Add defer/stream validation rules

Author: robrichard

spec/Section 5 -- Validation.md

@@ -422,6 +422,7 @@ FieldsInSetCanMerge(set):
   {set} including visiting fragments and inline fragments.
 - Given each pair of members {fieldA} and {fieldB} in {fieldsForName}:
   - {SameResponseShape(fieldA, fieldB)} must be true.
+  - {SameStreamDirective(fieldA, fieldB)} must be true.
   - If the parent types of {fieldA} and {fieldB} are equal or if either is not
     an Object Type:
     - {fieldA} and {fieldB} must have identical field names.
@@ -456,6 +457,16 @@ SameResponseShape(fieldA, fieldB):
   - If {SameResponseShape(subfieldA, subfieldB)} is {false}, return {false}.
 - Return {true}.
 
+SameStreamDirective(fieldA, fieldB):
+
+- If neither {fieldA} nor {fieldB} has a directive named `stream`.
+  - Return {true}.
+- If both {fieldA} and {fieldB} have a directive named `stream`.
+  - Let {streamA} be the directive named `stream` on {fieldA}.
+  - Let {streamB} be the directive named `stream` on {fieldB}.
+  - If {streamA} and {streamB} have identical sets of arguments, return {true}.
+- Return {false}.
+
 Note: In prior versions of the spec the term "composite" was used to signal a
 type that is either an Object, Interface or Union type.
 
@@ -1521,6 +1532,174 @@ query ($foo: Boolean = true, $bar: Boolean = false) {
 }
 ```
 
+### Defer And Stream Directives Are Used On Valid Root Field
+
+** Formal Specification **
+
+- For every {directive} in a document.
+- Let {directiveName} be the name of {directive}.
+- Let {mutationType} be the root Mutation type in {schema}.
+- Let {subscriptionType} be the root Subscription type in {schema}.
+- If {directiveName} is "defer" or "stream":
+  - The parent type of {directive} must not be {mutationType} or
+    {subscriptionType}.
+
+**Explanatory Text**
+
+The defer and stream directives are not allowed to be used on root fields of the
+mutation or subscription type.
+
+For example, the following document will not pass validation because `@defer`
+has been used on a root mutation field:
+
+```raw graphql counter-example
+mutation {
+  ... @defer {
+    mutationField
+  }
+}
+```
+
+### Defer And Stream Directives Are Used On Valid Operations
+
+** Formal Specification **
+
+- Let {subscriptionFragments} be the empty set.
+- For each {operation} in a document:
+  - If {operation} is a subscription operation:
+    - Let {fragments} be every fragment referenced by that {operation}
+      transitively.
+    - For each {fragment} in {fragments}:
+      - Let {fragmentName} be the name of {fragment}.
+      - Add {fragmentName} to {subscriptionFragments}.
+- For every {directive} in a document:
+  - If {directiveName} is not "defer" or "stream":
+    - Continue to the next {directive}.
+  - Let {ancestor} be the ancestor operation or fragment definition of
+    {directive}.
+  - If {ancestor} is a fragment definition:
+    - If the fragment name of {ancestor} is not present in
+      {subscriptionFragments}:
+      - Continue to the next {directive}.
+  - If {ancestor} is not a subscription operation:
+    - Continue to the next {directive}.
+  - Let {if} be the argument named "if" on {directive}.
+  - {if} must be defined.
+  - Let {argumentValue} be the value passed to {if}.
+  - {argumentValue} must be a variable, or the boolean value "false".
+
+**Explanatory Text**
+
+The defer and stream directives can not be used to defer or stream data in
+subscription operations. If these directives appear in a subscription operation
+they must be disabled using the "if" argument. This rule will not permit any
+defer or stream directives on a subscription operation that cannot be disabled
+using the "if" argument.
+
+For example, the following document will not pass validation because `@defer`
+has been used in a subscription operation with no "if" argument defined:
+
+```raw graphql counter-example
+subscription sub {
+  newMessage {
+    ... @defer {
+      body
+    }
+  }
+}
+```
+
+### Defer And Stream Directive Labels Are Unique
+
+** Formal Specification **
+
+- Let {labelValues} be an empty set.
+- For every {directive} in the document:
+  - Let {directiveName} be the name of {directive}.
+  - If {directiveName} is "defer" or "stream":
+    - For every {argument} in {directive}:
+      - Let {argumentName} be the name of {argument}.
+      - Let {argumentValue} be the value passed to {argument}.
+      - If {argumentName} is "label":
+        - {argumentValue} must not be a variable.
+        - {argumentValue} must not be present in {labelValues}.
+        - Append {argumentValue} to {labelValues}.
+
+**Explanatory Text**
+
+The `@defer` and `@stream` directives each accept an argument "label". This
+label may be used by GraphQL clients to uniquely identify response payloads. If
+a label is passed, it must not be a variable and it must be unique within all
+other `@defer` and `@stream` directives in the document.
+
+For example the following document is valid:
+
+```graphql example
+{
+  dog {
+    ...fragmentOne
+    ...fragmentTwo @defer(label: "dogDefer")
+  }
+  pets @stream(label: "petStream") {
+    name
+  }
+}
+
+fragment fragmentOne on Dog {
+  name
+}
+
+fragment fragmentTwo on Dog {
+  owner {
+    name
+  }
+}
+```
+
+For example, the following document will not pass validation because the same
+label is used in different `@defer` and `@stream` directives.:
+
+```raw graphql counter-example
+{
+  dog {
+    ...fragmentOne @defer(label: "MyLabel")
+  }
+  pets @stream(label: "MyLabel") {
+    name
+  }
+}
+
+fragment fragmentOne on Dog {
+  name
+}
+```
+
+### Stream Directives Are Used On List Fields
+
+**Formal Specification**
+
+- For every {directive} in a document.
+- Let {directiveName} be the name of {directive}.
+- If {directiveName} is "stream":
+  - Let {adjacent} be the AST node the directive affects.
+  - {adjacent} must be a List type.
+
+**Explanatory Text**
+
+GraphQL directive locations do not provide enough granularity to distinguish the
+type of fields used in a GraphQL document. Since the stream directive is only
+valid on list fields, an additional validation rule must be used to ensure it is
+used correctly.
+
+For example, the following document will only pass validation if `field` is
+defined as a List type in the associated schema.
+
+```graphql counter-example
+query {
+  field @stream(initialCount: 0)
+}
+```
+
 ## Variables
 
 ### Variable Uniqueness