNumber(1071819)
+		Expect(err).ToNot(HaveOccurred())
 		Expect(block.Reward).To(Equal(5.31355))
 	})
 
 	It("calculates an uncle reward for a real block", func() {
 		blockchain := geth.NewBlockchain(test_config.InfuraClient.IPCPath)
-		block := blockchain.GetBlockByNumber(1071819)
+		block, err := blockchain.GetBlockByNumber(1071819)
+		Expect(err).ToNot(HaveOccurred())
 		Expect(block.UnclesReward).To(Equal(6.875))
 	})
 
diff --git a/integration_test/geth_blockchain_test.go b/integration_test/geth_blockchain_test.go
index 8eab9382..964f463b 100644
--- a/integration_test/geth_blockchain_test.go
+++ b/integration_test/geth_blockchain_test.go
@@ -3,6 +3,7 @@ package integration_test
 import (
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/gomega"
+	"github.com/vulcanize/vulcanizedb/pkg/core"
 	"github.com/vulcanize/vulcanizedb/pkg/datastore/inmemory"
 	"github.com/vulcanize/vulcanizedb/pkg/geth"
 	"github.com/vulcanize/vulcanizedb/pkg/history"
@@ -28,8 +29,10 @@ var _ = Describe("Reading from the Geth blockchain", func() {
 	}, 30)
 
 	It("retrieves the genesis block and first block", func(done Done) {
-		genesisBlock := blockchain.GetBlockByNumber(int64(0))
-		firstBlock := blockchain.GetBlockByNumber(int64(1))
+		genesisBlock, err := blockchain.GetBlockByNumber(int64(0))
+		Expect(err).ToNot(HaveOccurred())
+		firstBlock, err := blockchain.GetBlockByNumber(int64(1))
+		Expect(err).ToNot(HaveOccurred())
 		lastBlockNumber := blockchain.LastBlock()
 
 		Expect(genesisBlock.Number).To(Equal(int64(0)))
@@ -40,14 +43,27 @@ var _ = Describe("Reading from the Geth blockchain", func() {
 
 	It("retrieves the node info", func(done Done) {
 		node := blockchain.Node()
-		devNetworkNodeId := float64(1)
+		mainnetID := float64(1)
 
 		Expect(node.GenesisBlock).ToNot(BeNil())
-		Expect(node.NetworkID).To(Equal(devNetworkNodeId))
+		Expect(node.NetworkID).To(Equal(mainnetID))
 		Expect(len(node.ID)).ToNot(BeZero())
 		Expect(node.ClientName).ToNot(BeZero())
 
 		close(done)
 	}, 15)
 
+	//Benchmarking test: remove skip to test performance of block retrieval
+	XMeasure("retrieving n blocks", func(b Benchmarker) {
+		b.Time("runtime", func() {
+			var blocks []core.Block
+			n := 10
+			for i := 5327459; i > 5327459-n; i-- {
+				block, err := blockchain.GetBlockByNumber(int64(i))
+				Expect(err).ToNot(HaveOccurred())
+				blocks = append(blocks, block)
+			}
+			Expect(len(blocks)).To(Equal(n))
+		})
+	}, 10)
 })
diff --git a/pkg/core/blockchain.go b/pkg/core/blockchain.go
index b68aee2e..e67af221 100644
--- a/pkg/core/blockchain.go
+++ b/pkg/core/blockchain.go
@@ -4,7 +4,7 @@ import "math/big"
 
 type Blockchain interface {
 	ContractDataFetcher
-	GetBlockByNumber(blockNumber int64) Block
+	GetBlockByNumber(blockNumber int64) (Block, error)
 	GetLogs(contract Contract, startingBlockNumber *big.Int, endingBlockNumber *big.Int) ([]Log, error)
 	LastBlock() *big.Int
 	Node() Node
diff --git a/pkg/datastore/postgres/postgres_suite_test.go b/pkg/datastore/postgres/postgres_suite_test.go
index 642065b6..d2b9876e 100644
--- a/pkg/datastore/postgres/postgres_suite_test.go
+++ b/pkg/datastore/postgres/postgres_suite_test.go
@@ -3,10 +3,17 @@ package postgres_test
 import (
 	"testing"
 
+	"io/ioutil"
+	"log"
+
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/gomega"
 )
 
+func init() {
+	log.SetOutput(ioutil.Discard)
+}
+
 func TestPostgres(t *testing.T) {
 	RegisterFailHandler(Fail)
 	RunSpecs(t, "Postgres Suite")
diff --git a/pkg/datastore/postgres/postgres_test.go b/pkg/datastore/postgres/postgres_test.go
index 5c512ca1..523b8f7f 100644
--- a/pkg/datastore/postgres/postgres_test.go
+++ b/pkg/datastore/postgres/postgres_test.go
@@ -4,9 +4,6 @@ import (
 	"fmt"
 	"strings"
 
-	"io/ioutil"
-	"log"
-
 	"math/big"
 
 	"github.com/jmoiron/sqlx"
@@ -20,10 +17,6 @@ import (
 	"github.com/vulcanize/vulcanizedb/test_config"
 )
 
-func init() {
-	log.SetOutput(ioutil.Discard)
-}
-
 var _ = Describe("Postgres DB", func() {
 	var sqlxdb *sqlx.DB
 
diff --git a/pkg/fakes/blockchain.go b/pkg/fakes/blockchain.go
index 3c3c3901..ffc8d1ab 100644
--- a/pkg/fakes/blockchain.go
+++ b/pkg/fakes/blockchain.go
@@ -14,6 +14,7 @@ type Blockchain struct {
 	WasToldToStop       bool
 	node                core.Node
 	ContractReturnValue []byte
+	err                 error
 }
 
 func (blockchain *Blockchain) FetchContractData(abiJSON string, address string, method string, methodArg interface{}, result interface{}, blockNumber int64) error {
@@ -42,12 +43,13 @@ func (blockchain *Blockchain) Node() core.Node {
 	return blockchain.node
 }
 
-func NewBlockchain() *Blockchain {
+func NewBlockchain(err error) *Blockchain {
 	return &Blockchain{
 		blocks:             make(map[int64]core.Block),
 		logs:               make(map[string][]core.Log),
 		contractAttributes: make(map[string]map[string]string),
 		node:               core.Node{GenesisBlock: "GENESIS", NetworkID: 1, ID: "x123", ClientName: "Geth"},
+		err:                err,
 	}
 }
 
@@ -61,8 +63,11 @@ func NewBlockchainWithBlocks(blocks []core.Block) *Blockchain {
 	}
 }
 
-func (blockchain *Blockchain) GetBlockByNumber(blockNumber int64) core.Block {
-	return blockchain.blocks[blockNumber]
+func (blockchain *Blockchain) GetBlockByNumber(blockNumber int64) (core.Block, error) {
+	if blockchain.err != nil {
+		return core.Block{}, blockchain.err
+	}
+	return blockchain.blocks[blockNumber], nil
 }
 
 func (blockchain *Blockchain) AddBlock(block core.Block) {
diff --git a/pkg/geth/block_to_core_block.go b/pkg/geth/block_to_core_block.go
index 905d0656..17a5d355 100644
--- a/pkg/geth/block_to_core_block.go
+++ b/pkg/geth/block_to_core_block.go
@@ -10,6 +10,7 @@ import (
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/vulcanize/vulcanizedb/pkg/core"
 	"golang.org/x/net/context"
+	"golang.org/x/sync/errgroup"
 )
 
 type Client interface {
@@ -17,8 +18,11 @@ type Client interface {
 	TransactionReceipt(ctx context.Context, txHash common.Hash) (*types.Receipt, error)
 }
 
-func ToCoreBlock(gethBlock *types.Block, client Client) core.Block {
-	transactions := convertTransactionsToCore(gethBlock, client)
+func ToCoreBlock(gethBlock *types.Block, client Client) (core.Block, error) {
+	transactions, err := convertTransactionsToCore(gethBlock, client)
+	if err != nil {
+		return core.Block{}, err
+	}
 	coreBlock := core.Block{
 		Difficulty:   gethBlock.Difficulty().Int64(),
 		ExtraData:    hexutil.Encode(gethBlock.Extra()),
@@ -36,35 +40,48 @@ func ToCoreBlock(gethBlock *types.Block, client Client) core.Block {
 	}
 	coreBlock.Reward = CalcBlockReward(coreBlock, gethBlock.Uncles())
 	coreBlock.UnclesReward = CalcUnclesReward(coreBlock, gethBlock.Uncles())
-	return coreBlock
+	return coreBlock, nil
 }
 
-func convertTransactionsToCore(gethBlock *types.Block, client Client) []core.Transaction {
-	transactions := make([]core.Transaction, 0)
-	for i, gethTransaction := range gethBlock.Transactions() {
-		from, err := client.TransactionSender(context.Background(), gethTransaction, gethBlock.Hash(), uint(i))
-		if err != nil {
-			log.Println(err)
-		}
-		transaction := transToCoreTrans(gethTransaction, &from)
-		transaction, err = appendReceiptToTransaction(client, transaction)
-		if err != nil {
-			log.Println(err)
-		}
-		transactions = append(transactions, transaction)
+func convertTransactionsToCore(gethBlock *types.Block, client Client) ([]core.Transaction, error) {
+	var g errgroup.Group
+	coreTransactions := make([]core.Transaction, len(gethBlock.Transactions()))
+
+	for gethTransactionIndex, gethTransaction := range gethBlock.Transactions() {
+		//https://golang.org/doc/faq#closures_and_goroutines
+		transaction := gethTransaction
+		transactionIndex := uint(gethTransactionIndex)
+		g.Go(func() error {
+			from, err := client.TransactionSender(context.Background(), transaction, gethBlock.Hash(), transactionIndex)
+			if err != nil {
+				log.Println("transaction sender: ", err)
+				return err
+			}
+			coreTransaction := transToCoreTrans(transaction, &from)
+			coreTransaction, err = appendReceiptToTransaction(client, coreTransaction)
+			if err != nil {
+				log.Println("receipt: ", err)
+				return err
+			}
+			coreTransactions[transactionIndex] = coreTransaction
+			return nil
+		})
 	}
-	return transactions
+	if err := g.Wait(); err != nil {
+		log.Println("transactions: ", err)
+		return coreTransactions, err
+	}
+	return coreTransactions, nil
 }
 
 func appendReceiptToTransaction(client Client, transaction core.Transaction) (core.Transaction, error) {
 	gethReceipt, err := client.TransactionReceipt(context.Background(), common.HexToHash(transaction.Hash))
 	if err != nil {
-		log.Println(err)
 		return transaction, err
 	}
 	receipt := ReceiptToCoreReceipt(gethReceipt)
 	transaction.Receipt = receipt
-	return transaction, err
+	return transaction, nil
 }
 
 func transToCoreTrans(transaction *types.Transaction, from *common.Address) core.Transaction {
diff --git a/pkg/geth/block_to_core_block_test.go b/pkg/geth/block_to_core_block_test.go
index 0b661084..733aeabd 100644
--- a/pkg/geth/block_to_core_block_test.go
+++ b/pkg/geth/block_to_core_block_test.go
@@ -5,6 +5,13 @@ import (
 
 	"context"
 
