[feywind]

Stack Structure (Stack #36644)

Layer	Branch	PR
1	hedging/options-and-token-bucket	#36639
2	hedging/coordinator-and-scheduler	#36640 (this PR)
3	hedging/queue-integration	#36641
4	hedging/queue-unit-tests	#36642
5	hedging/system-tests	#36643

Summary

Implements Layer 2 of Cloud Pub/Sub Publish Hedging (go/sdk:pubsub-hedging) for google-cloud-pubsub:

• AsyncPublisher::AttemptHandle & AsyncPublisher::CancellationSharer (lib/google/cloud/pubsub/async_publisher/cancellation_sharer.rb):
  • Coordinates original (attempt 0) and hedged (attempt 1..N) publish attempts per batch.
  • Enforces first-wins sibling cancellation and callback completion outside the coordinator mutex.
  • Immediately clears @batch = nil upon completion so batch message payloads can be garbage-collected while scheduled HedgedRequest entries remain queued.
  • Implements asymmetric error handling: errors from hedged attempts (attempt >= 1) are ignored while attempt 0 is in flight; terminal failure of attempt 0 immediately cancels running hedged attempts and fails the batch.
• AsyncPublisher::HedgedRequest & AsyncPublisher::HedgingScheduler (lib/google/cloud/pubsub/async_publisher/hedging_scheduler.rb):
  • Event-driven single-timer FIFO scheduler that pops due items under @mutex and dispatches outside @mutex to prevent lock-order inversion.
  • Emits debug log messages on pubsub:publish-hedged ("Publishing hedged attempt <n>" and "Hedging rate limited due to lack of tokens.").
• Unit tests (test/google/cloud/pubsub/async_publisher/cancellation_sharer_test.rb, test/google/cloud/pubsub/async_publisher/hedging_scheduler_test.rb).