Fix false positive validation error from fragment cycle when unknown fragment is used

Closes #36.

Author: jhgg

graphql/core/validation/rules/no_fragment_cycles.py

@@ -22,7 +22,9 @@ def enter_FragmentDefinition(self, node, key, parent, path, ancestors):
         spread_path = []
 
         def detect_cycle_recursive(fragment_name):
-            spread_nodes = self.spreads_in_fragment[fragment_name]
+            spread_nodes = self.spreads_in_fragment.get(fragment_name)
+            if not spread_nodes:
+                return
 
             for spread_node in spread_nodes:
                 if spread_node in self.known_to_lead_to_cycle:

tests/core_validation/test_no_fragment_cycles.py

@@ -45,6 +45,14 @@ def test_double_spread_within_abstract_types():
     ''')
 
 
+def test_does_not_raise_false_positive_on_unknown_fragment():
+    expect_passes_rule(NoFragmentCycles, '''
+      fragment nameFragment on Pet {
+        ...UnknownFragment
+      }
+    ''')
+
+
 def test_spreading_recursively_within_field_fails():
     expect_fails_rule(NoFragmentCycles, '''
     fragment fragA on Human { relatives { ...fragA } },