+	"fmt"
+
+	"io/ioutil"
+	"log"
+
+	"os"
+
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/common/hexutil"
 	"github.com/ethereum/go-ethereum/core/types"
@@ -15,11 +22,25 @@ import (
 
 type FakeGethClient struct {
 	receipts map[string]*types.Receipt
+	err      error
 }
 
-func NewFakeClient() *FakeGethClient {
+type TransActionReceiptError struct{}
+
+func (tarErr TransActionReceiptError) Error() string {
+	return fmt.Sprintf("transaction receipt error")
+}
+
+type TransactionSenderError struct{}
+
+func (tasErr TransactionSenderError) Error() string {
+	return fmt.Sprintf("transaction sender error")
+}
+
+func NewFakeClient(err error) *FakeGethClient {
 	return &FakeGethClient{
 		receipts: make(map[string]*types.Receipt),
+		err:      err,
 	}
 }
 
@@ -30,6 +51,9 @@ func (client *FakeGethClient) AddReceipts(receipts []*types.Receipt) {
 }
 
 func (client *FakeGethClient) TransactionReceipt(ctx context.Context, txHash common.Hash) (*types.Receipt, error) {
+	if err, ok := client.err.(TransActionReceiptError); ok {
+		return &types.Receipt{}, err
+	}
 	if gasUsed, ok := client.receipts[txHash.Hex()]; ok {
 		return gasUsed, nil
 	}
@@ -37,6 +61,9 @@ func (client *FakeGethClient) TransactionReceipt(ctx context.Context, txHash com
 }
 
 func (client *FakeGethClient) TransactionSender(ctx context.Context, tx *types.Transaction, block common.Hash, index uint) (common.Address, error) {
+	if err, ok := client.err.(TransactionSenderError); ok {
+		return common.Address{}, err
+	}
 	return common.HexToAddress("0x123"), nil
 }
 
@@ -66,8 +93,9 @@ var _ = Describe("Conversion of GethBlock to core.Block", func() {
 		}
 		block := types.NewBlock(&header, []*types.Transaction{}, []*types.Header{}, []*types.Receipt{})
 		client := &FakeGethClient{}
-		gethBlock := geth.ToCoreBlock(block, client)
+		gethBlock, err := geth.ToCoreBlock(block, client)
 
+		Expect(err).ToNot(HaveOccurred())
 		Expect(gethBlock.Difficulty).To(Equal(difficulty.Int64()))
 		Expect(gethBlock.GasLimit).To(Equal(gasLimit))
 		Expect(gethBlock.Miner).To(Equal(miner.Hex()))
@@ -104,7 +132,7 @@ var _ = Describe("Conversion of GethBlock to core.Block", func() {
 			}
 			receipts := []*types.Receipt{&receipt}
 
-			client := NewFakeClient()
+			client := NewFakeClient(nil)
 			client.AddReceipts(receipts)
 
 			number := int64(1071819)
@@ -113,8 +141,9 @@ var _ = Describe("Conversion of GethBlock to core.Block", func() {
 			}
 			uncles := []*types.Header{{Number: big.NewInt(1071817)}, {Number: big.NewInt(1071818)}}
 			block := types.NewBlock(&header, transactions, uncles, []*types.Receipt{&receipt})
-			coreBlock := geth.ToCoreBlock(block, client)
+			coreBlock, err := geth.ToCoreBlock(block, client)
 
+			Expect(err).ToNot(HaveOccurred())
 			Expect(geth.CalcBlockReward(coreBlock, block.Uncles())).To(Equal(5.31355))
 		})
 
@@ -144,11 +173,12 @@ var _ = Describe("Conversion of GethBlock to core.Block", func() {
 			}
 			block := types.NewBlock(&header, transactions, uncles, receipts)
 
-			client := NewFakeClient()
+			client := NewFakeClient(nil)
 			client.AddReceipts(receipts)
 
-			coreBlock := geth.ToCoreBlock(block, client)
+			coreBlock, err := geth.ToCoreBlock(block, client)
 
+			Expect(err).ToNot(HaveOccurred())
 			Expect(geth.CalcUnclesReward(coreBlock, block.Uncles())).To(Equal(6.875))
 		})
 
@@ -190,10 +220,11 @@ var _ = Describe("Conversion of GethBlock to core.Block", func() {
 			var uncles []*types.Header
 			block := types.NewBlock(&header, transactions, uncles, receipts)
 
-			client := NewFakeClient()
+			client := NewFakeClient(nil)
 			client.AddReceipts(receipts)
-			coreBlock := geth.ToCoreBlock(block, client)
+			coreBlock, err := geth.ToCoreBlock(block, client)
 
+			Expect(err).ToNot(HaveOccurred())
 			Expect(geth.CalcBlockReward(coreBlock, block.Uncles())).To(Equal(3.024990672))
 		})
 	})
@@ -203,8 +234,9 @@ var _ = Describe("Conversion of GethBlock to core.Block", func() {
 			header := types.Header{}
 			block := types.NewBlock(&header, []*types.Transaction{}, []*types.Header{}, []*types.Receipt{})
 			client := &FakeGethClient{}
-			coreBlock := geth.ToCoreBlock(block, client)
+			coreBlock, err := geth.ToCoreBlock(block, client)
 
+			Expect(err).ToNot(HaveOccurred())
 			Expect(len(coreBlock.Transactions)).To(Equal(0))
 		})
 
@@ -227,7 +259,7 @@ var _ = Describe("Conversion of GethBlock to core.Block", func() {
 				TxHash:            gethTransaction.Hash(),
 			}
 
-			client := NewFakeClient()
+			client := NewFakeClient(nil)
 			client.AddReceipts([]*types.Receipt{gethReceipt})
 
 			header := types.Header{}
@@ -237,8 +269,9 @@ var _ = Describe("Conversion of GethBlock to core.Block", func() {
 				[]*types.Header{},
 				[]*types.Receipt{gethReceipt},
 			)
-			coreBlock := geth.ToCoreBlock(gethBlock, client)
+			coreBlock, err := geth.ToCoreBlock(gethBlock, client)
 
+			Expect(err).ToNot(HaveOccurred())
 			Expect(len(coreBlock.Transactions)).To(Equal(1))
 			coreTransaction := coreBlock.Transactions[0]
 			Expect(coreTransaction.Data).To(Equal("0xf7d8c8830000000000000000000000000000000000000000000000000000000000037788000000000000000000000000000000000000000000000000000000000003bd14"))
@@ -271,7 +304,7 @@ var _ = Describe("Conversion of GethBlock to core.Block", func() {
 				ContractAddress:   common.HexToAddress("0x1023342345"),
 			}
 
-			client := NewFakeClient()
+			client := NewFakeClient(nil)
 			client.AddReceipts([]*types.Receipt{gethReceipt})
 
 			gethBlock := types.NewBlock(
@@ -281,8 +314,9 @@ var _ = Describe("Conversion of GethBlock to core.Block", func() {
 				[]*types.Receipt{gethReceipt},
 			)
 
-			coreBlock := geth.ToCoreBlock(gethBlock, client)
+			coreBlock, err := geth.ToCoreBlock(gethBlock, client)
 
+			Expect(err).ToNot(HaveOccurred())
 			coreTransaction := coreBlock.Transactions[0]
 			Expect(coreTransaction.To).To(Equal(""))
 
@@ -292,4 +326,50 @@ var _ = Describe("Conversion of GethBlock to core.Block", func() {
 		})
 	})
 
+	Describe("transaction error handling", func() {
+		var gethTransaction *types.Transaction
+		var gethReceipt *types.Receipt
+		var header *types.Header
+		var gethBlock *types.Block
+
+		BeforeEach(func() {
+			log.SetOutput(ioutil.Discard)
+			gethTransaction = types.NewTransaction(
+				uint64(0),
+				common.Address{},
+				big.NewInt(0),
+				uint64(0),
+				big.NewInt(0),
+				[]byte{},
+			)
+			gethReceipt = &types.Receipt{}
+			header = &types.Header{}
+			gethBlock = types.NewBlock(
+				header,
+				[]*types.Transaction{gethTransaction},
+				[]*types.Header{},
+				[]*types.Receipt{gethReceipt},
+			)
+
+		})
+
+		AfterEach(func() {
+			defer log.SetOutput(os.Stdout)
+		})
+
+		It("returns an error when transaction sender call fails", func() {
+			client := NewFakeClient(TransactionSenderError{})
+			client.AddReceipts([]*types.Receipt{})
+			_, err := geth.ToCoreBlock(gethBlock, client)
+			Expect(err).To(Equal(TransactionSenderError{}))
+		})
+
+		It("returns an error when transaction receipt call fails", func() {
+			client := NewFakeClient(TransActionReceiptError{})
+			client.AddReceipts([]*types.Receipt{})
+			_, err := geth.ToCoreBlock(gethBlock, client)
+			Expect(err).To(Equal(TransActionReceiptError{}))
+		})
+	})
+
 })
diff --git a/pkg/geth/blockchain.go b/pkg/geth/blockchain.go
index ab07a41c..a6a12695 100644
--- a/pkg/geth/blockchain.go
+++ b/pkg/geth/blockchain.go
@@ -58,9 +58,16 @@ func (blockchain *Blockchain) Node() core.Node {
 	return blockchain.node
 }
 
-func (blockchain *Blockchain) GetBlockByNumber(blockNumber int64) core.Block {
-	gethBlock, _ := blockchain.client.BlockByNumber(context.Background(), big.NewInt(blockNumber))
-	return ToCoreBlock(gethBlock, blockchain.client)
+func (blockchain *Blockchain) GetBlockByNumber(blockNumber int64) (core.Block, error) {
+	gethBlock, err := blockchain.client.BlockByNumber(context.Background(), big.NewInt(blockNumber))
+	if err != nil {
+		return core.Block{}, err
+	}
+	block, err := ToCoreBlock(gethBlock, blockchain.client)
+	if err != nil {
+		return core.Block{}, err
+	}
+	return block, nil
 }
 
 func (blockchain *Blockchain) LastBlock() *big.Int {
diff --git a/pkg/geth/node/node.go b/pkg/geth/node/node.go
index f7fc3ba1..daa20e76 100644
--- a/pkg/geth/node/node.go
+++ b/pkg/geth/node/node.go
@@ -9,6 +9,8 @@ import (
 
 	"strings"
 
+	"log"
+
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/p2p"
 	"github.com/vulcanize/vulcanizedb/pkg/core"
@@ -79,7 +81,10 @@ func MakeNode(wrapper ClientWrapper) core.Node {
 
 func (client ClientWrapper) NetworkId() float64 {
 	var version string
-	client.CallContext(context.Background(), &version, "net_version")
+	err := client.CallContext(context.Background(), &version, "net_version")
+	if err != nil {
+		log.Println(err)
+	}
 	networkId, _ := strconv.ParseFloat(version, 64)
 	return networkId
 }
diff --git a/pkg/history/history_suite_test.go b/pkg/history/history_suite_test.go
index 98ce5532..6bf3894c 100644
--- a/pkg/history/history_suite_test.go
+++ b/pkg/history/history_suite_test.go
@@ -4,9 +4,15 @@ import (
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/gomega"
 
+	"io/ioutil"
+	"log"
 	"testing"
 )
 
