fix(router): allow null value for nullable scalar types while validating variables & coerce list variables

[ardatan]

Ref ROUTER-155
Ref ROUTER-156

• When validate_runtime_value takes null for a non-nullable scalar type, it was throwing like;
  Failed to collect GraphQL variables: Expected a string for type 'String', got Null

• When the list input value receives non-list value, it should be treated as a one-item list.

type Query {
  foo(bar: [String]): String
}

In this case if you have an operation like below;

{
   "query": "query Foo($bar: [String]) {
       foo(bar: $bar)
   }
}

And then variables like { "bar": "BAR" }
it should be treated as ["BAR"]

[gemini-code-assist]

Summary of Changes

Hello @ardatan, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an issue where the GraphQL variable validation process incorrectly rejected null values for types that are defined as nullable. The changes ensure that null is properly accepted during runtime value validation for both scalar and list types, aligning the validator's behavior with GraphQL's nullable type specifications and resolving the reported error (ROUTER-155).

Highlights

• Variable Validation Logic: Modified the validate_runtime_value function to correctly handle null values for nullable scalar and list types. Previously, providing null for these types would result in an error, even if the schema allowed it. The updated logic now explicitly permits null values to pass validation for TypeNode::Named and TypeNode::List.
• Test Coverage: Added a new unit test, allow_null_values_for_scalar_non_nullable_types, to verify that null values are successfully validated for various scalar types (String, Int, Float, Boolean, ID), ensuring the fix behaves as expected.

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[ardatan]

@gemini-code-assist review and summary

[Copilot]

Pull Request Overview

This PR fixes a bug in variable validation where null values were incorrectly rejected for nullable scalar types. The validation function now properly accepts null values for scalar types like String, Int, Float, Boolean, and ID when they are not wrapped in a NonNull type.

Key changes:

• Added early null checks for Named and List type nodes to allow null values
• Added comprehensive test coverage for null validation on scalar types

---

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[gemini-code-assist]

Code Review

This pull request correctly adjusts variable validation to allow null values for nullable scalar and list types, aligning with GraphQL's nullability rules. The changes in validate_runtime_value are logical and are accompanied by a new test case. My review includes a suggestion to rename the test for clarity and another to add a test case for nullable list types to ensure full coverage of the changes. The PR description seems to have a minor mix-up between 'nullable' and 'non-nullable', but the code implementation is correct.

[gemini-code-assist]

Code Review

This pull request correctly allows null values for nullable scalar and list types during variable validation, which fixes a bug where they were incorrectly rejected. My review includes a suggestion to refactor the implementation to reduce code duplication, a correction for a misleading test name, and a recommendation to add a missing test case for nullable list types to improve test coverage.

[github-actions]

✅ k6-benchmark results

     ✓ response code was 200
     ✓ no graphql errors
     ✓ valid response structure

     █ setup

     checks.........................: 100.00% ✓ 223401      ✗ 0    
     data_received..................: 6.5 GB  217 MB/s
     data_sent......................: 87 MB   2.9 MB/s
     http_req_blocked...............: avg=3.87µs   min=661ns  med=1.75µs  max=10.54ms  p(90)=2.53µs  p(95)=2.93µs  
     http_req_connecting............: avg=1.03µs   min=0s     med=0s      max=8.31ms   p(90)=0s      p(95)=0s      
     http_req_duration..............: avg=19.66ms  min=1.9ms  med=18.71ms max=100.64ms p(90)=27.12ms p(95)=30.44ms 
       { expected_response:true }...: avg=19.66ms  min=1.9ms  med=18.71ms max=100.64ms p(90)=27.12ms p(95)=30.44ms 
     http_req_failed................: 0.00%   ✓ 0           ✗ 74487
     http_req_receiving.............: avg=143.11µs min=25.7µs med=40.29µs max=76.61ms  p(90)=90.01µs p(95)=389.03µs
     http_req_sending...............: avg=25.84µs  min=5.88µs med=10.86µs max=23.82ms  p(90)=16.77µs p(95)=28.25µs 
     http_req_tls_handshaking.......: avg=0s       min=0s     med=0s      max=0s       p(90)=0s      p(95)=0s      
     http_req_waiting...............: avg=19.49ms  min=1.84ms med=18.58ms max=80.69ms  p(90)=26.86ms p(95)=30.09ms 
     http_reqs......................: 74487   2478.239516/s
     iteration_duration.............: avg=20.13ms  min=4.28ms med=19.07ms max=237.05ms p(90)=27.57ms p(95)=30.9ms  
     iterations.....................: 74467   2477.574101/s
     vus............................: 50      min=50        max=50 
     vus_max........................: 50      min=50        max=50 

[github-actions]

🐋 This PR was built and pushed to the following Docker images:

Image Names: ghcr.io/graphql-hive/router

Platforms: linux/amd64,linux/arm64

Image Tags: ghcr.io/graphql-hive/router:pr-483 ghcr.io/graphql-hive/router:sha-4d3751b

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