fix(federation): fixed a QP panic saying "would create a cycle"

[duckki]

The merge_fetches_to_same_subgraph_and_same_inputs function must merge decendant nodes into ancestor nodes and not the other way around (to avoid cycles).

This PR fixes it by topologically sorting the fetch dependency graph nodes before collecting nodes to merge. So, their merger is always topologically ordered.

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Checklist

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

• Changes are compatible[^1]
• Documentation[^2] completed
• Performance impact assessed and acceptable
• Tests added and passing[^3]
  • Unit Tests
  • Integration Tests
  • Manual Tests

[github-actions]

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

[router-perf]

CI performance tests

• 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

[lrlna]

non-blocking comment: what's the performance cost of doing a topological sort here, do you know?

[goto-bus-stop]

I tried a few queries with known-big fetch dependency graphs, and this function doesn't really show up in a flame graph. I think because it only happens once, as the absolute last step, when the graph has already been reduced as much as it can.

[lrlna]

excellent excellent!!

[duckki]

Thanks for that analysis. I didn't observe any noticeable slowdown during corpus comparison runs, either.