@@ -7,6 +7,7 @@ import (

 	"io/ioutil"

 	"os"

 	"regexp"

+	"sync"

 )

 // Record Structure for a single host record

@@ -21,14 +22,47 @@ func (r Record) WriteTo(w io.Writer) (int64, error) {

 	return int64(n), err

 }

-// Default hosts config records slice

-var defaultContent = []Record{

-	{Hosts: "localhost", IP: "127.0.0.1"},

-	{Hosts: "localhost ip6-localhost ip6-loopback", IP: "::1"},

-	{Hosts: "ip6-localnet", IP: "fe00::0"},

-	{Hosts: "ip6-mcastprefix", IP: "ff00::0"},

-	{Hosts: "ip6-allnodes", IP: "ff02::1"},

-	{Hosts: "ip6-allrouters", IP: "ff02::2"},

+var (

+	// Default hosts config records slice

+	defaultContent = []Record{

+		{Hosts: "localhost", IP: "127.0.0.1"},

+		{Hosts: "localhost ip6-localhost ip6-loopback", IP: "::1"},

+		{Hosts: "ip6-localnet", IP: "fe00::0"},

+		{Hosts: "ip6-mcastprefix", IP: "ff00::0"},

+		{Hosts: "ip6-allnodes", IP: "ff02::1"},

+		{Hosts: "ip6-allrouters", IP: "ff02::2"},

+	}

+

+	// A cache of path level locks for synchronizing /etc/hosts

+	// updates on a file level

+	pathMap = make(map[string]*sync.Mutex)

+

+	// A package level mutex to synchronize the cache itself

+	pathMutex sync.Mutex

+)

+

+func pathLock(path string) func() {

+	pathMutex.Lock()

+	defer pathMutex.Unlock()

+

+	pl, ok := pathMap[path]

+	if !ok {

+		pl = &sync.Mutex{}

+		pathMap[path] = pl

+	}

+

+	pl.Lock()

+	return func() {

+		pl.Unlock()

+	}

+}

+

+// Drop drops the path string from the path cache

+func Drop(path string) {

+	pathMutex.Lock()

+	defer pathMutex.Unlock()

+

+	delete(pathMap, path)

 }

 // Build function

@@ -36,6 +70,8 @@ var defaultContent = []Record{

 // IP, hostname, and domainname set main record leave empty for no master record

 // extraContent is an array of extra host records.

 func Build(path, IP, hostname, domainname string, extraContent []Record) error {

+	defer pathLock(path)()

+

 	content := bytes.NewBuffer(nil)

 	if IP != "" {

 		//set main record

@@ -68,6 +104,8 @@ func Build(path, IP, hostname, domainname string, extraContent []Record) error {

 // Add adds an arbitrary number of Records to an already existing /etc/hosts file

 func Add(path string, recs []Record) error {

+	defer pathLock(path)()

+

 	if len(recs) == 0 {

 		return nil

 	}

@@ -95,6 +133,8 @@ func Add(path string, recs []Record) error {

 // Delete deletes an arbitrary number of Records already existing in /etc/hosts file

 func Delete(path string, recs []Record) error {

+	defer pathLock(path)()

+

 	if len(recs) == 0 {

 		return nil

 	}

@@ -118,6 +158,8 @@ func Delete(path string, recs []Record) error {

 // IP is new IP address

 // hostname is hostname to search for to replace IP

 func Update(path, IP, hostname string) error {

+	defer pathLock(path)()

+

 	old, err := ioutil.ReadFile(path)

 	if err != nil {

 		return err

@@ -2,8 +2,10 @@ package etchosts

 import (

 	"bytes"

+	"fmt"

 	"io/ioutil"

 	"os"

+	"sync"

 	"testing"

 	_ "github.com/docker/libnetwork/testutils"

@@ -247,3 +249,61 @@ func TestDelete(t *testing.T) {

 		t.Fatalf("Did not expect to find '%s' got '%s'", expected, content)

 	}

 }

+

+func TestConcurrentWrites(t *testing.T) {

+	file, err := ioutil.TempFile("", "")

+	if err != nil {

+		t.Fatal(err)

+	}

+	defer os.Remove(file.Name())

+

+	err = Build(file.Name(), "", "", "", nil)

+	if err != nil {

+		t.Fatal(err)

+	}

+

+	if err := Add(file.Name(), []Record{

+		Record{

+			Hosts: "inithostname",

+			IP:    "172.17.0.1",

+		},

+	}); err != nil {

+		t.Fatal(err)

+	}

+

+	var wg sync.WaitGroup

+	for i := 0; i < 10; i++ {

+		wg.Add(1)

+		go func() {

+			defer wg.Done()

+

+			rec := []Record{

+				Record{

+					IP:    fmt.Sprintf("%d.%d.%d.%d", i, i, i, i),

+					Hosts: fmt.Sprintf("testhostname%d", i),

+				},

+			}

+

+			for j := 0; j < 25; j++ {

+				if err := Add(file.Name(), rec); err != nil {

+					t.Fatal(err)

+				}

+

+				if err := Delete(file.Name(), rec); err != nil {

+					t.Fatal(err)

+				}

+			}

+		}()

+	}

+

+	wg.Wait()

+

+	content, err := ioutil.ReadFile(file.Name())

+	if err != nil {

+		t.Fatal(err)

+	}

+

+	if expected := "172.17.0.1\tinithostname\n"; !bytes.Contains(content, []byte(expected)) {

+		t.Fatalf("Expected to find '%s' got '%s'", expected, content)

+	}

+}

@@ -182,6 +182,10 @@ func (sb *sandbox) Delete() error {

 		}

 	}

+	// Container is going away. Path cache in etchosts is most

+	// likely not required any more. Drop it.

+	etchosts.Drop(sb.config.hostsPath)

+

 	if sb.osSbox != nil {

 		sb.osSbox.Destroy()

 	}