Extract common logic from ExecuteQuery, ExecuteMutation and ExecuteSubscriptionEvent

Author: benjie

spec/Section 6 -- Execution.md

@@ -131,12 +131,8 @@ ExecuteQuery(query, schema, variableValues, initialValue):
 - Let {queryType} be the root Query type in {schema}.
 - Assert: {queryType} is an Object type.
 - Let {selectionSet} be the top level Selection Set in {query}.
-- Let {data} be the result of running {ExecuteSelectionSet(selectionSet,
-  queryType, initialValue, variableValues)} _normally_ (allowing
-  parallelization).
-- Let {errors} be the list of all _field error_ raised while executing the
-  selection set.
-- Return an unordered map containing {data} and {errors}.
+- Return {ExecuteRootSelectionSet(variableValues, initialValue, queryType,
+  selectionSet)}.
 
 ### Mutation
 
@@ -153,11 +149,8 @@ ExecuteMutation(mutation, schema, variableValues, initialValue):
 - Let {mutationType} be the root Mutation type in {schema}.
 - Assert: {mutationType} is an Object type.
 - Let {selectionSet} be the top level Selection Set in {mutation}.
-- Let {data} be the result of running {ExecuteSelectionSet(selectionSet,
-  mutationType, initialValue, variableValues)} _serially_.
-- Let {errors} be the list of all _field error_ raised while executing the
-  selection set.
-- Return an unordered map containing {data} and {errors}.
+- Return {ExecuteRootSelectionSet(variableValues, initialValue, mutationType,
+  selectionSet, true)}.
 
 ### Subscription
 
@@ -300,12 +293,8 @@ ExecuteSubscriptionEvent(subscription, schema, variableValues, initialValue):
 - Let {subscriptionType} be the root Subscription type in {schema}.
 - Assert: {subscriptionType} is an Object type.
 - Let {selectionSet} be the top level Selection Set in {subscription}.
-- Let {data} be the result of running {ExecuteSelectionSet(selectionSet,
-  subscriptionType, initialValue, variableValues)} _normally_ (allowing
-  parallelization).
-- Let {errors} be the list of all _field error_ raised while executing the
-  selection set.
-- Return an unordered map containing {data} and {errors}.
+- Return {ExecuteRootSelectionSet(variableValues, initialValue,
+  subscriptionType, selectionSet)}.
 
 Note: The {ExecuteSubscriptionEvent()} algorithm is intentionally similar to
 {ExecuteQuery()} since this is how each event result is produced.
@@ -321,6 +310,27 @@ Unsubscribe(responseStream):
 
 - Cancel {responseStream}
 
+## Executing the Root Selection Set
+
+To execute the root selection set, the object value being evaluated and the
+object type need to be known, as well as whether it must be executed serially,
+or may be executed in parallel.
+
+Executing the root selection set works similarly for queries (parallel),
+mutations (serial), and subscriptions (where it is executed for each event in
+the underlying Source Stream).
+
+ExecuteRootSelectionSet(variableValues, initialValue, objectType, selectionSet,
+serial):
+
+- If {serial} is not provided, initialize it to {false}.
+- Let {data} be the result of running {ExecuteSelectionSet(selectionSet,
+  objectType, initialValue, variableValues)} _serially_ if {serial} is {true},
+  _normally_ (allowing parallelization) otherwise.
+- Let {errors} be the list of all _field error_ raised while executing the
+  selection set.
+- Return an unordered map containing {data} and {errors}.
+
 ## Executing Selection Sets
 
 To execute a selection set, the object value being evaluated and the object type