+func init() {
+	log.SetOutput(ioutil.Discard)
+}
+
 func TestHistory(t *testing.T) {
 	RegisterFailHandler(Fail)
 	RunSpecs(t, "History Suite")
diff --git a/pkg/history/populate_blocks.go b/pkg/history/populate_blocks.go
index 19d63e72..09d81a8c 100644
--- a/pkg/history/populate_blocks.go
+++ b/pkg/history/populate_blocks.go
@@ -18,7 +18,11 @@ func PopulateMissingBlocks(blockchain core.Blockchain, blockRepository datastore
 
 func RetrieveAndUpdateBlocks(blockchain core.Blockchain, blockRepository datastore.BlockRepository, blockNumbers []int64) int {
 	for _, blockNumber := range blockNumbers {
-		block := blockchain.GetBlockByNumber(blockNumber)
+		block, err := blockchain.GetBlockByNumber(blockNumber)
+		if err != nil {
+			log.Printf("failed to retrieve block number: %d\n", blockNumber)
+			return 0
+		}
 		blockRepository.CreateOrUpdateBlock(block)
 	}
 	return len(blockNumbers)
diff --git a/pkg/history/populate_blocks_test.go b/pkg/history/populate_blocks_test.go
index 14e6d61f..a98c76aa 100644
--- a/pkg/history/populate_blocks_test.go
+++ b/pkg/history/populate_blocks_test.go
@@ -1,6 +1,8 @@
 package history_test
 
 import (
+	"errors"
+
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/gomega"
 	"github.com/vulcanize/vulcanizedb/pkg/core"
@@ -100,4 +102,12 @@ var _ = Describe("Populating blocks", func() {
 		Expect(blockRepository.CreateOrUpdateBlockCallCount).To(Equal(3))
 	})
 
+	It("does not call repository create block when there is an error", func() {
+		blockchain := fakes.NewBlockchain(errors.New("error getting block"))
+		blocks := history.MakeRange(1, 10)
+		history.RetrieveAndUpdateBlocks(blockchain, blockRepository, blocks)
+		Expect(blockRepository.BlockCount()).To(Equal(0))
+		Expect(blockRepository.CreateOrUpdateBlockCallCount).To(Equal(0))
+	})
+
 })
diff --git a/pkg/history/validate_blocks_test.go b/pkg/history/validate_blocks_test.go
index 056e84e1..fc92521e 100644
--- a/pkg/history/validate_blocks_test.go
+++ b/pkg/history/validate_blocks_test.go
@@ -3,9 +3,6 @@ package history_test
 import (
 	"bytes"
 
-	"io/ioutil"
-	"log"
-
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/gomega"
 	"github.com/vulcanize/vulcanizedb/pkg/core"
@@ -14,10 +11,6 @@ import (
 	"github.com/vulcanize/vulcanizedb/pkg/history"
 )
 
-func init() {
-	log.SetOutput(ioutil.Discard)
-}
-
 var _ = Describe("Blocks validator", func() {
 
