Beagle SCM

DIS-008: GET #~N commit-ancestor rewind unimplemented

GET documents be get '#~1' as "rewinds cur's ref by one commit and resets the wt, staying attached to cur", but no ~N ancestor operator exists in fragment handling — the ~1 is resolved as an ordinary ref/hex and fails. The goal is the documented relative-ancestor rewind. See GET, URI.

WIP

Design decisions

• ~N is verb payload INSIDE the #fragment (free-form per URI), not URI structure — parsed by a tiny DOGFragRewind predicate (typed u64decdrain, no hand-rolled scanner), NOT the URI grammar.
• Walk-back reuses the keeper first-parent body-parse pattern (GITu8sDrainCommit + GIT_FIELD_PARENT) as a new KEEPResolveAncestor in keeper/RESOLVE.c — works on unindexed history (no DAG needed).
• Stay attached by feeding GETCheckout source ?<cur_branch> (empty => trunk ?): GETCheckout already derives both the recorded row shape AND the REFS tip advance from source, so the ref rewinds in place.
• Past-root → KEEPResolveAncestor returns RESLVNONE → GET refuses CLEANLY before any wt mutation (the wt is left untouched).

TODOs

• Parse #~N (default N=1) on the GET fragment path; reject ~0/junk.
• Walk cur's first-parent chain back N commits to the target sha.
• Reset the wt and rewind cur's ref staying attached.
• Table cases: #~1 parent, #~2 two back, ?#~1 spelling, #~N past root refuses (test/get/65 + DOGtest FragRewind table).

Blockers and bummers

None — the first-parent walk-back is now exposed via KEEPResolveAncestor.

Outcome

LANDED (audited 2026-07-07): 25851a5a (beagle) + dd0a8f4b (dog) "DIS-008 be get #~N ancestor rewind", 26Jun.

• Root cause: ~N was never recognised — #~1 fell through to the bare resume (silently re-checked-out cur tip, no rewind); ?#~1 hit GET's "not a valid commit ref" refusal. No first-parent walk was wired in.
• Fix: dog/DOG.c:486 DOGFragRewind (+ dog/DOG.h decl); keeper/RESOLVE.c:301 KEEPResolveAncestor (+ RESOLVE.h decl); sniff/GET.c:2076 SNIFFGetURI rewind arm (detect → resolve → attached GETCheckout).
• Tests: test/get/65-rewind-ancestor (auto-globbed) — #~1/#~2/?#~1 rewind + attached-post, past-root refuses cleanly; dog/test/DOG.c DOGTestFragRewind table. Full ctest 377/377 green.