do not initiate multiple streams at the same path

spec/Section 6 -- Execution.md

@@ -134,21 +134,22 @@ ExecuteQuery(query, schema, variableValues, initialValue):
 
 - Let {subsequentPayloads} be an empty list.
 - Initialize {branches} to the empty set.
+- Initialize {streams} to the empty set.
 - 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 {groupedFieldSet} and {newDeferDepth} be the result of
   {CollectRootFields(queryType, selectionSet, variableValues)}.
 - Let {data} be the result of running {ExecuteGroupedFieldSet(groupedFieldSet,
-  queryType, initialValue, variableValues, subsequentPayloads, branches)}
-  _normally_ (allowing parallelization).
+  queryType, initialValue, variableValues, subsequentPayloads, branches,
+  streams)} _normally_ (allowing parallelization).
 - Let {errors} be the list of all _field error_ raised while executing the
   selection set.
 - If {newDeferDepth} is defined and {ShouldBranch(branches, groupedFieldSet,
   path)} is {true}:
   - Call {ExecuteDeferredFragment(objectType, objectValue, groupedFieldSet,
     path, newDeferDepth, variableValues, asyncRecord, subsequentPayloads,
-    branches)}
+    branches, streams)}
 - If {subsequentPayloads} is empty:
   - Return an unordered map containing {data} and {errors}.
 - If {subsequentPayloads} is not empty:
@@ -177,21 +178,22 @@ ExecuteMutation(mutation, schema, variableValues, initialValue):
 
 - Let {subsequentPayloads} be an empty list.
 - Initialize {branches} to the empty set.
+- Initialize {streams} to the empty set.
 - 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 {groupedFieldSet} and {newDeferDepth} be the result of
   {CollectRootFields(queryType, selectionSet, variableValues)}.
 - Let {data} be the result of running {ExecuteGroupedFieldSet(groupedFieldSet,
-  mutationType, initialValue, variableValues, subsequentPayloads, branches)}
-  _serially_.
+  mutationType, initialValue, variableValues, subsequentPayloads, branches,
+  streams)} _serially_.
 - Let {errors} be the list of all _field error_ raised while executing the
   selection set.
 - If {newDeferDepth} is defined and {ShouldBranch(branches, groupedFieldSet,
   path)} is {true}:
   - Call {ExecuteDeferredFragment(objectType, objectValue,
     deferredGroupFieldSet, path, newDeferDepth, variableValues, asyncRecord,
-    subsequentPayloads, branches)}
+    subsequentPayloads, branches, streams)}
 - If {subsequentPayloads} is empty:
   - Return an unordered map containing {data} and {errors}.
 - If {subsequentPayloads} is not empty:
@@ -429,7 +431,7 @@ Each represented field in the grouped field set produces an entry into a
 response map.
 
 ExecuteGroupedFieldSet(groupedFieldSet, objectType, objectValue, variableValues,
-path, subsequentPayloads, branches, asyncRecord):
+path, subsequentPayloads, branches, streams, asyncRecord):
 
 - If {path} is not provided, initialize it to an empty list, unique for this
   execution.
@@ -445,7 +447,7 @@ path, subsequentPayloads, branches, asyncRecord):
   - If {fieldType} is defined:
     - If {ShouldExecute(fieldGroup, asyncRecord)} is {true}:
       - Let {responseValue} be {ExecuteField(objectType, objectValue, fieldType,
-        fieldGroup, variableValues, path, subsequentPayloads, branches,
+        fieldGroup, variableValues, path, subsequentPayloads, branches, streams,
         asyncRecord)}.
       - Set {responseValue} as the value for {responseKey} in {resultMap}.
 - Return {resultMap}.
@@ -807,7 +809,8 @@ All Async Payload Records are structures containing:
 #### Execute Deferred Fragment
 
 ExecuteDeferredFragment(objectType, objectValue, groupedFieldSet, path,
-deferDepth, variableValues, parentRecord, subsequentPayloads, branches):
+deferDepth, variableValues, parentRecord, subsequentPayloads, branches,
+streams):
 
 - Let {deferRecord} be an async payload record created from {path} and
   {deferDepth}.
@@ -825,7 +828,7 @@ deferDepth, variableValues, parentRecord, subsequentPayloads, branches):
     - If {fieldType} is defined:
       - Let {responseValue} be {ExecuteField(objectType, objectValue, fieldType,
         fieldGroup, variableValues, path, subsequentPayloads, asyncRecord,
-        branches)}.
+        branches, streams)}.
       - Set {responseValue} as the value for {responseKey} in {resultMap}.
   - Append any encountered field errors to {errors}.
   - If {parentRecord} is defined:
@@ -890,7 +893,7 @@ finally completes that value either by recursively executing another selection
 set or coercing a scalar value.
 