 	It("creates a ValidationWindow equal to (HEAD-windowSize, HEAD)", func() {
diff --git a/scripts/install-postgres-10.sh b/scripts/install-postgres-10.sh
new file mode 100755
index 00000000..9d7e124c
--- /dev/null
+++ b/scripts/install-postgres-10.sh
@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# Travis doesn't support postgres 10
+# https://github.com/travis-ci/travis-ci/issues/8537
+
+set -ex
+
+echo "Installing Postgres 10"
+sudo service postgresql stop
+sudo apt-get remove -q 'postgresql-*'
+sudo apt-get update -q
+sudo apt-get install -q postgresql-10 postgresql-client-10
+sudo cp /etc/postgresql/{9.6,10}/main/pg_hba.conf
+
+echo "Restarting Postgres 10"
+sudo service postgresql restart
+
+sudo psql -c 'CREATE ROLE travis SUPERUSER LOGIN CREATEDB;' -U postgres
\ No newline at end of file
diff --git a/vendor/golang.org/x/sync/AUTHORS b/vendor/golang.org/x/sync/AUTHORS
new file mode 100644
index 00000000..15167cd7
--- /dev/null
+++ b/vendor/golang.org/x/sync/AUTHORS
@@ -0,0 +1,3 @@
+# This source code refers to The Go Authors for copyright purposes.
+# The master list of authors is in the main Go distribution,
+# visible at http://tip.golang.org/AUTHORS.
diff --git a/vendor/golang.org/x/sync/CONTRIBUTING.md b/vendor/golang.org/x/sync/CONTRIBUTING.md
new file mode 100644
index 00000000..d0485e88
--- /dev/null
+++ b/vendor/golang.org/x/sync/CONTRIBUTING.md
@@ -0,0 +1,26 @@
+# Contributing to Go
+
+Go is an open source project.
+
+It is the work of hundreds of contributors. We appreciate your help!
+
+## Filing issues
+
+When [filing an issue](https://golang.org/issue/new), make sure to answer these five questions:
+
+1.  What version of Go are you using (`go version`)?
+2.  What operating system and processor architecture are you using?
+3.  What did you do?
+4.  What did you expect to see?
+5.  What did you see instead?
+
+General questions should go to the [golang-nuts mailing list](https://groups.google.com/group/golang-nuts) instead of the issue tracker.
+The gophers there will answer or ask you to file an issue if you've tripped over a bug.
+
+## Contributing code
+
+Please read the [Contribution Guidelines](https://golang.org/doc/contribute.html)
+before sending patches.
+
+Unless otherwise noted, the Go source files are distributed under
+the BSD-style license found in the LICENSE file.
diff --git a/vendor/golang.org/x/sync/CONTRIBUTORS b/vendor/golang.org/x/sync/CONTRIBUTORS
new file mode 100644
index 00000000..1c4577e9
--- /dev/null
+++ b/vendor/golang.org/x/sync/CONTRIBUTORS
@@ -0,0 +1,3 @@
+# This source code was written by the Go contributors.
+# The master list of contributors is in the main Go distribution,
+# visible at http://tip.golang.org/CONTRIBUTORS.
diff --git a/vendor/golang.org/x/sync/LICENSE b/vendor/golang.org/x/sync/LICENSE
new file mode 100644
index 00000000..6a66aea5
--- /dev/null
+++ b/vendor/golang.org/x/sync/LICENSE
@@ -0,0 +1,27 @@
+Copyright (c) 2009 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/vendor/golang.org/x/sync/PATENTS b/vendor/golang.org/x/sync/PATENTS
new file mode 100644
index 00000000..73309904
--- /dev/null
+++ b/vendor/golang.org/x/sync/PATENTS
@@ -0,0 +1,22 @@
+Additional IP Rights Grant (Patents)
+
+"This implementation" means the copyrightable works distributed by
+Google as part of the Go project.
+
+Google hereby grants to You a perpetual, worldwide, non-exclusive,
+no-charge, royalty-free, irrevocable (except as stated in this section)
+patent license to make, have made, use, offer to sell, sell, import,
+transfer and otherwise run, modify and propagate the contents of this
+implementation of Go, where such license applies only to those patent
+claims, both currently owned or controlled by Google and acquired in
+the future, licensable by Google that are necessarily infringed by this
+implementation of Go.  This grant does not include claims that would be
+infringed only as a consequence of further modification of this
+implementation.  If you or your agent or exclusive licensee institute or
+order or agree to the institution of patent litigation against any
+entity (including a cross-claim or counterclaim in a lawsuit) alleging
+that this implementation of Go or any code incorporated within this
+implementation of Go constitutes direct or contributory patent
+infringement, or inducement of patent infringement, then any patent
+rights granted to you under this License for this implementation of Go
+shall terminate as of the date such litigation is filed.
diff --git a/vendor/golang.org/x/sync/README.md b/vendor/golang.org/x/sync/README.md
new file mode 100644
index 00000000..1f8436cc
--- /dev/null
+++ b/vendor/golang.org/x/sync/README.md
@@ -0,0 +1,18 @@
+# Go Sync
+
+This repository provides Go concurrency primitives in addition to the
+ones provided by the language and "sync" and "sync/atomic" packages.
+
+## Download/Install
+
+The easiest way to install is to run `go get -u golang.org/x/sync`. You can
+also manually git clone the repository to `$GOPATH/src/golang.org/x/sync`.
+
+## Report Issues / Send Patches
+
+This repository uses Gerrit for code changes. To learn how to submit changes to
+this repository, see https://golang.org/doc/contribute.html.
+
+The main issue tracker for the sync repository is located at
+https://github.com/golang/go/issues. Prefix your issue with "x/sync:" in the
+subject line, so it is easy to find.
diff --git a/vendor/golang.org/x/sync/codereview.cfg b/vendor/golang.org/x/sync/codereview.cfg
new file mode 100644
index 00000000..3f8b14b6
--- /dev/null
+++ b/vendor/golang.org/x/sync/codereview.cfg
@@ -0,0 +1 @@
+issuerepo: golang/go
diff --git a/vendor/golang.org/x/sync/errgroup/errgroup.go b/vendor/golang.org/x/sync/errgroup/errgroup.go
new file mode 100644
index 00000000..533438d9
--- /dev/null
+++ b/vendor/golang.org/x/sync/errgroup/errgroup.go
@@ -0,0 +1,67 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package errgroup provides synchronization, error propagation, and Context
+// cancelation for groups of goroutines working on subtasks of a common task.
+package errgroup
+
+import (
+	"sync"
+
+	"golang.org/x/net/context"
+)
+
+// A Group is a collection of goroutines working on subtasks that are part of
+// the same overall task.
+//
+// A zero Group is valid and does not cancel on error.
+type Group struct {
+	cancel func()
+
+	wg sync.WaitGroup
+
+	errOnce sync.Once
+	err     error
+}
+
+// WithContext returns a new Group and an associated Context derived from ctx.
+//
+// The derived Context is canceled the first time a function passed to Go
+// returns a non-nil error or the first time Wait returns, whichever occurs
+// first.
+func WithContext(ctx context.Context) (*Group, context.Context) {
+	ctx, cancel := context.WithCancel(ctx)
+	return &Group{cancel: cancel}, ctx
+}
+
+// Wait blocks until all function calls from the Go method have returned, then
+// returns the first non-nil error (if any) from them.
+func (g *Group) Wait() error {
+	g.wg.Wait()
+	if g.cancel != nil {
+		g.cancel()
+	}
+	return g.err
+}
+
+// Go calls the given function in a new goroutine.
+//
+// The first call to return a non-nil error cancels the group; its error will be
+// returned by Wait.
+func (g *Group) Go(f func() error) {
+	g.wg.Add(1)
+
+	go func() {
+		defer g.wg.Done()
+
+		if err := f(); err != nil {
+			g.errOnce.Do(func() {
+				g.err = err
+				if g.cancel != nil {
+					g.cancel()
+				}
+			})
+		}
+	}()
+}
diff --git a/vendor/golang.org/x/sync/errgroup/errgroup_example_md5all_test.go b/vendor/golang.org/x/sync/errgroup/errgroup_example_md5all_test.go
new file mode 100644
index 00000000..714b5aea
--- /dev/null
+++ b/vendor/golang.org/x/sync/errgroup/errgroup_example_md5all_test.go
@@ -0,0 +1,101 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errgroup_test
+
+import (
+	"crypto/md5"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"path/filepath"
+
+	"golang.org/x/net/context"
+	"golang.org/x/sync/errgroup"
+)
+
+// Pipeline demonstrates the use of a Group to implement a multi-stage
+// pipeline: a version of the MD5All function with bounded parallelism from
+// https://blog.golang.org/pipelines.
+func ExampleGroup_pipeline() {
+	m, err := MD5All(context.Background(), ".")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	for k, sum := range m {
+		fmt.Printf("%s:\t%x\n", k, sum)
+	}
+}
+
+type result struct {
+	path string
+	sum  [md5.Size]byte
+}
+
+// MD5All reads all the files in the file tree rooted at root and returns a map
+// from file path to the MD5 sum of the file's contents. If the directory walk
+// fails or any read operation fails, MD5All returns an error.
+func MD5All(ctx context.Context, root string) (map[string][md5.Size]byte, error) {
+	// ctx is canceled when g.Wait() returns. When this version of MD5All returns
+	// - even in case of error! - we know that all of the goroutines have finished
+	// and the memory they were using can be garbage-collected.
+	g, ctx := errgroup.WithContext(ctx)
+	paths := make(chan string)
+
+	g.Go(func() error {
+		defer close(paths)
+		return filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
+			if err != nil {
+				return err
+			}
+			if !info.Mode().IsRegular() {
+				return nil
+			}
+			select {
+			case paths <- path:
+			case <-ctx.Done():
+				return ctx.Err()
+			}
+			return nil
+		})
+	})
+
+	// Start a fixed number of goroutines to read and digest files.
+	c := make(chan result)
+	const numDigesters = 20
+	for i := 0; i < numDigesters; i++ {
+		g.Go(func() error {
+			for path := range paths {
+				data, err := ioutil.ReadFile(path)
+				if err != nil {
+					return err
+				}
+				select {
+				case c <- result{path, md5.Sum(data)}:
+				case <-ctx.Done():
+					return ctx.Err()
+				}
+			}
+			return nil
+		})
+	}
+	go func() {
+		g.Wait()
+		close(c)
+	}()
+
+	m := make(map[string][md5.Size]byte)
+	for r := range c {
+		m[r.path] = r.sum
+	}
+	// Check whether any of the goroutines failed. Since g is accumulating the
+	// errors, we don't need to send them (or check for them) in the individual
+	// results sent on the channel.
+	if err := g.Wait(); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
diff --git a/vendor/golang.org/x/sync/errgroup/errgroup_test.go b/vendor/golang.org/x/sync/errgroup/errgroup_test.go
new file mode 100644
index 00000000..6a9696ef
--- /dev/null
+++ b/vendor/golang.org/x/sync/errgroup/errgroup_test.go
@@ -0,0 +1,176 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errgroup_test
+
+import (
+	"errors"
+	"fmt"
+	"net/http"
+	"os"
+	"testing"
+
+	"golang.org/x/net/context"
+	"golang.org/x/sync/errgroup"
+)
+
+var (
+	Web   = fakeSearch("web")
+	Image = fakeSearch("image")
+	Video = fakeSearch("video")
+)
+
+type Result string
+type Search func(ctx context.Context, query string) (Result, error)
+
+func fakeSearch(kind string) Search {
+	return func(_ context.Context, query string) (Result, error) {
+		return Result(fmt.Sprintf("%s result for %q", kind, query)), nil
+	}
+}
+
+// JustErrors illustrates the use of a Group in place of a sync.WaitGroup to
+// simplify goroutine counting and error handling. This example is derived from
+// the sync.WaitGroup example at https://golang.org/pkg/sync/#example_WaitGroup.
+func ExampleGroup_justErrors() {
+	var g errgroup.Group
