Move content negotiation to a plugin

[carodewig]

Combines accept, content-type, and vary header processing (for both requests and responses) into a single plugin on the router service.

This PR mostly just extracts logic from other areas of the code and combines them into one module. This should make it easier to both reason about the code now, and modify it in the future.

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Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• Changes are compatible¹
• Documentation² completed
• Performance impact assessed and acceptable
• Tests added and passing³
  • Unit Tests
  • Integration Tests
  • Manual Tests

Exceptions

Note any exceptions here

Notes

Footnotes

1. It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this. ↩

2. Configuration is an important part of many changes. Where applicable please try to document configuration examples. ↩

3. Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions. ↩

[svc-apollo-docs]

✅ Docs preview has no changes

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Build ID: 4c1e66604f3b03a9226543d1

[github-actions]

@carodewig, please consider creating a changeset entry in /.changesets/. These instructions describe the process and tooling.

[router-perf]

CI performance tests

• connectors-const - Connectors stress test that runs with a constant number of users
• const - Basic stress test that runs with a constant number of users
• demand-control-instrumented - A copy of the step test, but with demand control monitoring and metrics enabled
• demand-control-uninstrumented - A copy of the step test, but with demand control monitoring enabled
• enhanced-signature - Enhanced signature enabled
• events - Stress test for events with a lot of users and deduplication ENABLED
• events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• events_big_cap_high_rate_callback - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity using callback mode
• events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• extended-reference-mode - Extended reference mode enabled
• large-request - Stress test with a 1 MB request payload
• no-tracing - Basic stress test, no tracing
• reload - Reload test over a long period of time at a constant rate of users
• step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• step-local-metrics - Field stats that are generated from the router rather than FTV1
• step-with-prometheus - A copy of the step test with the Prometheus metrics exporter enabled
• step - Basic stress test that steps up the number of users over time
• xlarge-request - Stress test with 10 MB request payload
• xxlarge-request - Stress test with 100 MB request payload

[glasser]

@carodewig just out of pure passerby curiosity (as somebody who wanted to make a change to this code and was confused by the duplication) were you able to figure out why this code was apparently duplicated in router?

[lrlna]

This is so much easier to read and reason about now! Thank you for tackling this 🙏

[carodewig]

@carodewig just out of pure passerby curiosity (as somebody who wanted to make a change to this code and was confused by the duplication) were you able to figure out why this code was apparently duplicated in router?

@glasser Unfortunately I wasn't able to come to a satisfactory conclusion - my working theory is that the code wasn't fully moved when the router_service was first created (#2170). It does have to happen at the router service at the moment, though - that's where decisions are made about the type of streaming response, etc.

[martinbonnin]

Cool stuff! Does that unlock #6851? There is some movement in the spec repo lately and I'd like to go back to Benjie and the rest of the wg with a confirmation that the new media type is working for us. Or reasons why it's not working for us.

[carodewig]

Does that unlock #6851? There is some movement in the spec repo lately and I'd like to go back to Benjie and the rest of the wg with a confirmation that the new media type is working for us. Or reasons why it's not working for us.

@martinbonnin Yes, absolutely! The purpose of moving the code into a plugin was to make those kinds of fixes easier to reason about, implement, and test.

[abernix]

We're going to revert this because it's caused a couple regressions at this point, including #7555. See #7725 for the revert.

[glasser]

OK. Should we just try to implement #6851 (perhaps as opt-in for now for compatibility reasons) on top of the existing code structure, then?

[carodewig]

OK. Should we just try to implement #6851 (perhaps as opt-in for now for compatibility reasons) on top of the existing code structure, then?

@glasser Yes, I think that's a good idea. Unfortunately I will not have the bandwidth to get back to this and address the edge cases I missed anytime soon.