Extract sockets initialization to a package.

Because I just used it somewhere else and it would be nice if I didn't have to copy and paste the code.

Signed-off-by: David Calavera <david.calavera@gmail.com>

sockets/unix_socket.go

@@ -0,0 +1,80 @@
+// +build linux
+
+package sockets
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"strconv"
+	"syscall"
+
+	"github.com/Sirupsen/logrus"
+	"github.com/docker/docker/pkg/listenbuffer"
+	"github.com/docker/libcontainer/user"
+)
+
+func NewUnixSocket(path, group string, activate <-chan struct{}) (net.Listener, error) {
+	if err := syscall.Unlink(path); err != nil && !os.IsNotExist(err) {
+		return nil, err
+	}
+	mask := syscall.Umask(0777)
+	defer syscall.Umask(mask)
+	l, err := listenbuffer.NewListenBuffer("unix", path, activate)
+	if err != nil {
+		return nil, err
+	}
+	if err := setSocketGroup(path, group); err != nil {
+		l.Close()
+		return nil, err
+	}
+	if err := os.Chmod(path, 0660); err != nil {
+		l.Close()
+		return nil, err
+	}
+	return l, nil
+}
+
+func setSocketGroup(path, group string) error {
+	if group == "" {
+		return nil
+	}
+	if err := changeGroup(path, group); err != nil {
+		if group != "docker" {
+			return err
+		}
+		logrus.Debugf("Warning: could not change group %s to docker: %v", path, err)
+	}
+	return nil
+}
+
+func changeGroup(path string, nameOrGid string) error {
+	gid, err := lookupGidByName(nameOrGid)
+	if err != nil {
+		return err
+	}
+	logrus.Debugf("%s group found. gid: %d", nameOrGid, gid)
+	return os.Chown(path, 0, gid)
+}
+
+func lookupGidByName(nameOrGid string) (int, error) {
+	groupFile, err := user.GetGroupPath()
+	if err != nil {
+		return -1, err
+	}
+	groups, err := user.ParseGroupFileFilter(groupFile, func(g user.Group) bool {
+		return g.Name == nameOrGid || strconv.Itoa(g.Gid) == nameOrGid
+	})
+	if err != nil {
+		return -1, err
+	}
+	if groups != nil && len(groups) > 0 {
+		return groups[0].Gid, nil
+	}
+	gid, err := strconv.Atoi(nameOrGid)
+	if err == nil {
+		logrus.Warnf("Could not find GID %d", gid)
+		return gid, nil
+	}
+	return -1, fmt.Errorf("Group %s not found", nameOrGid)
+}