Field 2026-08-28, iPad Pro on iOS 27: the picture froze twice in one
session while audio and input kept running, and only reconnecting
cleared it. pf-present logged decode at full rate against ok=0,
noDrawable=120 and no vendLeadMs samples at all, so the
CAMetalDisplayLink had stopped calling back.
Stage-4 has no drawable source of its own: every drawable arrives as
update.drawable. A link that goes quiet is therefore a stream that
never presents again, and nothing noticed. Stage-3's
PresentGate.staleAfter insurance does not reach this path, and the
link is only invalidated at session teardown.
The render thread now tracks how long it has gone without a vend and,
past 250 ms with frames still decoding, retires the link generation
and starts a fresh one. Invalidating the old link also returns the
drawables it holds, so this covers an exhausted pool as well as a
dead link. The relink is logged to the send-logs ring.
Also holds the link delegate for the link thread's lifetime. It is
stored weakly and was kept alive only by a local whose last use is
that store, which ARC is free to release on the spot.
enricobuehler