chore(verify): remove the dead same_commit helper; record a measured ablation

CHANGELOG.md

@@ -7,6 +7,13 @@ breaking changes may land in a minor release.
 
 ## [Unreleased]
 
+### Removed
+
+- **`verify.same_commit` is gone — nothing called it.** #645's `_canonical_commit_oid` displaced
+  its last call site and compares canonical full object ids exactly. The helper's leftover
+  prefix-tolerant equality — either argument a prefix of the other once both reach 7 characters —
+  would have handed that looseness to whichever caller reached for it next.
+
 ### Fixed
 
 - **psmux: a hand-back that succeeded no longer reports as failed — or undoes itself (#659).**

src/bmad_loop/verify.py

@@ -252,8 +252,8 @@ def git_bytes(
 
 def rev_parse_head(repo: Path) -> str:
     """The sha HEAD resolves to. Reads stdout alone (`_git_out`): git exits 0 while
-    still warning on stderr, and a warning-suffixed "sha" flows into `same_commit`
-    comparisons and into persisted run baselines (#442)."""
+    still warning on stderr, and a warning-suffixed "sha" flows into every commit
+    comparison and into persisted run baselines (#442)."""
     rc, out, detail = _git_out(repo, "rev-parse", "HEAD")
     if rc != 0:
         raise GitError(f"git rev-parse HEAD failed in {repo}: {detail}")
@@ -316,17 +316,6 @@ def worktree_clean(repo: Path) -> bool:
     return proc.stdout.strip() == ""
 
 
-def same_commit(a: str, b: str) -> bool:
-    """Hash equality tolerant of abbreviated forms (>= 7 chars); sessions
-    sometimes report `git rev-parse --short HEAD`. The 7 is git's *minimum* auto
-    abbreviation, not a fixed default: `core.abbrev` defaults to `auto`, which
-    scales the length with repository size and clamps upward to 7 only for small
-    repos, so there is no single "default --short length" to mirror."""
-    if len(a) < 7 or len(b) < 7:
-        return a == b
-    return a.startswith(b) or b.startswith(a)
-
-
 def is_ancestor(repo: Path, ancestor: str, descendant: str) -> bool:
     """True when `ancestor` is an ancestor of (or equal to) `descendant`.
 
@@ -346,7 +335,9 @@ def is_ancestor(repo: Path, ancestor: str, descendant: str) -> bool:
 # session stamped it. Hex spelling is necessary but insufficient: Git also permits
 # all-hex ref names. The stamp is `git rev-parse HEAD` output by contract, so requiring
 # a uniquely disambiguated direct commit costs a well-behaved session nothing. Length
-# floor mirrors `same_commit`'s 7; ceiling admits sha256.
+# floor is git's shortest auto abbreviation: with `core.abbrev` unset the length scales
+# with the repository's object count and clamps upward to 7 only for small repos, so
+# nothing git abbreviates on its own is shorter. Ceiling admits sha256.
 _OBJECT_ID = re.compile(r"\A[0-9a-fA-F]{7,64}\Z")
 
 

tests/test_verify.py

@@ -122,7 +122,17 @@ def test_attempt_dirty_tracked_change(project):
 
 
 def test_file_bytes_at_revision_distinguishes_blob_absence_tree_and_git_failure(project):
-    """The baseline oracle returns only proven blob bytes, never tree listings."""
+    """The baseline oracle returns only proven blob bytes, never tree listings.
+
+    Ablation: drop either side of ``entry is None or entry[1] != "blob"`` from
+    either oracle — four mutations, each reddening exactly one of the four ``is
+    None`` assertions. The absence side raises ``TypeError`` on the ``missing.bin``
+    case; the type side reddens the ``oracle`` case, and only there do the two
+    oracles differ. Plain ``cat-file blob`` is refused by git on a tree oid, so
+    that mutation still fails loudly; ``cat-file --filters`` instead renders the
+    tree's listing and hands it back as file content, which nothing but this
+    clause keeps out of a baseline comparison.
+    """
     repo = project.project
     nested = repo / "oracle" / "spec.bin"
     nested.parent.mkdir()
@@ -2001,10 +2011,10 @@ def test_verify_dev_bundle_single_char_ref_baseline_is_refused(project):
 def test_verify_dev_bundle_below_floor_abbreviation_is_refused(project):
     """Characterizes the deliberate 7-character floor on the bundle path: an
     abbreviation git itself resolves is still refused when it is shorter than
-    ``_OBJECT_ID``'s floor. That floor mirrors ``same_commit``'s 7; it is not a
-    length git derives (``core.abbrev`` defaults to ``auto``, which scales with
-    repository size and clamps upward to 7 only for small repos, so there is no
-    fixed default to mirror). The stamp is ``git rev-parse HEAD`` output by
+    ``_OBJECT_ID``'s floor. That 7 is the gate's own constant, set where git's
+    auto abbreviation bottoms out (``core.abbrev`` defaults to ``auto``, which
+    scales with repository size and clamps upward to 7 only for small repos, so
+    there is no fixed default to mirror). The stamp is ``git rev-parse HEAD`` output by
     contract, so the floor costs a well-behaved session nothing, and
     accepting shorter claims would re-admit prefix collisions the gate cannot
     distinguish from drift. The refusal is the floor's doing, not an
@@ -3682,7 +3692,7 @@ def noisy(cmd, repo, **kw):
 
 
 def test_rev_parse_head_reads_stdout_alone_under_host_noise(project):
-    """A warning-suffixed "sha" is not a sha. It reaches `same_commit` comparisons
+    """A warning-suffixed "sha" is not a sha. It reaches every commit comparison
     and the baselines persisted in run state, so a resume grades a warning-carrying
     string against a clean one and reads "moved" — silent, with a plausible-looking
     value.