Resurrecting test_reading_legacy_dataset test

It was removed by mistake. The test actually tests an ability to read a
dataset created with previous version of petastorm.

petastorm/etl/safe_pickle.py

@@ -23,6 +23,8 @@
     "pyspark",
     "decimal",
     "builtins",
+    "copy_reg",
+    "__builtin__",
 }
 
 

petastorm/tests/test_reading_legacy_datasets.py

@@ -0,0 +1,38 @@
+#  Copyright (c) 2017-2018 Uber Technologies, Inc.
+#
+# 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.
+import os
+
+import pytest
+
+from petastorm import make_reader
+
+
+def dataset_urls():
+    """Returns a list of legacy datasets available for testing"""
+    legacy_data_directory = os.path.join(os.path.dirname(__file__), 'data', 'legacy')
+    versions = os.listdir(legacy_data_directory)
+    urls = ['file://' + os.path.join(legacy_data_directory, v) for v in versions]
+    return urls
+
+
+@pytest.mark.parametrize('legacy_dataset_url', dataset_urls())
+def test_reading_legacy_dataset(legacy_dataset_url):
+    """The test runs for a single legacy dataset. Opens the dataset using `make_reader` and reads all records from it"""
+    with make_reader(legacy_dataset_url, workers_count=1) as reader:
+        all_data = list(reader)
+
+        # Some basic check on the data
+        assert len(all_data) == 100
+        assert len(all_data[0]._fields) > 5
+        assert all_data[0].matrix.shape == (32, 16, 3)