Bring FakeData utility over from glance

This brings over the small-footprint FakeData generator from glance and
refactors the rbd resize tests to use it instead of a static string.

Change-Id: I842578ca933294e4340ba080be488999e7910727

3 files changed, 82 insertions, 2 deletions

diff --git a/glance_store/tests/unit/test_rbd_store.py b/glance_store/tests/unit/test_rbd_store.py
index 4f44bc2..fe6a2ac 100644
--- a/glance_store/tests/unit/test_rbd_store.py
+++ b/glance_store/tests/unit/test_rbd_store.py
@@ -24,6 +24,7 @@ from glance_store import exceptions
 from glance_store import location as g_location
 from glance_store.tests import base
 from glance_store.tests.unit import test_store_capabilities
+from glance_store.tests import utils as test_utils
 
 
 class TestException(Exception):
@@ -189,9 +190,8 @@ class TestReSize(base.StoreBaseTest,
 
     def test_add_w_image_size_zero_less_resizes(self):
         """Assert that correct size is returned even though 0 was provided."""
-        # TODO(jokke): use the FakeData iterator once it exists.
         data_len = 57 * units.Mi
-        data_iter = six.BytesIO(b'*' * data_len)
+        data_iter = test_utils.FakeData(data_len)
         with mock.patch.object(rbd_store.rbd.Image, 'resize') as resize:
             with mock.patch.object(rbd_store.rbd.Image, 'write') as write:
                 ret = self.store.add(
diff --git a/glance_store/tests/unit/test_test_utils.py b/glance_store/tests/unit/test_test_utils.py
new file mode 100644
index 0000000..81495a2
--- /dev/null
+++ b/glance_store/tests/unit/test_test_utils.py
@@ -0,0 +1,38 @@
+# Copyright 2020 Red Hat, Inc
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from glance_store.tests import base
+from glance_store.tests import utils as test_utils
+
+
+class TestFakeData(base.StoreBaseTest):
+    def test_via_read(self):
+        fd = test_utils.FakeData(1024)
+        data = []
+        for i in range(0, 1025, 256):
+            chunk = fd.read(256)
+            data.append(chunk)
+            if not chunk:
+                break
+
+        self.assertEqual(5, len(data))
+        # Make sure we got a zero-length final read
+        self.assertEqual(b'', data[-1])
+        # Make sure we only got 1024 bytes
+        self.assertEqual(1024, len(b''.join(data)))
+
+    def test_via_iter(self):
+        data = b''.join(list(test_utils.FakeData(1024)))
+        self.assertEqual(1024, len(data))
diff --git a/glance_store/tests/utils.py b/glance_store/tests/utils.py
index 2f3a90f..5180bdd 100644
--- a/glance_store/tests/utils.py
+++ b/glance_store/tests/utils.py
@@ -16,6 +16,7 @@
 import six
 from six.moves import urllib
 
+from oslo_utils import units
 import requests
 
 
@@ -73,3 +74,44 @@ def fake_response(status_code=200, headers=None, content=None, **kwargs):
     r.headers = headers or {}
     r.raw = FakeHTTPResponse(status_code, headers, content, kwargs)
     return r
+
+
+class FakeData(object):
+    """Generate a bunch of data without storing it in memory.
+
+    This acts like a read-only file object which generates fake data
+    in chunks when read() is called or it is used as a generator. It
+    can generate an arbitrary amount of data without storing it in
+    memory.
+
+    :param length: The number of bytes to generate
+    :param chunk_size: The chunk size to return in iteration mode, or when
+                       read() is called unbounded
+
+    """
+    def __init__(self, length, chunk_size=64 * units.Ki):
+        self._max = length
+        self._chunk_size = chunk_size
+        self._len = 0
+
+    def read(self, length=None):
+        if length is None:
+            length = self._chunk_size
+
+        length = min(length, self._max - self._len)
+
+        self._len += length
+        if length == 0:
+            return b''
+        else:
+            return b'0' * length
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        r = self.read()
+        if len(r) == 0:
+            raise StopIteration()
+        else:
+            return r