 ExecuteField(objectType, objectValue, fieldType, fieldGroup, variableValues,
-path, subsequentPayloads, branches, asyncRecord):
+path, subsequentPayloads, branches, streams, asyncRecord):
 
 - Let {taggedField} be the value of the first entry in {fieldGroup}.
 - Let {field} be the corresponding entry within {taggedField}.
@@ -903,7 +906,7 @@ path, subsequentPayloads, branches, asyncRecord):
   argumentValues)}.
 - Let {result} be the result of calling {CompleteValue(fieldType, fieldGroup,
   resolvedValue, variableValues, fieldPath, subsequentPayloads, branches,
-  asyncRecord)}.
+  streams, asyncRecord)}.
 - Return {result}.
 
 ### Coercing Field Arguments
@@ -1007,7 +1010,7 @@ yielded items satisfies `initialCount` specified on the `@stream` directive.
 #### Execute Stream Field
 
 ExecuteStreamField(iterator, index, fieldGroup, innerType, path, deferDepth,
-parentRecord, variableValues, subsequentPayloads, branches):
+parentRecord, variableValues, subsequentPayloads, branches, streams):
 
 - If {parentRecord} is defined:
   - Let {deferDepth} be equal to the corresponding entry on {asyncRecord}.
@@ -1028,12 +1031,12 @@ parentRecord, variableValues, subsequentPayloads, branches):
   - Otherwise:
     - Let {item} be the item retrieved from {iterator}.
     - Let {data} be the result of calling {CompleteValue(innerType, fieldGroup,
-      item, variableValues, itemPath, subsequentPayloads, branches,
+      item, variableValues, itemPath, subsequentPayloads, branches, streams,
       parentRecord)}.
     - Append any encountered field errors to {errors}.
     - Increment {index}.
     - Call {ExecuteStreamField(iterator, index, fieldGroup, innerType, path,
-      streamRecord, variableValues, subsequentPayloads, branches)}.
+      streamRecord, variableValues, subsequentPayloads, branches, streams)}.
     - If a field error was raised, causing a {null} to be propagated to {data},
       and {innerType} is a Non-Nullable type:
       - Add an entry to {payload} named `items` with the value {null}.
@@ -1050,7 +1053,7 @@ parentRecord, variableValues, subsequentPayloads, branches):
 - Append {streamRecord} to {subsequentPayloads}.
 
 CompleteValue(fieldType, fieldGroup, result, variableValues, path,
-subsequentPayloads, asyncRecord, branches):
+subsequentPayloads, asyncRecord, branches, streams):
 
 - If the {fieldType} is a Non-Null type:
   - Let {innerType} be the inner type of {fieldType}.
@@ -1080,8 +1083,9 @@ subsequentPayloads, asyncRecord, branches):
   - While {result} is not closed:
     - If {streamDirective} is defined and {index} is greater than or equal to
       {initialCount}:
-      - Call {ExecuteStreamField(iterator, index, fieldGroup, innerType, path,
-        asyncRecord, subsequentPayloads, branches)}.
+      - If {ShouldStream(streams, fieldGroup)}:
+        - Call {ExecuteStreamField(iterator, index, fieldGroup, innerType, path,
+          asyncRecord, subsequentPayloads, branches, streams)}.
       - Return {items}.
     - Otherwise:
       - Wait for the next item from {result} via the {iterator}.
@@ -1091,7 +1095,7 @@ subsequentPayloads, asyncRecord, branches):
         {index} appended.
       - Let {resolvedItem} be the result of calling {CompleteValue(innerType,
         fieldGroup, resultItem, variableValues, itemPath, subsequentPayloads,
-        branches, asyncRecord)}.
+        branches, streams, asyncRecord)}.
       - Append {resolvedItem} to {items}.
       - Increment {index}.
   - Return {items}.
@@ -1107,12 +1111,12 @@ subsequentPayloads, asyncRecord, branches):
   - Let {resultMap} be the result of evaluating
     {ExecuteGroupedFieldSet(groupedSubfieldSet, deferredGroupedSubfieldsList
     objectType, result, variableValues, path, subsequentPayloads, branches,
-    asyncRecord)} _normally_ (allowing for parallelization).
+    streams, asyncRecord)} _normally_ (allowing for parallelization).
   - If {newDeferDepth} is defined and {ShouldBranch(branches, groupedFieldSet,
     path)} is {true}:
     - Call {ExecuteDeferredFragment(objectType, objectValue, groupedFieldSet,
       path, newDeferDepth, variableValues, asyncRecord, subsequentPayloads,
-      branches)},
+      branches, streams)},
   - Return {resultMap}.
 
 ShouldBranch(branches, path):
@@ -1121,6 +1125,12 @@ ShouldBranch(branches, path):
 - Add {path} to {branches}.
 - Return {true}.
 
+ShouldStream(streams, path):
+
+- If {streams} contains {path}, return {false}.
+- Add {path} to {streams}.
+- Return {true}.
+
 ShouldExecute(fieldGroup, asyncPayloadRecord):
 
 - Let {hasDepth} equal {false}.