`placement.choose` runs in a thread and raises `pod.inflight` the moment it
picks a pod. If the awaiting coroutine was cancelled after the executor had
produced the Lease but before the value was delivered, the object was dropped
with the counter already raised -- and nothing released it, because the page
was gone and no `finally` ran.

A client that gives up on a batch cancels every page of it at once, so the leak
arrives in thousands rather than one at a time.

Measured on a live run, once `max_inflight_per_pod: 64` made it visible: the
pool reported all 52 healthy pods at their cap -- 3,328 pages "in flight" --
while `ss` inside the container showed 3 to 5 open sockets per pod, the cards
sat at `GPU KV cache usage: 0.2%` with `Running: 1 reqs`, and nothing was
preempted. Roughly 60 of every 64 pages did not exist. `_pick` then refused
every pod, correctly, reporting "all 52 serving pod(s) are at their in-flight
cap", and the run fell to 0.33 pages/s.

It had been leaking since the pod pipeline was written. With
`max_inflight_per_pod: 0` an inflated counter blocks nothing, so the only
symptom was DEMAND -- the in-flight counts of 2,700 and 4,096 that earlier runs
reported, and that the scaler dutifully rented pods to serve. The cap did not
cause this; it converted a silent accounting error into a hard stop, which is
how it was finally found. Three runs were spent on its consequences.

`_acquire_lease` shields the acquisition, so it completes even when the page
waiting on it is cancelled, and a done-callback hands the lease straight back.
Orphans are counted and said once a minute, because the entire cost of this bug
was that it was silent.

Note on the analysis: the gap between `lease = await ...` and the `try` that
releases it is NOT a window -- CancelledError is only delivered at an await
point, and there is none in between. The acquisition itself was the only one.

Proven rather than argued: the same test, same cancellation, against the
unshielded form and the shielded one -- granted=1 released=0 LEAKED=1, against
granted=1 released=1 LEAKED=0.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KnfkxBpcZ1v6ypRFFyhrVM