Don't extend the passed in targets

If the caller passes in a value for targets, copy it before extending
it with our own targets.  Because the argument is a reference, our
extending the list here may cause the list to be extended again if
the caller creates another ReadSubunit instance reusing the same
argument.

This caused "ValueError: I/O operation on closed file" errors because
the pares_outcome callback of every previous instantiation was being
called for each new outcome.

Change-Id: Iadd7b063e61590869dbea6ce9db9ffe611fbae0a

subunit2sql/read_subunit.py

@@ -37,6 +37,8 @@ class ReadSubunit(object):
                  targets=None):
         if targets is None:
             targets = []
+        else:
+            targets = targets[:]
         self.stream_file = stream_file
         self.stream = subunit.ByteStreamToStreamResult(self.stream_file)
         starts = testtools.StreamResult()

subunit2sql/tests/test_read_subunit.py

@@ -175,3 +175,18 @@ class TestReadSubunit(base.TestCase):
     def test_targets_added_to_result(self, ttc_mock):
         subunit.ReadSubunit(mock.MagicMock(), targets=['foo'])
         self.assertIn('foo', ttc_mock.call_args[0][0])
+
+    @mock.patch('testtools.CopyStreamResult')
+    def test_targets_not_modified(self, ttc_mock):
+        # A regression test that verifies that the subunit reader does
+        # not modify the passed in value of targets.
+        targets = ['foo']
+
+        subunit.ReadSubunit(mock.MagicMock(), targets=targets)
+        ntargets1 = len(ttc_mock.call_args[0][0])
+
+        subunit.ReadSubunit(mock.MagicMock(), targets=targets)
+        ntargets2 = len(ttc_mock.call_args[0][0])
+
+        self.assertEqual(ntargets1, ntargets2)
+        self.assertEqual(targets, ['foo'])