D571 - The call ring is circular, with a separate opening record

Status: decided Date: 2026-09-04

The recorded-call ring wraps, so CallTrace::tail holds the calls before the fault; the opening of the run is kept in its own record that is never overwritten. Each slot stores its call's sequence number, readers order by sequence, and a return is written only if the slot still belongs to that call.

Why: the tail exists for the neighbourhood of the wall, and a ring that fills once and stops shows calls from early boot for any long run. The opening has its own reader, which a small fixed record serves, so neither question is traded for the other. After a wrap slot order says nothing about the guest, and writing a return into a slot a later call owns attributes it to the wrong function.

Rejected:

• Keeping the first calls and calling them the tail: the report claims more than it measured.
• One ring for both ends: the opening and the fault compete for the same slots.