+	var urls = []string{
+		"http://www.golang.org/",
+		"http://www.google.com/",
+		"http://www.somestupidname.com/",
+	}
+	for _, url := range urls {
+		// Launch a goroutine to fetch the URL.
+		url := url // https://golang.org/doc/faq#closures_and_goroutines
+		g.Go(func() error {
+			// Fetch the URL.
+			resp, err := http.Get(url)
+			if err == nil {
+				resp.Body.Close()
+			}
+			return err
+		})
+	}
+	// Wait for all HTTP fetches to complete.
+	if err := g.Wait(); err == nil {
+		fmt.Println("Successfully fetched all URLs.")
+	}
+}
+
+// Parallel illustrates the use of a Group for synchronizing a simple parallel
+// task: the "Google Search 2.0" function from
+// https://talks.golang.org/2012/concurrency.slide#46, augmented with a Context
+// and error-handling.
+func ExampleGroup_parallel() {
+	Google := func(ctx context.Context, query string) ([]Result, error) {
+		g, ctx := errgroup.WithContext(ctx)
+
+		searches := []Search{Web, Image, Video}
+		results := make([]Result, len(searches))
+		for i, search := range searches {
+			i, search := i, search // https://golang.org/doc/faq#closures_and_goroutines
+			g.Go(func() error {
+				result, err := search(ctx, query)
+				if err == nil {
+					results[i] = result
+				}
+				return err
+			})
+		}
+		if err := g.Wait(); err != nil {
+			return nil, err
+		}
+		return results, nil
+	}
+
+	results, err := Google(context.Background(), "golang")
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		return
+	}
+	for _, result := range results {
+		fmt.Println(result)
+	}
+
+	// Output:
+	// web result for "golang"
+	// image result for "golang"
+	// video result for "golang"
+}
+
+func TestZeroGroup(t *testing.T) {
+	err1 := errors.New("errgroup_test: 1")
+	err2 := errors.New("errgroup_test: 2")
+
+	cases := []struct {
+		errs []error
+	}{
+		{errs: []error{}},
+		{errs: []error{nil}},
+		{errs: []error{err1}},
+		{errs: []error{err1, nil}},
+		{errs: []error{err1, nil, err2}},
+	}
+
+	for _, tc := range cases {
+		var g errgroup.Group
+
+		var firstErr error
+		for i, err := range tc.errs {
+			err := err
+			g.Go(func() error { return err })
+
+			if firstErr == nil && err != nil {
+				firstErr = err
+			}
+
+			if gErr := g.Wait(); gErr != firstErr {
+				t.Errorf("after %T.Go(func() error { return err }) for err in %v\n"+
+					"g.Wait() = %v; want %v",
+					g, tc.errs[:i+1], err, firstErr)
+			}
+		}
+	}
+}
+
+func TestWithContext(t *testing.T) {
+	errDoom := errors.New("group_test: doomed")
+
+	cases := []struct {
+		errs []error
+		want error
+	}{
+		{want: nil},
+		{errs: []error{nil}, want: nil},
+		{errs: []error{errDoom}, want: errDoom},
+		{errs: []error{errDoom, nil}, want: errDoom},
+	}
+
+	for _, tc := range cases {
+		g, ctx := errgroup.WithContext(context.Background())
+
+		for _, err := range tc.errs {
+			err := err
+			g.Go(func() error { return err })
+		}
+
+		if err := g.Wait(); err != tc.want {
+			t.Errorf("after %T.Go(func() error { return err }) for err in %v\n"+
+				"g.Wait() = %v; want %v",
+				g, tc.errs, err, tc.want)
+		}
+
+		canceled := false
+		select {
+		case <-ctx.Done():
+			canceled = true
+		default:
+		}
+		if !canceled {
+			t.Errorf("after %T.Go(func() error { return err }) for err in %v\n"+
+				"ctx.Done() was not closed",
+				g, tc.errs)
+		}
+	}
+}
diff --git a/vendor/golang.org/x/sync/semaphore/semaphore.go b/vendor/golang.org/x/sync/semaphore/semaphore.go
new file mode 100644
index 00000000..e9d2d79a
--- /dev/null
+++ b/vendor/golang.org/x/sync/semaphore/semaphore.go
@@ -0,0 +1,131 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package semaphore provides a weighted semaphore implementation.
+package semaphore // import "golang.org/x/sync/semaphore"
+
+import (
+	"container/list"
+	"sync"
+
+	// Use the old context because packages that depend on this one
+	// (e.g. cloud.google.com/go/...) must run on Go 1.6.
+	// TODO(jba): update to "context" when possible.
+	"golang.org/x/net/context"
+)
+
+type waiter struct {
+	n     int64
+	ready chan<- struct{} // Closed when semaphore acquired.
+}
+
+// NewWeighted creates a new weighted semaphore with the given
+// maximum combined weight for concurrent access.
+func NewWeighted(n int64) *Weighted {
+	w := &Weighted{size: n}
+	return w
+}
+
+// Weighted provides a way to bound concurrent access to a resource.
+// The callers can request access with a given weight.
+type Weighted struct {
+	size    int64
+	cur     int64
+	mu      sync.Mutex
+	waiters list.List
+}
+
+// Acquire acquires the semaphore with a weight of n, blocking only until ctx
+// is done. On success, returns nil. On failure, returns ctx.Err() and leaves
+// the semaphore unchanged.
+//
+// If ctx is already done, Acquire may still succeed without blocking.
+func (s *Weighted) Acquire(ctx context.Context, n int64) error {
+	s.mu.Lock()
+	if s.size-s.cur >= n && s.waiters.Len() == 0 {
+		s.cur += n
+		s.mu.Unlock()
+		return nil
+	}
+
+	if n > s.size {
+		// Don't make other Acquire calls block on one that's doomed to fail.
+		s.mu.Unlock()
+		<-ctx.Done()
+		return ctx.Err()
+	}
+
+	ready := make(chan struct{})
+	w := waiter{n: n, ready: ready}
+	elem := s.waiters.PushBack(w)
+	s.mu.Unlock()
+
+	select {
+	case <-ctx.Done():
+		err := ctx.Err()
+		s.mu.Lock()
+		select {
+		case <-ready:
+			// Acquired the semaphore after we were canceled.  Rather than trying to
+			// fix up the queue, just pretend we didn't notice the cancelation.
+			err = nil
+		default:
+			s.waiters.Remove(elem)
+		}
+		s.mu.Unlock()
+		return err
+
+	case <-ready:
+		return nil
+	}
+}
+
+// TryAcquire acquires the semaphore with a weight of n without blocking.
+// On success, returns true. On failure, returns false and leaves the semaphore unchanged.
+func (s *Weighted) TryAcquire(n int64) bool {
+	s.mu.Lock()
+	success := s.size-s.cur >= n && s.waiters.Len() == 0
+	if success {
+		s.cur += n
+	}
+	s.mu.Unlock()
+	return success
+}
+
+// Release releases the semaphore with a weight of n.
+func (s *Weighted) Release(n int64) {
+	s.mu.Lock()
+	s.cur -= n
+	if s.cur < 0 {
+		s.mu.Unlock()
+		panic("semaphore: bad release")
+	}
+	for {
+		next := s.waiters.Front()
+		if next == nil {
+			break // No more waiters blocked.
+		}
+
+		w := next.Value.(waiter)
+		if s.size-s.cur < w.n {
+			// Not enough tokens for the next waiter.  We could keep going (to try to
+			// find a waiter with a smaller request), but under load that could cause
+			// starvation for large requests; instead, we leave all remaining waiters
+			// blocked.
+			//
+			// Consider a semaphore used as a read-write lock, with N tokens, N
+			// readers, and one writer.  Each reader can Acquire(1) to obtain a read
+			// lock.  The writer can Acquire(N) to obtain a write lock, excluding all
+			// of the readers.  If we allow the readers to jump ahead in the queue,
+			// the writer will starve — there is always one token available for every
+			// reader.
+			break
+		}
+
+		s.cur += w.n
+		s.waiters.Remove(next)
+		close(w.ready)
+	}
+	s.mu.Unlock()
+}
diff --git a/vendor/golang.org/x/sync/semaphore/semaphore_bench_test.go b/vendor/golang.org/x/sync/semaphore/semaphore_bench_test.go
new file mode 100644
index 00000000..1e3ab75f
--- /dev/null
+++ b/vendor/golang.org/x/sync/semaphore/semaphore_bench_test.go
@@ -0,0 +1,131 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build go1.7
+
+package semaphore_test
+
+import (
+	"fmt"
+	"testing"
+
+	"golang.org/x/net/context"
+	"golang.org/x/sync/semaphore"
+)
+
+// weighted is an interface matching a subset of *Weighted.  It allows
+// alternate implementations for testing and benchmarking.
+type weighted interface {
+	Acquire(context.Context, int64) error
+	TryAcquire(int64) bool
+	Release(int64)
+}
+
+// semChan implements Weighted using a channel for
+// comparing against the condition variable-based implementation.
+type semChan chan struct{}
+
+func newSemChan(n int64) semChan {
+	return semChan(make(chan struct{}, n))
+}
+
+func (s semChan) Acquire(_ context.Context, n int64) error {
+	for i := int64(0); i < n; i++ {
+		s <- struct{}{}
+	}
+	return nil
+}
+
+func (s semChan) TryAcquire(n int64) bool {
+	if int64(len(s))+n > int64(cap(s)) {
+		return false
+	}
+
+	for i := int64(0); i < n; i++ {
+		s <- struct{}{}
+	}
+	return true
+}
+
+func (s semChan) Release(n int64) {
+	for i := int64(0); i < n; i++ {
+		<-s
+	}
+}
+
+// acquireN calls Acquire(size) on sem N times and then calls Release(size) N times.
+func acquireN(b *testing.B, sem weighted, size int64, N int) {
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for j := 0; j < N; j++ {
+			sem.Acquire(context.Background(), size)
+		}
+		for j := 0; j < N; j++ {
+			sem.Release(size)
+		}
+	}
+}
+
+// tryAcquireN calls TryAcquire(size) on sem N times and then calls Release(size) N times.
+func tryAcquireN(b *testing.B, sem weighted, size int64, N int) {
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for j := 0; j < N; j++ {
+			if !sem.TryAcquire(size) {
+				b.Fatalf("TryAcquire(%v) = false, want true", size)
+			}
+		}
+		for j := 0; j < N; j++ {
+			sem.Release(size)
+		}
+	}
+}
+
+func BenchmarkNewSeq(b *testing.B) {
+	for _, cap := range []int64{1, 128} {
+		b.Run(fmt.Sprintf("Weighted-%d", cap), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				_ = semaphore.NewWeighted(cap)
+			}
+		})
+		b.Run(fmt.Sprintf("semChan-%d", cap), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				_ = newSemChan(cap)
+			}
+		})
+	}
+}
+
+func BenchmarkAcquireSeq(b *testing.B) {
+	for _, c := range []struct {
+		cap, size int64
+		N         int
+	}{
+		{1, 1, 1},
+		{2, 1, 1},
+		{16, 1, 1},
+		{128, 1, 1},
+		{2, 2, 1},
+		{16, 2, 8},
+		{128, 2, 64},
+		{2, 1, 2},
+		{16, 8, 2},
+		{128, 64, 2},
+	} {
+		for _, w := range []struct {
+			name string
+			w    weighted
+		}{
+			{"Weighted", semaphore.NewWeighted(c.cap)},
+			{"semChan", newSemChan(c.cap)},
+		} {
+			b.Run(fmt.Sprintf("%s-acquire-%d-%d-%d", w.name, c.cap, c.size, c.N), func(b *testing.B) {
+				acquireN(b, w.w, c.size, c.N)
+			})
+			b.Run(fmt.Sprintf("%s-tryAcquire-%d-%d-%d", w.name, c.cap, c.size, c.N), func(b *testing.B) {
+				tryAcquireN(b, w.w, c.size, c.N)
+			})
+		}
+	}
+}
diff --git a/vendor/golang.org/x/sync/semaphore/semaphore_example_test.go b/vendor/golang.org/x/sync/semaphore/semaphore_example_test.go
new file mode 100644
index 00000000..e75cd79f
--- /dev/null
+++ b/vendor/golang.org/x/sync/semaphore/semaphore_example_test.go
@@ -0,0 +1,84 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package semaphore_test
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"runtime"
+
+	"golang.org/x/sync/semaphore"
+)
+
+// Example_workerPool demonstrates how to use a semaphore to limit the number of
+// goroutines working on parallel tasks.
+//
+// This use of a semaphore mimics a typical “worker pool” pattern, but without
+// the need to explicitly shut down idle workers when the work is done.
+func Example_workerPool() {
+	ctx := context.TODO()
+
+	var (
+		maxWorkers = runtime.GOMAXPROCS(0)
+		sem        = semaphore.NewWeighted(int64(maxWorkers))
+		out        = make([]int, 32)
+	)
+
+	// Compute the output using up to maxWorkers goroutines at a time.
+	for i := range out {
+		// When maxWorkers goroutines are in flight, Acquire blocks until one of the
