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fix(ios): regain healthy timer leeway after the fast foreground beat - #21478
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The fast foreground beat arms first=1s/repeating=4s with 50ms leeway (one leeway serves the whole DispatchSourceTimer schedule), but recorded the armed interval as 4s, so the tick handler's desired==armed check never retargeted and every later healthy beat kept the 50 ms coalescing tolerance for the rest of the session. Track the pending fast first fire explicitly, retarget to the armed cadence after it emits (deferring to the desired-cadence branch when the state changed while the beat was pending), and clear the flag on cancel. Invariant: none triggered (liveness deadlines unchanged; coalescing tolerance only).
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Summary
Answers the one unresolved non-outdated thread on #21406 ("Restore healthy leeway after the fast foreground beat"):
appBecameActivearmsfirst: 1s, repeating: 4s, and aDispatchSourceTimerrecords a single leeway for the whole schedule — 50 ms for the fast first fire. BecauseprogressIntervalArmedwas recorded as 4 s, the tick handler'sdesired == armedcheck never retargeted, so every later healthy beat for the rest of the session kept the 50 ms coalescing tolerance the fast beat needed.The pending fast first fire is now tracked explicitly (
progressFastFirePending). After it emits, the timer retargets to the armed repeating cadence, regaining the 500 ms healthy leeway. If the capture state changed while the beat was pending, the retarget defers to the existing desired-cadence branch instead, preserving the fault-path behaviors pinned by the #21406 review (a preserved fast fire through recovery, a second fault before the fire).cancelProgressTimerclears the flag with the rest of the timer state.Product invariants
Failure class
Failure-Class: none
Test plan
syncProgressTimer/applyProgressCadencestate machine; flag lifecycle covers arm/retarget/cancel