diff --git a/pkt-line.c b/pkt-line.c
index 7296731cf3..555eb2a507 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -215,6 +215,22 @@ void packet_buf_write(struct strbuf *buf, const char *fmt, ...)
 	va_end(args);
 }
 
+void packet_buf_write_len(struct strbuf *buf, const char *data, size_t len)
+{
+	size_t orig_len, n;
+
+	orig_len = buf->len;
+	strbuf_addstr(buf, "0000");
+	strbuf_add(buf, data, len);
+	n = buf->len - orig_len;
+
+	if (n > LARGE_PACKET_MAX)
+		die("protocol error: impossibly long line");
+
+	set_packet_header(&buf->buf[orig_len], n);
+	packet_trace(data, len, 1);
+}
+
 int write_packetized_from_fd(int fd_in, int fd_out)
 {
 	static char buf[LARGE_PACKET_DATA_MAX];
diff --git a/pkt-line.h b/pkt-line.h
index 9570bd7a0a..5b28d43472 100644
--- a/pkt-line.h
+++ b/pkt-line.h
@@ -26,6 +26,7 @@ void packet_buf_flush(struct strbuf *buf);
 void packet_buf_delim(struct strbuf *buf);
 void packet_write(int fd_out, const char *buf, size_t size);
 void packet_buf_write(struct strbuf *buf, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
+void packet_buf_write_len(struct strbuf *buf, const char *data, size_t len);
 int packet_flush_gently(int fd);
 int packet_write_fmt_gently(int fd, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
 int write_packetized_from_fd(int fd_in, int fd_out);