+		// workers finishes.
+		if err := sem.Acquire(ctx, 1); err != nil {
+			log.Printf("Failed to acquire semaphore: %v", err)
+			break
+		}
+
+		go func(i int) {
+			defer sem.Release(1)
+			out[i] = collatzSteps(i + 1)
+		}(i)
+	}
+
+	// Acquire all of the tokens to wait for any remaining workers to finish.
+	//
+	// If you are already waiting for the workers by some other means (such as an
+	// errgroup.Group), you can omit this final Acquire call.
+	if err := sem.Acquire(ctx, int64(maxWorkers)); err != nil {
+		log.Printf("Failed to acquire semaphore: %v", err)
+	}
+
+	fmt.Println(out)
+
+	// Output:
+	// [0 1 7 2 5 8 16 3 19 6 14 9 9 17 17 4 12 20 20 7 7 15 15 10 23 10 111 18 18 18 106 5]
+}
+
+// collatzSteps computes the number of steps to reach 1 under the Collatz
+// conjecture. (See https://en.wikipedia.org/wiki/Collatz_conjecture.)
+func collatzSteps(n int) (steps int) {
+	if n <= 0 {
+		panic("nonpositive input")
+	}
+
+	for ; n > 1; steps++ {
+		if steps < 0 {
+			panic("too many steps")
+		}
+
+		if n%2 == 0 {
+			n /= 2
+			continue
+		}
+
+		const maxInt = int(^uint(0) >> 1)
+		if n > (maxInt-1)/3 {
+			panic("overflow")
+		}
+		n = 3*n + 1
+	}
+
+	return steps
+}
diff --git a/vendor/golang.org/x/sync/semaphore/semaphore_test.go b/vendor/golang.org/x/sync/semaphore/semaphore_test.go
new file mode 100644
index 00000000..2541b906
--- /dev/null
+++ b/vendor/golang.org/x/sync/semaphore/semaphore_test.go
@@ -0,0 +1,171 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package semaphore_test
+
+import (
+	"math/rand"
+	"runtime"
+	"sync"
+	"testing"
+	"time"
+
+	"golang.org/x/net/context"
+	"golang.org/x/sync/errgroup"
+	"golang.org/x/sync/semaphore"
+)
+
+const maxSleep = 1 * time.Millisecond
+
+func HammerWeighted(sem *semaphore.Weighted, n int64, loops int) {
+	for i := 0; i < loops; i++ {
+		sem.Acquire(context.Background(), n)
+		time.Sleep(time.Duration(rand.Int63n(int64(maxSleep/time.Nanosecond))) * time.Nanosecond)
+		sem.Release(n)
+	}
+}
+
+func TestWeighted(t *testing.T) {
+	t.Parallel()
+
+	n := runtime.GOMAXPROCS(0)
+	loops := 10000 / n
+	sem := semaphore.NewWeighted(int64(n))
+	var wg sync.WaitGroup
+	wg.Add(n)
+	for i := 0; i < n; i++ {
+		i := i
+		go func() {
+			defer wg.Done()
+			HammerWeighted(sem, int64(i), loops)
+		}()
+	}
+	wg.Wait()
+}
+
+func TestWeightedPanic(t *testing.T) {
+	t.Parallel()
+
+	defer func() {
+		if recover() == nil {
+			t.Fatal("release of an unacquired weighted semaphore did not panic")
+		}
+	}()
+	w := semaphore.NewWeighted(1)
+	w.Release(1)
+}
+
+func TestWeightedTryAcquire(t *testing.T) {
+	t.Parallel()
+
+	ctx := context.Background()
+	sem := semaphore.NewWeighted(2)
+	tries := []bool{}
+	sem.Acquire(ctx, 1)
+	tries = append(tries, sem.TryAcquire(1))
+	tries = append(tries, sem.TryAcquire(1))
+
+	sem.Release(2)
+
+	tries = append(tries, sem.TryAcquire(1))
+	sem.Acquire(ctx, 1)
+	tries = append(tries, sem.TryAcquire(1))
+
+	want := []bool{true, false, true, false}
+	for i := range tries {
+		if tries[i] != want[i] {
+			t.Errorf("tries[%d]: got %t, want %t", i, tries[i], want[i])
+		}
+	}
+}
+
+func TestWeightedAcquire(t *testing.T) {
+	t.Parallel()
+
+	ctx := context.Background()
+	sem := semaphore.NewWeighted(2)
+	tryAcquire := func(n int64) bool {
+		ctx, cancel := context.WithTimeout(ctx, 10*time.Millisecond)
+		defer cancel()
+		return sem.Acquire(ctx, n) == nil
+	}
+
+	tries := []bool{}
+	sem.Acquire(ctx, 1)
+	tries = append(tries, tryAcquire(1))
+	tries = append(tries, tryAcquire(1))
+
+	sem.Release(2)
+
+	tries = append(tries, tryAcquire(1))
+	sem.Acquire(ctx, 1)
+	tries = append(tries, tryAcquire(1))
+
+	want := []bool{true, false, true, false}
+	for i := range tries {
+		if tries[i] != want[i] {
+			t.Errorf("tries[%d]: got %t, want %t", i, tries[i], want[i])
+		}
+	}
+}
+
+func TestWeightedDoesntBlockIfTooBig(t *testing.T) {
+	t.Parallel()
+
+	const n = 2
+	sem := semaphore.NewWeighted(n)
+	{
+		ctx, cancel := context.WithCancel(context.Background())
+		defer cancel()
+		go sem.Acquire(ctx, n+1)
+	}
+
+	g, ctx := errgroup.WithContext(context.Background())
+	for i := n * 3; i > 0; i-- {
+		g.Go(func() error {
+			err := sem.Acquire(ctx, 1)
+			if err == nil {
+				time.Sleep(1 * time.Millisecond)
+				sem.Release(1)
+			}
+			return err
+		})
+	}
+	if err := g.Wait(); err != nil {
+		t.Errorf("semaphore.NewWeighted(%v) failed to AcquireCtx(_, 1) with AcquireCtx(_, %v) pending", n, n+1)
+	}
+}
+
+// TestLargeAcquireDoesntStarve times out if a large call to Acquire starves.
+// Merely returning from the test function indicates success.
+func TestLargeAcquireDoesntStarve(t *testing.T) {
+	t.Parallel()
+
+	ctx := context.Background()
+	n := int64(runtime.GOMAXPROCS(0))
+	sem := semaphore.NewWeighted(n)
+	running := true
+
+	var wg sync.WaitGroup
+	wg.Add(int(n))
+	for i := n; i > 0; i-- {
+		sem.Acquire(ctx, 1)
+		go func() {
+			defer func() {
+				sem.Release(1)
+				wg.Done()
+			}()
+			for running {
+				time.Sleep(1 * time.Millisecond)
+				sem.Release(1)
+				sem.Acquire(ctx, 1)
+			}
+		}()
+	}
+
+	sem.Acquire(ctx, n)
+	running = false
+	sem.Release(n)
+	wg.Wait()
+}
diff --git a/vendor/golang.org/x/sync/singleflight/singleflight.go b/vendor/golang.org/x/sync/singleflight/singleflight.go
new file mode 100644
index 00000000..9a4f8d59
--- /dev/null
+++ b/vendor/golang.org/x/sync/singleflight/singleflight.go
@@ -0,0 +1,111 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package singleflight provides a duplicate function call suppression
+// mechanism.
+package singleflight // import "golang.org/x/sync/singleflight"
+
+import "sync"
+
+// call is an in-flight or completed singleflight.Do call
+type call struct {
+	wg sync.WaitGroup
+
+	// These fields are written once before the WaitGroup is done
+	// and are only read after the WaitGroup is done.
+	val interface{}
+	err error
+
+	// These fields are read and written with the singleflight
+	// mutex held before the WaitGroup is done, and are read but
+	// not written after the WaitGroup is done.
+	dups  int
+	chans []chan<- Result
+}
+
+// Group represents a class of work and forms a namespace in
+// which units of work can be executed with duplicate suppression.
+type Group struct {
+	mu sync.Mutex       // protects m
+	m  map[string]*call // lazily initialized
+}
+
+// Result holds the results of Do, so they can be passed
+// on a channel.
+type Result struct {
+	Val    interface{}
+	Err    error
+	Shared bool
+}
+
+// Do executes and returns the results of the given function, making
+// sure that only one execution is in-flight for a given key at a
+// time. If a duplicate comes in, the duplicate caller waits for the
+// original to complete and receives the same results.
+// The return value shared indicates whether v was given to multiple callers.
+func (g *Group) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
+	g.mu.Lock()
+	if g.m == nil {
+		g.m = make(map[string]*call)
+	}
+	if c, ok := g.m[key]; ok {
+		c.dups++
+		g.mu.Unlock()
+		c.wg.Wait()
+		return c.val, c.err, true
+	}
+	c := new(call)
+	c.wg.Add(1)
+	g.m[key] = c
+	g.mu.Unlock()
+
+	g.doCall(c, key, fn)
+	return c.val, c.err, c.dups > 0
+}
+
+// DoChan is like Do but returns a channel that will receive the
+// results when they are ready.
+func (g *Group) DoChan(key string, fn func() (interface{}, error)) <-chan Result {
+	ch := make(chan Result, 1)
+	g.mu.Lock()
+	if g.m == nil {
+		g.m = make(map[string]*call)
+	}
+	if c, ok := g.m[key]; ok {
+		c.dups++
+		c.chans = append(c.chans, ch)
+		g.mu.Unlock()
+		return ch
+	}
+	c := &call{chans: []chan<- Result{ch}}
+	c.wg.Add(1)
+	g.m[key] = c
+	g.mu.Unlock()
+
+	go g.doCall(c, key, fn)
+
+	return ch
+}
+
+// doCall handles the single call for a key.
+func (g *Group) doCall(c *call, key string, fn func() (interface{}, error)) {
+	c.val, c.err = fn()
+	c.wg.Done()
+
+	g.mu.Lock()
+	delete(g.m, key)
+	for _, ch := range c.chans {
+		ch <- Result{c.val, c.err, c.dups > 0}
+	}
+	g.mu.Unlock()
+}
+
+// Forget tells the singleflight to forget about a key.  Future calls
+// to Do for this key will call the function rather than waiting for
+// an earlier call to complete.
+func (g *Group) Forget(key string) {
+	g.mu.Lock()
+	delete(g.m, key)
+	g.mu.Unlock()
+}
diff --git a/vendor/golang.org/x/sync/singleflight/singleflight_test.go b/vendor/golang.org/x/sync/singleflight/singleflight_test.go
new file mode 100644
index 00000000..5e6f1b32
--- /dev/null
+++ b/vendor/golang.org/x/sync/singleflight/singleflight_test.go
@@ -0,0 +1,87 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package singleflight
+
+import (
+	"errors"
+	"fmt"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestDo(t *testing.T) {
+	var g Group
+	v, err, _ := g.Do("key", func() (interface{}, error) {
+		return "bar", nil
+	})
+	if got, want := fmt.Sprintf("%v (%T)", v, v), "bar (string)"; got != want {
+		t.Errorf("Do = %v; want %v", got, want)
+	}
+	if err != nil {
+		t.Errorf("Do error = %v", err)
+	}
+}
+
+func TestDoErr(t *testing.T) {
+	var g Group
+	someErr := errors.New("Some error")
+	v, err, _ := g.Do("key", func() (interface{}, error) {
+		return nil, someErr
+	})
+	if err != someErr {
+		t.Errorf("Do error = %v; want someErr %v", err, someErr)
+	}
+	if v != nil {
+		t.Errorf("unexpected non-nil value %#v", v)
+	}
+}
+
+func TestDoDupSuppress(t *testing.T) {
+	var g Group
+	var wg1, wg2 sync.WaitGroup
+	c := make(chan string, 1)
+	var calls int32
+	fn := func() (interface{}, error) {
+		if atomic.AddInt32(&calls, 1) == 1 {
+			// First invocation.
+			wg1.Done()
+		}
+		v := <-c
+		c <- v // pump; make available for any future calls
+
+		time.Sleep(10 * time.Millisecond) // let more goroutines enter Do
+
+		return v, nil
+	}
+
+	const n = 10
+	wg1.Add(1)
+	for i := 0; i < n; i++ {
+		wg1.Add(1)
+		wg2.Add(1)
+		go func() {
+			defer wg2.Done()
+			wg1.Done()
+			v, err, _ := g.Do("key", fn)
+			if err != nil {
+				t.Errorf("Do error: %v", err)
+				return
+			}
+			if s, _ := v.(string); s != "bar" {
+				t.Errorf("Do = %T %v; want %q", v, v, "bar")
+			}
+		}()
+	}
+	wg1.Wait()
+	// At least one goroutine is in fn now and all of them have at
+	// least reached the line before the Do.
+	c <- "bar"
+	wg2.Wait()
+	if got := atomic.LoadInt32(&calls); got <= 0 || got >= n {
+		t.Errorf("number of calls = %d; want over 0 and less than %d", got, n)
+	}
+}
diff --git a/vendor/golang.org/x/sync/syncmap/map.go b/vendor/golang.org/x/sync/syncmap/map.go
new file mode 100644
index 00000000..80e15847
--- /dev/null
+++ b/vendor/golang.org/x/sync/syncmap/map.go
@@ -0,0 +1,372 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package syncmap provides a concurrent map implementation.
+// It is a prototype for a proposed addition to the sync package
+// in the standard library.
+// (https://golang.org/issue/18177)
+package syncmap
+
+import (
+	"sync"
+	"sync/atomic"
+	"unsafe"
+)
+
+// Map is a concurrent map with amortized-constant-time loads, stores, and deletes.
+// It is safe for multiple goroutines to call a Map's methods concurrently.
+//
+// The zero Map is valid and empty.
+//
+// A Map must not be copied after first use.
+type Map struct {
+	mu sync.Mutex
+
+	// read contains the portion of the map's contents that are safe for
