The lossy MPSC log ring published each record by writing its text and THEN storing
the slot's sequence. A consumer that passed the seq==generation check could then
read text while a producer 'capacity' generations later overwrote that same slot
(it wrote text before bumping seq), yielding a torn line. Diagnostics-only and
practically unreachable (it needs the consumer a full ring behind -- ~60k lines/s
between two host drains), but a real data race.
Make it a proper seqlock: the producer stores seq 0 (in-progress) and fences
BEFORE touching the record, then publishes the generation after the text; the
consumer copies the record out and re-checks seq, dropping the line if it changed.
The ring stays lossy, never torn.
Adds log_ring_test (previously zero coverage): a deterministic wrap-drop case plus
a threaded torn-read guard (4 producers + a slow consumer on a 32-slot ring) that
emits 0 torn lines out of ~300k produced. Closes both the cross-process item and
the log_ring coverage gap.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
BlackMark
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