flow like the river

BACK_BACK/node_modules/graphql/validation/rules/LoneSchemaDefinitionRule.js.flow

@@ -0,0 +1,43 @@
+// @flow strict
+import { GraphQLError } from '../../error/GraphQLError';
+import type { ASTVisitor } from '../../language/visitor';
+
+import type { SDLValidationContext } from '../ValidationContext';
+
+/**
+ * Lone Schema definition
+ *
+ * A GraphQL document is only valid if it contains only one schema definition.
+ */
+export function LoneSchemaDefinitionRule(
+  context: SDLValidationContext,
+): ASTVisitor {
+  const oldSchema = context.getSchema();
+  const alreadyDefined =
+    oldSchema?.astNode ??
+    oldSchema?.getQueryType() ??
+    oldSchema?.getMutationType() ??
+    oldSchema?.getSubscriptionType();
+
+  let schemaDefinitionsCount = 0;
+  return {
+    SchemaDefinition(node) {
+      if (alreadyDefined) {
+        context.reportError(
+          new GraphQLError(
+            'Cannot define a new schema within a schema extension.',
+            node,
+          ),
+        );
+        return;
+      }
+
+      if (schemaDefinitionsCount > 0) {
+        context.reportError(
+          new GraphQLError('Must provide only one schema definition.', node),
+        );
+      }
+      ++schemaDefinitionsCount;
+    },
+  };
+}