+	// concurrent access (with or without mu held).
+	//
+	// The read field itself is always safe to load, but must only be stored with
+	// mu held.
+	//
+	// Entries stored in read may be updated concurrently without mu, but updating
+	// a previously-expunged entry requires that the entry be copied to the dirty
+	// map and unexpunged with mu held.
+	read atomic.Value // readOnly
+
+	// dirty contains the portion of the map's contents that require mu to be
+	// held. To ensure that the dirty map can be promoted to the read map quickly,
+	// it also includes all of the non-expunged entries in the read map.
+	//
+	// Expunged entries are not stored in the dirty map. An expunged entry in the
+	// clean map must be unexpunged and added to the dirty map before a new value
+	// can be stored to it.
+	//
+	// If the dirty map is nil, the next write to the map will initialize it by
+	// making a shallow copy of the clean map, omitting stale entries.
+	dirty map[interface{}]*entry
+
+	// misses counts the number of loads since the read map was last updated that
+	// needed to lock mu to determine whether the key was present.
+	//
+	// Once enough misses have occurred to cover the cost of copying the dirty
+	// map, the dirty map will be promoted to the read map (in the unamended
+	// state) and the next store to the map will make a new dirty copy.
+	misses int
+}
+
+// readOnly is an immutable struct stored atomically in the Map.read field.
+type readOnly struct {
+	m       map[interface{}]*entry
+	amended bool // true if the dirty map contains some key not in m.
+}
+
+// expunged is an arbitrary pointer that marks entries which have been deleted
+// from the dirty map.
+var expunged = unsafe.Pointer(new(interface{}))
+
+// An entry is a slot in the map corresponding to a particular key.
+type entry struct {
+	// p points to the interface{} value stored for the entry.
+	//
+	// If p == nil, the entry has been deleted and m.dirty == nil.
+	//
+	// If p == expunged, the entry has been deleted, m.dirty != nil, and the entry
+	// is missing from m.dirty.
+	//
+	// Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty
+	// != nil, in m.dirty[key].
+	//
+	// An entry can be deleted by atomic replacement with nil: when m.dirty is
+	// next created, it will atomically replace nil with expunged and leave
+	// m.dirty[key] unset.
+	//
+	// An entry's associated value can be updated by atomic replacement, provided
+	// p != expunged. If p == expunged, an entry's associated value can be updated
+	// only after first setting m.dirty[key] = e so that lookups using the dirty
+	// map find the entry.
+	p unsafe.Pointer // *interface{}
+}
+
+func newEntry(i interface{}) *entry {
+	return &entry{p: unsafe.Pointer(&i)}
+}
+
+// Load returns the value stored in the map for a key, or nil if no
+// value is present.
+// The ok result indicates whether value was found in the map.
+func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
+	read, _ := m.read.Load().(readOnly)
+	e, ok := read.m[key]
+	if !ok && read.amended {
+		m.mu.Lock()
+		// Avoid reporting a spurious miss if m.dirty got promoted while we were
+		// blocked on m.mu. (If further loads of the same key will not miss, it's
+		// not worth copying the dirty map for this key.)
+		read, _ = m.read.Load().(readOnly)
+		e, ok = read.m[key]
+		if !ok && read.amended {
+			e, ok = m.dirty[key]
+			// Regardless of whether the entry was present, record a miss: this key
+			// will take the slow path until the dirty map is promoted to the read
+			// map.
+			m.missLocked()
+		}
+		m.mu.Unlock()
+	}
+	if !ok {
+		return nil, false
+	}
+	return e.load()
+}
+
+func (e *entry) load() (value interface{}, ok bool) {
+	p := atomic.LoadPointer(&e.p)
+	if p == nil || p == expunged {
+		return nil, false
+	}
+	return *(*interface{})(p), true
+}
+
+// Store sets the value for a key.
+func (m *Map) Store(key, value interface{}) {
+	read, _ := m.read.Load().(readOnly)
+	if e, ok := read.m[key]; ok && e.tryStore(&value) {
+		return
+	}
+
+	m.mu.Lock()
+	read, _ = m.read.Load().(readOnly)
+	if e, ok := read.m[key]; ok {
+		if e.unexpungeLocked() {
+			// The entry was previously expunged, which implies that there is a
+			// non-nil dirty map and this entry is not in it.
+			m.dirty[key] = e
+		}
+		e.storeLocked(&value)
+	} else if e, ok := m.dirty[key]; ok {
+		e.storeLocked(&value)
+	} else {
+		if !read.amended {
+			// We're adding the first new key to the dirty map.
+			// Make sure it is allocated and mark the read-only map as incomplete.
+			m.dirtyLocked()
+			m.read.Store(readOnly{m: read.m, amended: true})
+		}
+		m.dirty[key] = newEntry(value)
+	}
+	m.mu.Unlock()
+}
+
+// tryStore stores a value if the entry has not been expunged.
+//
+// If the entry is expunged, tryStore returns false and leaves the entry
+// unchanged.
+func (e *entry) tryStore(i *interface{}) bool {
+	p := atomic.LoadPointer(&e.p)
+	if p == expunged {
+		return false
+	}
+	for {
+		if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
+			return true
+		}
+		p = atomic.LoadPointer(&e.p)
+		if p == expunged {
+			return false
+		}
+	}
+}
+
+// unexpungeLocked ensures that the entry is not marked as expunged.
+//
+// If the entry was previously expunged, it must be added to the dirty map
+// before m.mu is unlocked.
+func (e *entry) unexpungeLocked() (wasExpunged bool) {
+	return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
+}
+
+// storeLocked unconditionally stores a value to the entry.
+//
+// The entry must be known not to be expunged.
+func (e *entry) storeLocked(i *interface{}) {
+	atomic.StorePointer(&e.p, unsafe.Pointer(i))
+}
+
+// LoadOrStore returns the existing value for the key if present.
+// Otherwise, it stores and returns the given value.
+// The loaded result is true if the value was loaded, false if stored.
+func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
+	// Avoid locking if it's a clean hit.
+	read, _ := m.read.Load().(readOnly)
+	if e, ok := read.m[key]; ok {
+		actual, loaded, ok := e.tryLoadOrStore(value)
+		if ok {
+			return actual, loaded
+		}
+	}
+
+	m.mu.Lock()
+	read, _ = m.read.Load().(readOnly)
+	if e, ok := read.m[key]; ok {
+		if e.unexpungeLocked() {
+			m.dirty[key] = e
+		}
+		actual, loaded, _ = e.tryLoadOrStore(value)
+	} else if e, ok := m.dirty[key]; ok {
+		actual, loaded, _ = e.tryLoadOrStore(value)
+		m.missLocked()
+	} else {
+		if !read.amended {
+			// We're adding the first new key to the dirty map.
+			// Make sure it is allocated and mark the read-only map as incomplete.
+			m.dirtyLocked()
+			m.read.Store(readOnly{m: read.m, amended: true})
+		}
+		m.dirty[key] = newEntry(value)
+		actual, loaded = value, false
+	}
+	m.mu.Unlock()
+
+	return actual, loaded
+}
+
+// tryLoadOrStore atomically loads or stores a value if the entry is not
+// expunged.
+//
+// If the entry is expunged, tryLoadOrStore leaves the entry unchanged and
+// returns with ok==false.
+func (e *entry) tryLoadOrStore(i interface{}) (actual interface{}, loaded, ok bool) {
+	p := atomic.LoadPointer(&e.p)
+	if p == expunged {
+		return nil, false, false
+	}
+	if p != nil {
+		return *(*interface{})(p), true, true
+	}
+
+	// Copy the interface after the first load to make this method more amenable
+	// to escape analysis: if we hit the "load" path or the entry is expunged, we
+	// shouldn't bother heap-allocating.
+	ic := i
+	for {
+		if atomic.CompareAndSwapPointer(&e.p, nil, unsafe.Pointer(&ic)) {
+			return i, false, true
+		}
+		p = atomic.LoadPointer(&e.p)
+		if p == expunged {
+			return nil, false, false
+		}
+		if p != nil {
+			return *(*interface{})(p), true, true
+		}
+	}
+}
+
+// Delete deletes the value for a key.
+func (m *Map) Delete(key interface{}) {
+	read, _ := m.read.Load().(readOnly)
+	e, ok := read.m[key]
+	if !ok && read.amended {
+		m.mu.Lock()
+		read, _ = m.read.Load().(readOnly)
+		e, ok = read.m[key]
+		if !ok && read.amended {
+			delete(m.dirty, key)
+		}
+		m.mu.Unlock()
+	}
+	if ok {
+		e.delete()
+	}
+}
+
+func (e *entry) delete() (hadValue bool) {
+	for {
+		p := atomic.LoadPointer(&e.p)
+		if p == nil || p == expunged {
+			return false
+		}
+		if atomic.CompareAndSwapPointer(&e.p, p, nil) {
+			return true
+		}
+	}
+}
+
+// Range calls f sequentially for each key and value present in the map.
+// If f returns false, range stops the iteration.
+//
+// Range does not necessarily correspond to any consistent snapshot of the Map's
+// contents: no key will be visited more than once, but if the value for any key
+// is stored or deleted concurrently, Range may reflect any mapping for that key
+// from any point during the Range call.
+//
+// Range may be O(N) with the number of elements in the map even if f returns
+// false after a constant number of calls.
+func (m *Map) Range(f func(key, value interface{}) bool) {
+	// We need to be able to iterate over all of the keys that were already
+	// present at the start of the call to Range.
+	// If read.amended is false, then read.m satisfies that property without
+	// requiring us to hold m.mu for a long time.
+	read, _ := m.read.Load().(readOnly)
+	if read.amended {
+		// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
+		// (assuming the caller does not break out early), so a call to Range
+		// amortizes an entire copy of the map: we can promote the dirty copy
+		// immediately!
+		m.mu.Lock()
+		read, _ = m.read.Load().(readOnly)
+		if read.amended {
+			read = readOnly{m: m.dirty}
+			m.read.Store(read)
+			m.dirty = nil
+			m.misses = 0
+		}
+		m.mu.Unlock()
+	}
+
+	for k, e := range read.m {
+		v, ok := e.load()
+		if !ok {
+			continue
+		}
+		if !f(k, v) {
+			break
+		}
+	}
+}
+
+func (m *Map) missLocked() {
+	m.misses++
+	if m.misses < len(m.dirty) {
+		return
+	}
+	m.read.Store(readOnly{m: m.dirty})
+	m.dirty = nil
+	m.misses = 0
+}
+
+func (m *Map) dirtyLocked() {
+	if m.dirty != nil {
+		return
+	}
+
+	read, _ := m.read.Load().(readOnly)
+	m.dirty = make(map[interface{}]*entry, len(read.m))
+	for k, e := range read.m {
+		if !e.tryExpungeLocked() {
+			m.dirty[k] = e
+		}
+	}
+}
+
+func (e *entry) tryExpungeLocked() (isExpunged bool) {
+	p := atomic.LoadPointer(&e.p)
+	for p == nil {
+		if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
+			return true
+		}
+		p = atomic.LoadPointer(&e.p)
+	}
+	return p == expunged
+}
diff --git a/vendor/golang.org/x/sync/syncmap/map_bench_test.go b/vendor/golang.org/x/sync/syncmap/map_bench_test.go
new file mode 100644
index 00000000..b279b4f7
--- /dev/null
+++ b/vendor/golang.org/x/sync/syncmap/map_bench_test.go
@@ -0,0 +1,216 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package syncmap_test
+
+import (
+	"fmt"
+	"reflect"
+	"sync/atomic"
+	"testing"
+
+	"golang.org/x/sync/syncmap"
+)
+
+type bench struct {
+	setup func(*testing.B, mapInterface)
+	perG  func(b *testing.B, pb *testing.PB, i int, m mapInterface)
+}
+
+func benchMap(b *testing.B, bench bench) {
+	for _, m := range [...]mapInterface{&DeepCopyMap{}, &RWMutexMap{}, &syncmap.Map{}} {
+		b.Run(fmt.Sprintf("%T", m), func(b *testing.B) {
+			m = reflect.New(reflect.TypeOf(m).Elem()).Interface().(mapInterface)
+			if bench.setup != nil {
+				bench.setup(b, m)
+			}
+
+			b.ResetTimer()
+
+			var i int64
+			b.RunParallel(func(pb *testing.PB) {
+				id := int(atomic.AddInt64(&i, 1) - 1)
