fix(GraphQL): This PR allow to use __typename in mutation. (#7285)

Fixes GRAPHQL-921

The first mutation below will now return the "Mutation" type, which previously was giving an error.
Mutation:

mutation {
                       __typename
			addpost1(input: [{title: "Dgraph", numLikes: 92233720 }]) {
				post1 {
					title
					numLikes
				}
			}
		}

`Response:`
      {
                "__typename":"Mutation",
		"addpost1": {
			"post1": [{
				"title": "Dgraph",
				"numLikes": 92233720

			}]
		}
	}

(cherry picked from commit b6edc7e)

graphql/e2e/common/common.go

@@ -688,7 +688,7 @@ func RunAll(t *testing.T) {
 	t.Run("numUids test", testNumUids)
 	t.Run("empty delete", mutationEmptyDelete)
 	t.Run("duplicate xid in single mutation", deepMutationDuplicateXIDsSameObjectTest)
-	t.Run("query typename in mutation payload", queryTypenameInMutationPayload)
+	t.Run("query typename in mutation", queryTypenameInMutation)
 	t.Run("ensure alias in mutation payload", ensureAliasInMutationPayload)
 	t.Run("mutations have extensions", mutationsHaveExtensions)
 	t.Run("alias works for mutations", mutationsWithAlias)

graphql/e2e/common/mutation.go

@@ -3483,9 +3483,11 @@ func getXidFilter(xidKey string, xidVals []string) map[string]interface{} {
 	return filter
 }
 
-func queryTypenameInMutationPayload(t *testing.T) {
+func queryTypenameInMutation(t *testing.T) {
 	addStateParams := &GraphQLParams{
 		Query: `mutation {
+            __typename
+			a:__typename
 			addState(input: [{xcode: "S1", name: "State1"}]) {
 				state {
 					__typename
@@ -3501,6 +3503,8 @@ func queryTypenameInMutationPayload(t *testing.T) {
 	RequireNoGQLErrors(t, gqlResponse)
 
 	addStateExpected := `{
+		"__typename":"Mutation",
+		"a":"Mutation",
 		"addState": {
 			"state": [{
 				"__typename": "State",

graphql/resolve/resolver.go

@@ -252,6 +252,17 @@ func (rf *resolverFactory) WithSchemaIntrospection() ResolverFactory {
 		WithQueryResolver("__typename",
 			func(q schema.Query) QueryResolver {
 				return QueryResolverFunc(resolveIntrospection)
+			}).
+		WithMutationResolver("__typename",
+			func(m schema.Mutation) MutationResolver {
+				return MutationResolverFunc(func(ctx context.Context, m schema.Mutation) (*Resolved, bool) {
+					return &Resolved{
+						Data:       map[string]interface{}{"__typename": "Mutation"},
+						Field:      m,
+						Err:        nil,
+						Extensions: nil,
+					}, resolverSucceeded
+				})
 			})
 }
 
@@ -368,7 +379,6 @@ func (rf *resolverFactory) queryResolverFor(query schema.Query) QueryResolver {
 	if resolver, ok := rf.queryResolvers[query.Name()]; ok {
 		return mws.Then(resolver(query))
 	}
-
 	return rf.queryError
 }
 
@@ -377,7 +387,6 @@ func (rf *resolverFactory) mutationResolverFor(mutation schema.Mutation) Mutatio
 	if resolver, ok := rf.mutationResolvers[mutation.Name()]; ok {
 		return mws.Then(resolver(mutation))
 	}
-
 	return rf.mutationError
 }