fix: handle null only for outer list

[Noroth]

Summary by CodeRabbit

• Bug Fixes

  • Improved handling of empty and null items in nested lists within gRPC responses, ensuring stricter error reporting and correct output for non-nullable and nullable fields.

• Tests

  • Added a test to verify correct handling of empty and nullable list items in GraphQL responses.
  • Enhanced mock data to include cases with empty lists and null entries for more comprehensive testing.

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.

[coderabbitai]

Walkthrough

A new subtest was added to the gRPC subgraph execution test suite to validate handling of empty and nullable list items. The mock gRPC service was updated to return empty and null user group lists. The gRPC datasource logic was modified to adjust error handling and output for invalid nested messages and lists during list flattening.

Changes

Cohort / File(s)	Change Summary
Test Addition: Subgraph Execution execution/engine/execution_engine_grpc_test.go	Added a subtest to TestGRPCSubgraphExecution that runs a query with nested lists containing empty and null items, asserting correct handling and response structure.
Mock Data Update: gRPC Test Service v2/pkg/grpctest/mockservice.go	Modified the mock service to append an empty list and a null value to the AuthorGroups field in the QueryAuthor response, enabling testing of empty and nullable nested lists.
List Flattening & Error Handling Logic v2/pkg/engine/datasource/grpc_datasource/grpc_datasource.go	Updated the traverseList function to: (1) return an error for invalid nested messages in non-nullable lists, (2) always return an empty array for invalid nested lists, and (3) clarify nullability handling in comments. No changes to exported function signatures.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

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📜 Recent review details

Configuration used: CodeRabbit UI
Review profile: CHILL
Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between ecdd9d7 and 5603361.

📒 Files selected for processing (3)

• execution/engine/execution_engine_grpc_test.go (1 hunks)
• v2/pkg/engine/datasource/grpc_datasource/grpc_datasource.go (2 hunks)
• v2/pkg/grpctest/mockservice.go (1 hunks)

🧰 Additional context used

🧠 Learnings (3)

📓 Common learnings

Learnt from: Noroth
PR: wundergraph/graphql-go-tools#1246
File: v2/pkg/engine/datasource/grpc_datasource/execution_plan_visitor.go:457-457
Timestamp: 2025-07-28T12:44:56.405Z
Learning: In the graphql-go-tools gRPC datasource, only non-null lists use protobuf's repeated field syntax directly. For nullable or nested lists, wrapper types are used because protobuf repeated fields cannot be nullable. The TypeIsNonNullList check ensures only appropriate list types are marked as repeated in the protobuf message structure.

Learnt from: SkArchon
PR: wundergraph/graphql-go-tools#1203
File: v2/pkg/engine/resolve/loader.go:63-67
Timestamp: 2025-07-02T15:28:02.122Z
Learning: In the graphql-go-tools codebase, result structs are consistently initialized with non-nil bytes.Buffer instances, making additional nil checks for res.out unnecessary defensive programming.

v2/pkg/engine/datasource/grpc_datasource/grpc_datasource.go (1)

Learnt from: Noroth
PR: #1246
File: v2/pkg/engine/datasource/grpc_datasource/execution_plan_visitor.go:457-457
Timestamp: 2025-07-28T12:44:56.405Z
Learning: In the graphql-go-tools gRPC datasource, only non-null lists use protobuf's repeated field syntax directly. For nullable or nested lists, wrapper types are used because protobuf repeated fields cannot be nullable. The TypeIsNonNullList check ensures only appropriate list types are marked as repeated in the protobuf message structure.

execution/engine/execution_engine_grpc_test.go (2)

Learnt from: Noroth
PR: #1246
File: v2/pkg/engine/datasource/grpc_datasource/execution_plan_visitor.go:457-457
Timestamp: 2025-07-28T12:44:56.405Z
Learning: In the graphql-go-tools gRPC datasource, only non-null lists use protobuf's repeated field syntax directly. For nullable or nested lists, wrapper types are used because protobuf repeated fields cannot be nullable. The TypeIsNonNullList check ensures only appropriate list types are marked as repeated in the protobuf message structure.

Learnt from: SkArchon
PR: #1203
File: v2/pkg/engine/resolve/loader.go:63-67
Timestamp: 2025-07-02T15:28:02.122Z
Learning: In the graphql-go-tools codebase, result structs are consistently initialized with non-nil bytes.Buffer instances, making additional nil checks for res.out unnecessary defensive programming.

🧬 Code Graph Analysis (3)

v2/pkg/engine/datasource/grpc_datasource/grpc_datasource.go (2)

v2/pkg/engine/datasource/grpc_datasource/execution_plan.go (1)

• LevelInfo (189-192)

v2/pkg/graphqljsonschema/jsonschema.go (1)

• NewNull (426-430)

execution/engine/execution_engine_grpc_test.go (1)

v2/pkg/grpctest/mapping/mapping.go (1)

• DefaultGRPCMapping (10-954)

v2/pkg/grpctest/mockservice.go (1)

v2/pkg/grpctest/productv1/product.pb.go (3)

• ListOfUser_List (7603-7608)
• ListOfUser_List (7621-7621)
• ListOfUser_List (7636-7638)

🔇 Additional comments (4)

v2/pkg/grpctest/mockservice.go (1)

1631-1634: LGTM! Well-structured test data for edge case validation.

These additions effectively test two important edge cases for nested list handling:

1. Empty list: Uses proper wrapper structure {List: &productv1.ListOfUser_List{}} rather than nil
2. Null item: Tests null handling at the outer list level, which aligns with the PR objective "handle null only for outer list"

The clear comments and proper protobuf structure make this excellent test data for validating the gRPC datasource's handling of empty and nullable nested lists.

execution/engine/execution_engine_grpc_test.go (1)

1436-1453: LGTM! Well-structured test for empty and nullable list handling.

This test effectively validates the handling of empty and nullable list items in gRPC subgraph execution. The test structure follows established patterns and the expected response correctly demonstrates nested list handling with both empty arrays ([]) and null values (null) within the authorGroups field.

v2/pkg/engine/datasource/grpc_datasource/grpc_datasource.go (2)

332-334: Approve the simplified list handling logic.

The change to always return an empty array for invalid lists is well-justified by the updated comment. Since nullability checking is handled at the outer list wrapper level, this simplification makes the code more predictable and easier to understand.

The comment clearly explains the rationale: "nullability is checked on the outer List wrapper type," which aligns with the retrieved learning about protobuf repeated fields not being nullable directly.

---

302-307: Review nested invalid wrapper handling for non-nullable lists

In traverseList (grpc_datasource.go lines ~303–307), when a nested wrapper message is invalid for a non-nullable list you now return an empty array (arena.NewArray()) plus an error. All other error branches in this function return arena.NewNull(), with or without an error. This is the only occurrence of NewArray(), fmt.Errorf(...).

Please confirm whether this deviation is intentional. If it is, consider adding a clarifying comment; otherwise you may want to return NewNull() for consistency or adjust the logic to match the intended JSON shape.

• File: v2/pkg/engine/datasource/grpc_datasource/grpc_datasource.go
Lines 303–307:

if !msg.IsValid() {
    if md.LevelInfo[level].Optional {
        return arena.NewNull(), nil
    }

    // Inconsistent with other error cases that return NewNull()
    return arena.NewArray(), fmt.Errorf("cannot add null item to response for non nullable list")
}

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• Commit unit tests in branch ludwig/improve-nullability

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[Noroth]

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[Noroth]

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