Allow index-stats to compare non-existing path

With this commit we make the index-stats operation more lenient so it
can check conditions for paths that don't (yet) exist. One use case for
this is to wait until a certain index shows up in the stats.

Relates elastic#925

esrally/driver/runner.py

@@ -608,10 +608,15 @@ class IndicesStats(Runner):
     """
 
     def _get(self, v, path):
-        if len(path) == 1:
-            return v[path[0]]
+        if v is None:
+            return None
+        elif len(path) == 1:
+            return v.get(path[0])
         else:
-            return self._get(v[path[0]], path[1:])
+            return self._get(v.get(path[0]), path[1:])
+
+    def _safe_string(self, v):
+        return str(v) if v is not None else None
 
     def __call__(self, es, params):
         index = params.get("index", "_all")
@@ -628,8 +633,8 @@ def __call__(self, es, params):
                 "condition": {
                     "path": path,
                     # avoid mapping issues in the ES metrics store by always rendering values as strings
-                    "actual-value": str(actual_value),
-                    "expected-value": str(expected_value)
+                    "actual-value": self._safe_string(actual_value),
+                    "expected-value": self._safe_string(expected_value)
                 },
                 # currently we only support "==" as a predicate but that might change in the future
                 "success": actual_value == expected_value

tests/driver/runner_test.py

@@ -907,6 +907,39 @@ def test_indices_stats_with_successful_condition(self, es):
 
         es.indices.stats.assert_called_once_with(index="logs-*", metric="_all")
 
+    @mock.patch("elasticsearch.Elasticsearch")
+    def test_indices_stats_with_non_existing_path(self, es):
+        es.indices.stats.return_value = {
+            "indices": {
+                "total": {
+                    "docs": {
+                        "current": 0
+                    }
+                }
+            }
+        }
+
+        indices_stats = runner.IndicesStats()
+
+        result = indices_stats(es, params={
+            "index": "logs-*",
+            "condition": {
+                # non-existing path
+                "path": "indices.my_index.total.docs.count",
+                "expected-value": 0
+            }
+        })
+        self.assertEqual(1, result["weight"])
+        self.assertEqual("ops", result["unit"])
+        self.assertFalse(result["success"])
+        self.assertDictEqual({
+            "path": "indices.my_index.total.docs.count",
+            "actual-value": None,
+            "expected-value": "0"
+        }, result["condition"])
+
+        es.indices.stats.assert_called_once_with(index="logs-*", metric="_all")
+
 
 class QueryRunnerTests(TestCase):
     @mock.patch("elasticsearch.Elasticsearch")