mergeTypeDefs produces incorrect output schema

[cdaringe]

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---

Describe the bug

mergeTypeDefs should yield correct output, but incorrectly combines SchemaExtensions

To Reproduce Steps to reproduce the behavior:

Given two type defs:

# schema 1
      extend schema
        @link(
          url: "https://specs.apollo.dev/federation/v2.3"
          import: ["@external", /** snip **/]
        )

# schema 2
      extend schema
        @link(
          url: "https://specs.apollo.dev/federation/v2.3"
          import: ["@external", /** snip **/]
        )
        @link(url: "file://foo.org/trackable/v2.3", import: ["@trackable"])

One would expect an output like the following from mergeTypeDefs:

      extend schema
        @link(
          url: "https://specs.apollo.dev/federation/v2.3"
          import: ["@external", /* snip */]
        )
        @link(url: "file://foo.org/trackable/v2.3", import: ["@trackable"])

But you actually receive:

    extend schema 
     @link(url: "file://foo.org/trackable/v2.3", import: ["@external", /* snip*/, "@trackable"])

The @link kinds are merged, likely in the internal mergeDirectives function.

The issue is very easily reproducible in this repositories tests: https://github.com/ardatan/graphql-tools/compare/master...cdaringe:graphql-tools:demo/merge-schema-extension-bug?expand=1

Expected behavior

See expected output type above.

Environment:

• OS:
• @graphql-tools/merge: main branch
• NodeJS: 22.x

Additional context

[cdaringe]

Ideas:

mergeSchemaDefs has the following return statement:

if (existingNode) {
    return {
      kind:
        node.kind === Kind.SCHEMA_DEFINITION || existingNode.kind === Kind.SCHEMA_DEFINITION
          ? Kind.SCHEMA_DEFINITION
          : Kind.SCHEMA_EXTENSION,
      description: node['description'] || existingNode['description'],
      directives: mergeDirectives(node.directives, existingNode.directives, config, directives),
      operationTypes: mergeOperationTypes(node.operationTypes, existingNode.operationTypes),
    } as any;
  }

where mergeDirectives is squashing all @links internally together.

I'm not sure schema extension directives really should be merged unless they are deeply, structurally equivalent. I wonder if we need to instead of merge, strictly run these thru a structural de-dupe instead.

[ardatan]

Interesting use case. Not sure how intelligent we can make mergeTypeDefs to handle undefined @link directive.
mergeTypeDefs today tries to merge directive definitions if they are not defined as repeatable. Any proposals with PRs are welcome.

[cdaringe]

I think I came up with a clean solve. #6964!