Fix how Mistral prepares data for evaluating a YAQL expression

* If we use the built-in YAQL function 'str' in a workflow then it
  doesn't represent lists as '[item1, item3, ...]' but instead
  creates '(item1, item2,...). This is because the standard YAQL
  function 'yaql_utils.convert_input_data', which is needed to
  convert a initial user data into an internal YAQL format,
  converts all sequences (except strings) into tuples.
  This patch overrides this behavior for sequences that are not
  strings and tuples so that they now get converted into lists.
  YAQL uses tuples because it needs to obtain a safe immutable
  structure to make calculations upon. But in Mistral list is
  more suitable because lots of users care about string
  representations. Immutability is not so important because
  Mistral code base guarantees that the initial data context
  for an expression won't be changed while an expression is
  being evaluated by YAQL.
* "str" YAQL function used to work well but it was broken in
  https://review.openstack.org/#/c/477816/ that added additional
  context preparation in order to fix the issue
  https://bugs.launchpad.net/mistral/+bug/1772864

Change-Id: I69d32f8772418d586d6c414842bb54aada217481
Closes-Bug: #1815710

mistral/tests/unit/engine/test_yaql_functions.py

@@ -13,6 +13,7 @@
 #    limitations under the License.
 
 from oslo_config import cfg
+import six
 
 from mistral.db.v2 import api as db_api
 from mistral.services import workflows as wf_service
@@ -431,3 +432,42 @@ def test_yaml_dump_function(self):
         self.assertIsNotNone(json_str)
         self.assertIn('"key1": "foo"', json_str)
         self.assertIn('"key2": "bar"', json_str)
+
+    def test_built_in_str_function(self):
+        wf_text = """---
+        version: '2.0'
+
+        wf:
+          input:
+            - my_list
+
+          tasks:
+            task1:
+              publish:
+                val: <% str($.my_list) %>
+        """
+
+        wf_service.create_workflows(wf_text)
+
+        wf_ex = self.engine.start_workflow(
+            'wf',
+            wf_input={
+                'my_list': [
+                    {
+                        'k1': 'v1',
+                        'k2': 'v2'
+                    }
+                ]
+            }
+        )
+
+        self.await_workflow_success(wf_ex.id)
+
+        with db_api.transaction(read_only=True):
+            wf_ex = db_api.get_workflow_execution(wf_ex.id)
+
+            val = wf_ex.task_executions[0].published['val']
+
+            self.assertIsInstance(val, six.string_types)
+            self.assertIn('[', val)
+            self.assertIn(']', val)

mistral/utils/expression_utils.py

@@ -14,6 +14,7 @@
 #    limitations under the License.
 
 from functools import partial
+import six
 import warnings
 
 from oslo_log import log as logging
@@ -39,6 +40,27 @@
 ROOT_YAQL_CONTEXT = None
 
 
+def _convert_yaql_input_data(obj, rec=None):
+    # NOTE(rakhmerov): We have to define our own wrapper function
+    # around the function 'convert_input_data' from 'yaql_utils'
+    # because the latter always converts all sequences (except strings)
+    # into tuples, and it in turn breaks a number of things. For example,
+    # if we use the built-in 'str' YAQL function with an argument of the
+    # type 'list' then the result will be '(item1, item2 ..., itemN,)'
+    # instead of '[item1, item2 ..., itemN]'.
+    # So we override this behavior for sequences that are not strings and
+    # tuples.
+    if rec is None:
+        rec = _convert_yaql_input_data
+
+    if (isinstance(obj, yaql_utils.SequenceType) and
+            not isinstance(obj, six.string_types) and
+            not isinstance(obj, tuple)):
+        return list(rec(t, rec) for t in obj)
+    else:
+        return yaql_utils.convert_input_data(obj, rec)
+
+
 def get_yaql_context(data_context):
     global ROOT_YAQL_CONTEXT
 
@@ -48,7 +70,7 @@ def get_yaql_context(data_context):
         _register_yaql_functions(ROOT_YAQL_CONTEXT)
 
     new_ctx = ROOT_YAQL_CONTEXT.create_child_context()
-    new_ctx['$'] = yaql_utils.convert_input_data(data_context)
+    new_ctx['$'] = _convert_yaql_input_data(data_context)
 
     if isinstance(data_context, dict):
         new_ctx['__env'] = data_context.get('__env')