enhance(executor): simplify subgraph execution error handling

[ardatan]

Extracted from #628

Now SubgraphExecutor can throw SubgraphExecutorError which will be coerced to PlanExecutorError.
So that the execution related errors and the errors propagated from the subgraph can be distinguished.
Combining this with #651, we no longer need to generate dummy subgraph response objects with GraphQLErrors. It was a bit complicated before because in different levels, helpers like error_to_graphql_bytes and internal_server_error_response were handling those errors in different places, so it was hard to debug it.

Right now the subgraph response will be the actual subgraph response while the rest of the execution errors thrown by us will be handled as PlanExecutionError.

Of course an error in a plan node will not stop the request entirely for such cases like a subgraph is down etc which is tested in an E2E test added with this PR.

To summarize the idea is to take the responsibility of error to response conversion from the subgraph executor to the plan executor.

I also removed unused methods;

• HeaderRuleCompileError.new_expression_build
• HeaderRuleRuntimeError.new_value_conversion

And removed unnecessary methods;

• HeaderRuleRuntimeError.new_expression_evaluation

[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 refactors the error handling mechanism within the SubgraphExecutor to provide more granular and robust error management. By introducing a distinct SubgraphExecutorError and integrating it seamlessly into the PlanExecutionError hierarchy, the system can now differentiate between various error sources more effectively. This change not only simplifies the internal error processing flow by eliminating redundant error object generation but also enhances the overall resilience of the router by ensuring that a single subgraph failure does not prevent the successful execution of other parts of a GraphQL query.

Highlights

• Dedicated Subgraph Error Type: Introduced SubgraphExecutorError to specifically represent errors originating from subgraph execution, improving error clarity and allowing for more precise error handling.
• Streamlined Error Propagation: Subgraph execution errors are now directly propagated as SubgraphExecutorError and then coerced into PlanExecutionError, centralizing error handling and removing the previous need for generating dummy GraphQLError objects at lower levels.
• Enhanced Resilience: Individual subgraph failures no longer halt the entire GraphQL request; partial data is returned alongside specific errors, as demonstrated by a newly added end-to-end test.
• Simplified Executor Logic: Removed error_to_graphql_bytes and internal_server_error_response helper functions from executors, making the code cleaner and more focused on direct error propagation.

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[gemini-code-assist]

Code Review

This pull request significantly simplifies the error handling within the subgraph executor by propagating Result types instead of manually creating GraphQLError responses at lower levels. This is an excellent improvement that makes the code more idiomatic, readable, and maintainable. The introduction of SubgraphExecutorError into PlanExecutionErrorKind and the use of with_plan_context correctly centralize error handling. The new E2E test effectively validates that the router can gracefully handle a failing subgraph and return partial data. My review includes one suggestion to refactor the new E2E test for better readability, in line with the repository's style guide.

[github-actions]

✅ k6-benchmark results

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

     █ setup

     checks.........................: 100.00% ✓ 219162      ✗ 0    
     data_received..................: 6.4 GB  213 MB/s
     data_sent......................: 86 MB   2.9 MB/s
     http_req_blocked...............: avg=3.93µs   min=691ns   med=1.84µs  max=5.76ms   p(90)=2.66µs  p(95)=3.03µs  
     http_req_connecting............: avg=1.18µs   min=0s      med=0s      max=3.83ms   p(90)=0s      p(95)=0s      
     http_req_duration..............: avg=20.05ms  min=2.01ms  med=19.05ms max=110.85ms p(90)=27.6ms  p(95)=30.99ms 
       { expected_response:true }...: avg=20.05ms  min=2.01ms  med=19.05ms max=110.85ms p(90)=27.6ms  p(95)=30.99ms 
     http_req_failed................: 0.00%   ✓ 0           ✗ 73074
     http_req_receiving.............: avg=146.14µs min=26.76µs med=41.76µs max=86.01ms  p(90)=97.83µs p(95)=413.47µs
     http_req_sending...............: avg=25.25µs  min=5.73µs  med=10.92µs max=20.23ms  p(90)=16.39µs p(95)=30.32µs 
     http_req_tls_handshaking.......: avg=0s       min=0s      med=0s      max=0s       p(90)=0s      p(95)=0s      
     http_req_waiting...............: avg=19.88ms  min=1.95ms  med=18.91ms max=105.25ms p(90)=27.32ms p(95)=30.68ms 
     http_reqs......................: 73074   2430.239366/s
     iteration_duration.............: avg=20.53ms  min=5.54ms  med=19.4ms  max=247.39ms p(90)=28.08ms p(95)=31.52ms 
     iterations.....................: 73054   2429.574222/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-659 ghcr.io/graphql-hive/router:sha-b6e549b

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[kamilkisiela]

All good, except the e2e test I find hard to understand and could be simpler