WIP rewrite the daemon

cmd subdir does not build on purpose, it's only left in tree to grab old
code and move it to github.com/zrepl/zrepl/daemon

daemon/nethelpers/helpers.go

@@ -0,0 +1,49 @@
+package nethelpers
+
+import (
+	"github.com/pkg/errors"
+	"net"
+	"os"
+	"path/filepath"
+)
+
+func PreparePrivateSockpath(sockpath string) error {
+	sockdir := filepath.Dir(sockpath)
+	sdstat, err := os.Stat(sockdir)
+	if err != nil {
+		return errors.Wrapf(err, "cannot stat(2) '%s'", sockdir)
+	}
+	if !sdstat.IsDir() {
+		return errors.Errorf("not a directory: %s", sockdir)
+	}
+	p := sdstat.Mode().Perm()
+	if p&0007 != 0 {
+		return errors.Errorf("socket directory must not be world-accessible: %s (permissions are %#o)", sockdir, p)
+	}
+
+	// Maybe things have not been cleaned up before
+	s, err := os.Stat(sockpath)
+	if os.IsNotExist(err) {
+		return nil
+	}
+	if err != nil {
+		return errors.Wrapf(err, "cannot stat(2) '%s'", sockpath)
+	}
+	if s.Mode()&os.ModeSocket == 0 {
+		return errors.Errorf("unexpected file type at path '%s'", sockpath)
+	}
+	err = os.Remove(sockpath)
+	if err != nil {
+		return errors.Wrapf(err, "cannot remove presumably stale socket '%s'", sockpath)
+	}
+	return nil
+}
+
+func ListenUnixPrivate(sockaddr *net.UnixAddr) (*net.UnixListener, error) {
+
+	if err := PreparePrivateSockpath(sockaddr.Name); err != nil {
+		return nil, err
+	}
+
+	return net.ListenUnix("unix", sockaddr)
+}