Fix inconsistent breaker state

[elijah-rou]

Implement comprehensive pod state management system in the activator to enable
graceful handling of pod lifecycle transitions and connection management.

Pod State Management:

• Add pod state tracking with states: podHealthy, podPending, podDraining, podRemoved
• Implement graceful draining with reference counting for in-flight requests
• Add proper synchronization for concurrent pod state transitions
• Ensure atomic updates to pod tracker assignments

Connection Management Improvements:

• Add connection-level metrics for monitoring pod health
• Track transport failures and timeouts per pod
• Implement proper error attribution for healthy vs unhealthy targets
• Add pending request tracking metrics

Breaker Integration:

• Update queue breaker to track pending vs in-flight requests
• Add metrics for requests waiting to acquire pod trackers
• Implement proper capacity management during state transitions

Testing:

• Add comprehensive tests for pod state transitions
• Add race condition tests for concurrent state updates
• Verify graceful draining behavior with in-flight requests
• Test proper capacity calculations with multiple activators

Performance:

• Optimize pod assignment using consistent hashing
• Reduce lock contention in hot paths
• Improve capacity update calculations

This change improves reliability by ensuring connections are properly managed
during pod lifecycle events, preventing request failures during scaling operations
and pod terminations.

This succeeds work initially done in #15811

[knative-prow]

Hi @elijah-rou. Thanks for your PR.

I'm waiting for a knative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: elijah-rou
Once this PR has been reviewed and has the lgtm label, please assign dprotaso for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS
• pkg/apis/OWNERS

Approvers can indicate their approval by writing /approve in a comment
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[codecov]

Codecov Report

❌ Patch coverage is 78.38617% with 75 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.99%. Comparing base (26a8cec) to head (2cc54da).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/activator/net/throttler.go	80.93%	45 Missing and 12 partials ⚠️
pkg/activator/handler/context.go	10.00%	17 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16083      +/-   ##
==========================================
- Coverage   80.20%   79.99%   -0.21%     
==========================================
  Files         214      214              
  Lines       16887    17111     +224     
==========================================
+ Hits        13544    13688     +144     
- Misses       2987     3054      +67     
- Partials      356      369      +13     

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[elijah-rou]

/retest

[knative-prow]

@elijah-rou: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[dprotaso]

/ok-to-test

[knative-prow]

There are empty aliases in OWNER_ALIASES, cleanup is advised.

[elijah-rou]

/retest

[elijah-rou]

@dprotaso what's happening with build-tests_serving_main? I am a bit confused what is broken

[dprotaso]

diff in the protobuf formatting it looks like - the build tests run this script

https://github.com/knative/serving/blob/main/hack/verify-codegen.sh

We're using this version of protoc - https://github.com/knative/infra/blob/8acbaddaa605095b52df9f569e97a631a412ecb8/images/prow-tests/Dockerfile#L25

If you're messing with proto files we have a flag in our ./hack/update-codegen.sh script here

serving/hack/update-codegen.sh

Line 35 in e85e3f6

--generate-protobufs) generate_protobufs=1 ;;

[elijah-rou]

@dprotaso any thoughts?