docs(code-runtime-python): register the open-merge split billing in the fd-3 note

The review's suggestion: the open-merge mechanism (incremental split billing on
both sides, host caps logBudget-1/logBudget+2, the sub-2-byte walk guard, the
child's _open_started-keyed billing) lived only in code comments. The wire
contract section of the note now states it, paired and re-recorded.
This commit is contained in:
Chinesezjc
2026-08-31 15:03:21 +08:00
committed by Tianyi Cui
parent 22e2dc454d
commit 1097bd3c3c
3 changed files with 4 additions and 4 deletions
@@ -24,7 +24,7 @@ The package ships the runtime alongside the protocol; it remains independently b
## Wire contract
Frames are JSON-lines on fd 3, one object per line, leaving stdout/stderr free for the program's own output. Child → host: `boot-ack`, `call`, `log`, `done`. Host → child: `boot` (first frame), `run` (after `boot-ack`), and one `reply` per `call`. The `log` frame's `truncated` flag marks the frame that IS the child ledger's own truncation marker, so the host stops capturing at the same point the child did instead of inferring it from its own budget. The `log` frame's `open` flag marks an unterminated line committed by an explicit flush: the host holds it (bounded by the ledger budget) and appends the next frame to the same entry, so an explicit flush followed by more text reads back as one line rather than a fake newline. `done.error.kind` is one of `exception`, `invalid-output`, `output-limit`; wall/CPU budgets, aborts, and substrate death are observed host-side, not carried as frames.
Frames are JSON-lines on fd 3, one object per line, leaving stdout/stderr free for the program's own output. Child → host: `boot-ack`, `call`, `log`, `done`. Host → child: `boot` (first frame), `run` (after `boot-ack`), and one `reply` per `call`. The `log` frame's `truncated` flag marks the frame that IS the child ledger's own truncation marker, so the host stops capturing at the same point the child did instead of inferring it from its own budget. The `log` frame's `open` flag marks an unterminated line committed by an explicit flush: the host holds it and appends the next frame to the same entry, so an explicit flush followed by more text reads back as one line rather than a fake newline. The merged entry's wire cost is billed exactly once, split incrementally across its fragments on both sides (O(k) for k fragments, never a re-walk of the whole hold): the FIRST fragment pays the full JSON-string cost plus the separator, each continuation and the closing frame pay only their content; the host's exact-cost caps are `logBudget - 1` for a first fragment (the ledger's reserved byte, matching `admit`) and `logBudget + 2` for a continuation or closing frame (billed without the two quotes), and `jsonStringCostUpTo` returns `undefined` below a 2-byte cap; the child keys its split billing off `_open_started` alone, so a closing frame bills as the merged tail. `done.error.kind` is one of `exception`, `invalid-output`, `output-limit`; wall/CPU budgets, aborts, and substrate death are observed host-side, not carried as frames.
## Mirror alignment