Lifecycle-aligned Cancellation

When handling concurrent async work (streaming, tees, semaphores/limits, watchdogs), ensure cancellation, timeouts, and accounting are tied to the actual consumer lifecycle—release/decrement/teardown at the point where the work is truly finished, and don’t block on cancellation promises that can’t resolve until another branch drains.

Practical rules:

• Don’t await cancellation of detached/tee branches if it may remain pending until another consumer drains; cancel and proceed.
• Release concurrency slots only after the stream is fully consumed (e.g., TransformStream.flush/end-of-stream), not when headers/initial chunks arrive.
• Prevent double accounting: if you decrement pending counters at multiple phases (e.g., flush + handler), gate with a flag or ensure only one phase decrements.
• Use abort timeouts for network calls and merge with any provided AbortSignal so requests can’t hang indefinitely.
• Guard watchdog math: if the baseline timestamp/listener wiring can be missing, seed it or skip instead of computing stall against undefined.

Example pattern (release on flush, not header arrival):

const slot = await acquireSemaphore();
let released = false;
const releaseOnce = () => {
  if (released) return;
  released = true;
  slot();
};

return new TransformStream<Uint8Array, Uint8Array>({
  transform(chunk, controller) {
    controller.enqueue(chunk);
  },
  flush() {
    // stream fully consumed (client drained)
    releaseOnce();
  }
});

These practices reduce race conditions, deadlocks, leaked capacity, and spurious failures in highly concurrent streaming systems.