tests/docker: fix update command due to python3 str/bytes distinction

Does this seem convoluted to you? It feels a little complicated to me.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200724064509.331-10-alex.bennee@linaro.org>

1 files changed, 7 insertions, 6 deletions

diff --git a/tests/docker/docker.py b/tests/docker/docker.py
index c9f20d8d09..356d7618f1 100755
--- a/tests/docker/docker.py
+++ b/tests/docker/docker.py
@@ -24,7 +24,7 @@ import tempfile
 import re
 import signal
 from tarfile import TarFile, TarInfo
-from io import StringIO
+from io import StringIO, BytesIO
 from shutil import copy, rmtree
 from pwd import getpwuid
 from datetime import datetime, timedelta
@@ -541,13 +541,14 @@ class UpdateCommand(SubCommand):
 
         # Create a Docker buildfile
         df = StringIO()
-        df.write("FROM %s\n" % args.tag)
-        df.write("ADD . /\n")
-        df.seek(0)
+        df.write(u"FROM %s\n" % args.tag)
+        df.write(u"ADD . /\n")
+
+        df_bytes = BytesIO(bytes(df.getvalue(), "UTF-8"))
 
         df_tar = TarInfo(name="Dockerfile")
-        df_tar.size = len(df.buf)
-        tmp_tar.addfile(df_tar, fileobj=df)
+        df_tar.size = df_bytes.getbuffer().nbytes
+        tmp_tar.addfile(df_tar, fileobj=df_bytes)
 
         tmp_tar.close()