The probe runs in the app's process, and a provider can fail by aborting
rather than by returning an error — XNNPACK did on SCRFD. A rung that does
that once would do it on every launch, before the first photograph is on
screen.
Every session build above the CPU, the probe's and each background
compile's, now writes what it is attempting to `attempt` in the cache
directory first and removes it after. After two launches in a row that
died inside the same attempt it is refused and recorded — a rung in
`failed`, an engine in the new `refused` — until the fingerprint changes.
Two, not one, because quitting during a TensorRT compile leaves the same
file.
dtourolle
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