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

// @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;
    },
  };
}