+				bench.perG(b, pb, id*b.N, m)
+			})
+		})
+	}
+}
+
+func BenchmarkLoadMostlyHits(b *testing.B) {
+	const hits, misses = 1023, 1
+
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			for i := 0; i < hits; i++ {
+				m.LoadOrStore(i, i)
+			}
+			// Prime the map to get it into a steady state.
+			for i := 0; i < hits*2; i++ {
+				m.Load(i % hits)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.Load(i % (hits + misses))
+			}
+		},
+	})
+}
+
+func BenchmarkLoadMostlyMisses(b *testing.B) {
+	const hits, misses = 1, 1023
+
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			for i := 0; i < hits; i++ {
+				m.LoadOrStore(i, i)
+			}
+			// Prime the map to get it into a steady state.
+			for i := 0; i < hits*2; i++ {
+				m.Load(i % hits)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.Load(i % (hits + misses))
+			}
+		},
+	})
+}
+
+func BenchmarkLoadOrStoreBalanced(b *testing.B) {
+	const hits, misses = 128, 128
+
+	benchMap(b, bench{
+		setup: func(b *testing.B, m mapInterface) {
+			if _, ok := m.(*DeepCopyMap); ok {
+				b.Skip("DeepCopyMap has quadratic running time.")
+			}
+			for i := 0; i < hits; i++ {
+				m.LoadOrStore(i, i)
+			}
+			// Prime the map to get it into a steady state.
+			for i := 0; i < hits*2; i++ {
+				m.Load(i % hits)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				j := i % (hits + misses)
+				if j < hits {
+					if _, ok := m.LoadOrStore(j, i); !ok {
+						b.Fatalf("unexpected miss for %v", j)
+					}
+				} else {
+					if v, loaded := m.LoadOrStore(i, i); loaded {
+						b.Fatalf("failed to store %v: existing value %v", i, v)
+					}
+				}
+			}
+		},
+	})
+}
+
+func BenchmarkLoadOrStoreUnique(b *testing.B) {
+	benchMap(b, bench{
+		setup: func(b *testing.B, m mapInterface) {
+			if _, ok := m.(*DeepCopyMap); ok {
+				b.Skip("DeepCopyMap has quadratic running time.")
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.LoadOrStore(i, i)
+			}
+		},
+	})
+}
+
+func BenchmarkLoadOrStoreCollision(b *testing.B) {
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			m.LoadOrStore(0, 0)
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.LoadOrStore(0, 0)
+			}
+		},
+	})
+}
+
+func BenchmarkRange(b *testing.B) {
+	const mapSize = 1 << 10
+
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			for i := 0; i < mapSize; i++ {
+				m.Store(i, i)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.Range(func(_, _ interface{}) bool { return true })
+			}
+		},
+	})
+}
+
+// BenchmarkAdversarialAlloc tests performance when we store a new value
+// immediately whenever the map is promoted to clean and otherwise load a
+// unique, missing key.
+//
+// This forces the Load calls to always acquire the map's mutex.
+func BenchmarkAdversarialAlloc(b *testing.B) {
+	benchMap(b, bench{
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			var stores, loadsSinceStore int64
+			for ; pb.Next(); i++ {
+				m.Load(i)
+				if loadsSinceStore++; loadsSinceStore > stores {
+					m.LoadOrStore(i, stores)
+					loadsSinceStore = 0
+					stores++
+				}
+			}
+		},
+	})
+}
+
+// BenchmarkAdversarialDelete tests performance when we periodically delete
+// one key and add a different one in a large map.
+//
+// This forces the Load calls to always acquire the map's mutex and periodically
+// makes a full copy of the map despite changing only one entry.
+func BenchmarkAdversarialDelete(b *testing.B) {
+	const mapSize = 1 << 10
+
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			for i := 0; i < mapSize; i++ {
+				m.Store(i, i)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.Load(i)
+
+				if i%mapSize == 0 {
+					m.Range(func(k, _ interface{}) bool {
+						m.Delete(k)
+						return false
+					})
+					m.Store(i, i)
+				}
+			}
+		},
+	})
+}
diff --git a/vendor/golang.org/x/sync/syncmap/map_reference_test.go b/vendor/golang.org/x/sync/syncmap/map_reference_test.go
new file mode 100644
index 00000000..923c51b7
--- /dev/null
+++ b/vendor/golang.org/x/sync/syncmap/map_reference_test.go
@@ -0,0 +1,151 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package syncmap_test
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+// This file contains reference map implementations for unit-tests.
+
+// mapInterface is the interface Map implements.
+type mapInterface interface {
+	Load(interface{}) (interface{}, bool)
+	Store(key, value interface{})
+	LoadOrStore(key, value interface{}) (actual interface{}, loaded bool)
+	Delete(interface{})
+	Range(func(key, value interface{}) (shouldContinue bool))
+}
+
+// RWMutexMap is an implementation of mapInterface using a sync.RWMutex.
+type RWMutexMap struct {
+	mu    sync.RWMutex
+	dirty map[interface{}]interface{}
+}
+
+func (m *RWMutexMap) Load(key interface{}) (value interface{}, ok bool) {
+	m.mu.RLock()
+	value, ok = m.dirty[key]
+	m.mu.RUnlock()
+	return
+}
+
+func (m *RWMutexMap) Store(key, value interface{}) {
+	m.mu.Lock()
+	if m.dirty == nil {
+		m.dirty = make(map[interface{}]interface{})
+	}
+	m.dirty[key] = value
+	m.mu.Unlock()
+}
+
+func (m *RWMutexMap) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
+	m.mu.Lock()
+	actual, loaded = m.dirty[key]
+	if !loaded {
+		actual = value
+		if m.dirty == nil {
+			m.dirty = make(map[interface{}]interface{})
+		}
+		m.dirty[key] = value
+	}
+	m.mu.Unlock()
+	return actual, loaded
+}
+
+func (m *RWMutexMap) Delete(key interface{}) {
+	m.mu.Lock()
+	delete(m.dirty, key)
+	m.mu.Unlock()
+}
+
+func (m *RWMutexMap) Range(f func(key, value interface{}) (shouldContinue bool)) {
+	m.mu.RLock()
+	keys := make([]interface{}, 0, len(m.dirty))
+	for k := range m.dirty {
+		keys = append(keys, k)
+	}
+	m.mu.RUnlock()
+
+	for _, k := range keys {
+		v, ok := m.Load(k)
+		if !ok {
+			continue
+		}
+		if !f(k, v) {
+			break
+		}
+	}
+}
+
+// DeepCopyMap is an implementation of mapInterface using a Mutex and
+// atomic.Value.  It makes deep copies of the map on every write to avoid
+// acquiring the Mutex in Load.
+type DeepCopyMap struct {
+	mu    sync.Mutex
+	clean atomic.Value
+}
+
+func (m *DeepCopyMap) Load(key interface{}) (value interface{}, ok bool) {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	value, ok = clean[key]
+	return value, ok
+}
+
+func (m *DeepCopyMap) Store(key, value interface{}) {
+	m.mu.Lock()
+	dirty := m.dirty()
+	dirty[key] = value
+	m.clean.Store(dirty)
+	m.mu.Unlock()
+}
+
+func (m *DeepCopyMap) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	actual, loaded = clean[key]
+	if loaded {
+		return actual, loaded
+	}
+
+	m.mu.Lock()
+	// Reload clean in case it changed while we were waiting on m.mu.
+	clean, _ = m.clean.Load().(map[interface{}]interface{})
+	actual, loaded = clean[key]
+	if !loaded {
+		dirty := m.dirty()
+		dirty[key] = value
+		actual = value
+		m.clean.Store(dirty)
+	}
+	m.mu.Unlock()
+	return actual, loaded
+}
+
+func (m *DeepCopyMap) Delete(key interface{}) {
+	m.mu.Lock()
+	dirty := m.dirty()
+	delete(dirty, key)
+	m.clean.Store(dirty)
+	m.mu.Unlock()
+}
+
+func (m *DeepCopyMap) Range(f func(key, value interface{}) (shouldContinue bool)) {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	for k, v := range clean {
+		if !f(k, v) {
+			break
+		}
+	}
+}
+
+func (m *DeepCopyMap) dirty() map[interface{}]interface{} {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	dirty := make(map[interface{}]interface{}, len(clean)+1)
+	for k, v := range clean {
+		dirty[k] = v
+	}
+	return dirty
+}
diff --git a/vendor/golang.org/x/sync/syncmap/map_test.go b/vendor/golang.org/x/sync/syncmap/map_test.go
new file mode 100644
index 00000000..c883f176
--- /dev/null
+++ b/vendor/golang.org/x/sync/syncmap/map_test.go
@@ -0,0 +1,172 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package syncmap_test
+
+import (
+	"math/rand"
+	"reflect"
+	"runtime"
+	"sync"
+	"testing"
+	"testing/quick"
+
+	"golang.org/x/sync/syncmap"
+)
+
+type mapOp string
+
+const (
+	opLoad        = mapOp("Load")
+	opStore       = mapOp("Store")
+	opLoadOrStore = mapOp("LoadOrStore")
+	opDelete      = mapOp("Delete")
+)
+
+var mapOps = [...]mapOp{opLoad, opStore, opLoadOrStore, opDelete}
+
+// mapCall is a quick.Generator for calls on mapInterface.
+type mapCall struct {
+	op   mapOp
+	k, v interface{}
+}
+
+func (c mapCall) apply(m mapInterface) (interface{}, bool) {
+	switch c.op {
+	case opLoad:
+		return m.Load(c.k)
+	case opStore:
+		m.Store(c.k, c.v)
+		return nil, false
+	case opLoadOrStore:
+		return m.LoadOrStore(c.k, c.v)
+	case opDelete:
+		m.Delete(c.k)
+		return nil, false
+	default:
+		panic("invalid mapOp")
+	}
+}
+
+type mapResult struct {
+	value interface{}
+	ok    bool
+}
+
+func randValue(r *rand.Rand) interface{} {
+	b := make([]byte, r.Intn(4))
+	for i := range b {
+		b[i] = 'a' + byte(rand.Intn(26))
+	}
+	return string(b)
+}
+
+func (mapCall) Generate(r *rand.Rand, size int) reflect.Value {
+	c := mapCall{op: mapOps[rand.Intn(len(mapOps))], k: randValue(r)}
+	switch c.op {
+	case opStore, opLoadOrStore:
+		c.v = randValue(r)
+	}
+	return reflect.ValueOf(c)
+}
+
+func applyCalls(m mapInterface, calls []mapCall) (results []mapResult, final map[interface{}]interface{}) {
+	for _, c := range calls {
+		v, ok := c.apply(m)
+		results = append(results, mapResult{v, ok})
+	}
+
+	final = make(map[interface{}]interface{})
+	m.Range(func(k, v interface{}) bool {
+		final[k] = v
+		return true
+	})
+
+	return results, final
+}
+
+func applyMap(calls []mapCall) ([]mapResult, map[interface{}]interface{}) {
+	return applyCalls(new(syncmap.Map), calls)
+}
+
+func applyRWMutexMap(calls []mapCall) ([]mapResult, map[interface{}]interface{}) {
+	return applyCalls(new(RWMutexMap), calls)
+}
+
+func applyDeepCopyMap(calls []mapCall) ([]mapResult, map[interface{}]interface{}) {
+	return applyCalls(new(DeepCopyMap), calls)
+}
+
+func TestMapMatchesRWMutex(t *testing.T) {
+	if err := quick.CheckEqual(applyMap, applyRWMutexMap, nil); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestMapMatchesDeepCopy(t *testing.T) {
+	if err := quick.CheckEqual(applyMap, applyDeepCopyMap, nil); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestConcurrentRange(t *testing.T) {
+	const mapSize = 1 << 10
+
+	m := new(syncmap.Map)
+	for n := int64(1); n <= mapSize; n++ {
+		m.Store(n, int64(n))
+	}
+
+	done := make(chan struct{})
+	var wg sync.WaitGroup
+	defer func() {
+		close(done)
+		wg.Wait()
+	}()
+	for g := int64(runtime.GOMAXPROCS(0)); g > 0; g-- {
+		r := rand.New(rand.NewSource(g))
+		wg.Add(1)
+		go func(g int64) {
+			defer wg.Done()
+			for i := int64(0); ; i++ {
+				select {
+				case <-done:
+					return
+				default:
+				}
+				for n := int64(1); n < mapSize; n++ {
+					if r.Int63n(mapSize) == 0 {
+						m.Store(n, n*i*g)
+					} else {
+						m.Load(n)
+					}
+				}
+			}
+		}(g)
+	}
+
+	iters := 1 << 10
+	if testing.Short() {
+		iters = 16
+	}
+	for n := iters; n > 0; n-- {
+		seen := make(map[int64]bool, mapSize)
+
+		m.Range(func(ki, vi interface{}) bool {
+			k, v := ki.(int64), vi.(int64)
+			if v%k != 0 {
+				t.Fatalf("while Storing multiples of %v, Range saw value %v", k, v)
+			}
+			if seen[k] {
+				t.Fatalf("Range visited key %v twice", k)
+			}
+			seen[k] = true
+			return true
+		})
+
+		if len(seen) != mapSize {
+			t.Fatalf("Range visited %v elements of %v-element Map", len(seen), mapSize)
+		}
+	}
+}