utils: Add function to allow copying with detach

Signed-off-by: Mrunal Patel <mpatel@redhat.com>

utils/utils.go

@@ -79,3 +79,63 @@ func newProp(name string, units interface{}) systemdDbus.Property {
 		Value: dbus.MakeVariant(units),
 	}
 }
+
+// DetachError is special error which returned in case of container detach.
+type DetachError struct{}
+
+func (DetachError) Error() string {
+	return "detached from container"
+}
+
+// CopyDetachable is similar to io.Copy but support a detach key sequence to break out.
+func CopyDetachable(dst io.Writer, src io.Reader, keys []byte) (written int64, err error) {
+	if len(keys) == 0 {
+		// Default keys : ctrl-p ctrl-q
+		keys = []byte{16, 17}
+	}
+
+	buf := make([]byte, 32*1024)
+	for {
+		nr, er := src.Read(buf)
+		if nr > 0 {
+			preservBuf := []byte{}
+			for i, key := range keys {
+				preservBuf = append(preservBuf, buf[0:nr]...)
+				if nr != 1 || buf[0] != key {
+					break
+				}
+				if i == len(keys)-1 {
+					// src.Close()
+					return 0, DetachError{}
+				}
+				nr, er = src.Read(buf)
+			}
+			var nw int
+			var ew error
+			if len(preservBuf) > 0 {
+				nw, ew = dst.Write(preservBuf)
+				nr = len(preservBuf)
+			} else {
+				nw, ew = dst.Write(buf[0:nr])
+			}
+			if nw > 0 {
+				written += int64(nw)
+			}
+			if ew != nil {
+				err = ew
+				break
+			}
+			if nr != nw {
+				err = io.ErrShortWrite
+				break
+			}
+		}
+		if er != nil {
+			if er != io.EOF {
+				err = er
+			}
+			break
+		}
+	}
+	return written, err
+}