refactor(executor, router): define ClientRequestDetails only once and avoid all clones, improve how coerce_variables is done

[dotansimha]

We've seen that ClientRequestDetails is already used in multiple places, and the current implementation tries to avoid re-creating it by doing lazy init only when it's really used - which might still lead to multiple creation.
The original reason behind it was an issue with the lifetime definition of ClientRequestDetails.

Since ClientRequestDetails is constructed with data from multiple places, it needs to declare multiple lifetimes, based on where the data actually came from. This is the new definition:

/// The `ClientRequestDetails` itself + `OperationDetails` that's inside are both defined inside `execute_pipeline`. So both has `'exec` lifetime. 
/// The rest of the parameters are based on the `'req` lifetimes because the originated in the `ntex::Request`. 
pub struct ClientRequestDetails<'exec, 'req> {
    pub method: &'req Method,
    pub url: &'req http::Uri,
    pub headers: &'req NtexHeaderMap,
    pub operation: OperationDetails<'exec>,
}

In addition, this PR removed the need to clone the query and put it into a Cow in order to pass it around during execution. To do that, I changed the way ExecutionRequest is passed around - coerce_variables no longer takes ownership, and the way we have variables (and no variables) is also improved now.

[gemini-code-assist]

Summary of Changes

Hello @dotansimha, 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 introduces a significant refactoring to how ClientRequestDetails are handled within the router and executor components. The primary goal is to optimize performance and enhance correctness by ensuring that request details are constructed only once and then passed by reference throughout the request processing pipeline. This change addresses previous issues with redundant object creation and improves lifetime management, leading to a more efficient and robust system for handling client request information.

Highlights

• Refactored ClientRequestDetails: The ClientRequestDetails struct is now defined once at the start of the request pipeline and passed by reference, eliminating redundant cloning and ensuring consistent usage across the application.
• Improved Lifetime Management: The ClientRequestDetails struct has been updated with two lifetime parameters, 'exec and 'req, to accurately reflect the origin and lifetime of its constituent data, enhancing memory safety and correctness.
• Function Signature Updates: Numerous functions and struct definitions across executor and router crates have been updated to accept ClientRequestDetails by reference and incorporate the new lifetime parameters, including execute_plan, request_override_context, evaluate, execute_query_plan, and various Executor and SubgraphExecutorMap methods.
• Dependency Cleanup: Unnecessary imports like std::borrow::Cow and OperationKind have been removed from bin/router/src/pipeline/execution.rs as they are no longer required with the refactored ClientRequestDetails handling.

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[gemini-code-assist]

Code Review

This pull request is a great refactoring that defines ClientRequestDetails once per request and passes it by reference, successfully avoiding many clones in the request pipeline. This improves performance and code clarity.

My review focuses on fully achieving the goal of "avoiding all clones". I've identified a couple of remaining heap allocations for the operation's name and query and provided a suggestion to eliminate them by adjusting the OperationDetails struct to use references. This change would make the request processing even more efficient, in line with the repository's performance-first style guide.

[github-actions]

✅ k6-benchmark results

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

     █ setup

     checks.........................: 100.00% ✓ 217872      ✗ 0    
     data_received..................: 6.4 GB  212 MB/s
     data_sent......................: 85 MB   2.8 MB/s
     http_req_blocked...............: avg=3.11µs   min=691ns   med=1.68µs  max=15.91ms  p(90)=2.32µs  p(95)=2.67µs  
     http_req_connecting............: avg=487ns    min=0s      med=0s      max=1.58ms   p(90)=0s      p(95)=0s      
     http_req_duration..............: avg=20.16ms  min=2.14ms  med=19.15ms max=106.35ms p(90)=27.75ms p(95)=31.04ms 
       { expected_response:true }...: avg=20.16ms  min=2.14ms  med=19.15ms max=106.35ms p(90)=27.75ms p(95)=31.04ms 
     http_req_failed................: 0.00%   ✓ 0           ✗ 72644
     http_req_receiving.............: avg=133.39µs min=25.46µs med=38.46µs max=87.62ms  p(90)=80.96µs p(95)=379.66µs
     http_req_sending...............: avg=21.31µs  min=5.27µs  med=10.56µs max=23.29ms  p(90)=15.18µs p(95)=25.85µs 
     http_req_tls_handshaking.......: avg=0s       min=0s      med=0s      max=0s       p(90)=0s      p(95)=0s      
     http_req_waiting...............: avg=20.01ms  min=2.07ms  med=19.04ms max=87.24ms  p(90)=27.52ms p(95)=30.73ms 
     http_reqs......................: 72644   2416.449244/s
     iteration_duration.............: avg=20.64ms  min=6.1ms   med=19.52ms max=237.39ms p(90)=28.21ms p(95)=31.52ms 
     iterations.....................: 72624   2415.783958/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-532 ghcr.io/graphql-hive/router:sha-410114f

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