watch: Avoid double-decoding in python3

Both iter_content and the first line of the loop in the
etcd3gw watch code were decoding the chunk received.

In python2, this works fine:
```
$ python2 -c "print(b'hello_world'.decode().decode('utf-8'))"
hello_world
```

In python3, it raises an AttributeError:
```
$ python3 -c "print(b'hello_world'.decode().decode('utf-8'))"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AttributeError: 'str' object has no attribute 'decode'
```

Thus, etcd3gw's watch is broken in python3.  This commit
fixes it by only decoding the line once.

Signed-off-by: Tyler J. Stachecki <tstachecki@bloomberg.net>
Change-Id: I203574a1ef4996a43860350be59fbe208457562d
Closes-Bug: 1950517

etcd3gw/tests/test_etcd3gw.py

@@ -17,12 +17,16 @@ test_etcd3-gateway
 Tests for `etcd3gw` module.
 """
 
+import base64
+import json
+import requests
 import six
 import threading
 import time
 import uuid
 
 from testtools.testcase import unittest
+from unittest import mock
 import urllib3
 
 from etcd3gw.client import Etcd3Client
@@ -404,3 +408,30 @@ class TestEtcd3Gateway(base.TestCase):
         keys = lease.keys()
         self.assertEqual(1, len(keys))
         self.assertIn(six.b(key), keys)
+
+    def my_iter_content(self, *args, **kwargs):
+        payload = json.dumps({
+            'result': {
+                'events': [{
+                    'kv': {'key': base64.b64encode(b'value').decode('utf-8')},
+                }]
+            }
+        })
+
+        if not kwargs.get('decode_unicode', False):
+            payload = payload.encode()
+        return [payload]
+
+    @mock.patch.object(requests.Response, 'iter_content', new=my_iter_content)
+    @mock.patch.object(requests.sessions.Session, 'post')
+    def test_watch_unicode(self, mock_post):
+        mocked_response = requests.Response()
+        mocked_response.connection = mock.Mock()
+        mock_post.return_value = mocked_response
+
+        try:
+            res = self.client.watch_once('/some/key', timeout=1)
+        except exceptions.WatchTimedOut:
+            self.fail("watch timed out when server responded with unicode")
+
+        self.assertEqual(res, {'kv': {'key': b'value'}})

etcd3gw/watch.py

@@ -19,7 +19,7 @@ from etcd3gw.utils import _get_threadpool_executor
 
 
 def _watch(resp, callback):
-    for line in resp.iter_content(chunk_size=None, decode_unicode=True):
+    for line in resp.iter_content(chunk_size=None, decode_unicode=False):
         decoded_line = line.decode('utf-8')
         payload = json.loads(decoded_line)
         if 'created' in payload['result']: