On a queue used with concurrencyKey, concurrencyLimit applies to each key
value independently, so nothing bounds the queue as a whole short of the
environment limit. The new totalConcurrencyLimit queue option caps in-flight
runs across all keys of the queue while each key still gets at most
concurrencyLimit.
Enforcement lives in the concurrency-key dequeue and enqueue fast-path
scripts, gated behind RUN_ENGINE_TOTAL_CONCURRENCY_LIMITS_ENABLED (default
off). A per-base-queue groupConcurrency set tracks total in-flight; every
release path mirrors its per-key removal into that set unconditionally so
the set stays correct across flag toggles.
Matt Aitken
3f35339d2c