cmd: extract listening on a UNIX socket in a private directory into a helper func

cmd/helpers.go

@@ -0,0 +1,26 @@
+package cmd
+
+import (
+	"github.com/pkg/errors"
+	"net"
+	"os"
+	"path/filepath"
+)
+
+func ListenUnixPrivate(sockaddr *net.UnixAddr) (*net.UnixListener, error) {
+
+	sockdir := filepath.Dir(sockaddr.Name)
+	sdstat, err := os.Stat(sockdir)
+	if err != nil {
+		return nil, errors.Wrapf(err, "cannot stat(2) '%s'", sockdir)
+	}
+	if !sdstat.IsDir() {
+		return nil, errors.Errorf("%s is not a directory: %s", sockdir)
+	}
+	p := sdstat.Mode().Perm()
+	if p&0007 != 0 {
+		return nil, errors.Errorf("%s must not be world-accessible (permissions are %#o)", p)
+	}
+
+	return net.ListenUnix("unix", sockaddr)
+}