fix(hooks): no run evidence for a merged name git reads as a tag or another ref

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

hooks/lib/merge-evidence.mjs

@@ -15,6 +15,18 @@
 // wrong type reads "no run evidence" or "not measured", never a green line.
 // Text taken from the file prints with every non-printable or non-ASCII
 // character as `?`.
+//
+// A merged name git would read as something other than refs/heads/<name>
+// gives "no run evidence": git looks a name up as `$GIT_DIR/<name>`,
+// `refs/<name>` and `refs/tags/<name>` before `refs/heads/<name>`, so the
+// merge would not take the branch head the evidence was written for. The name
+// is ambiguous when an entry of any kind (file, directory, link; lstat only)
+// sits at `<git dir>/<name>`, `<common>/<name>`, `<common>/refs/<name>` or
+// `<common>/refs/tags/<name>`, or when a `packed-refs` line (not a `^` peeled
+// line) names exactly `refs/<name>` or `refs/tags/<name>`. A `packed-refs`
+// that is there but cannot be read within its cap counts as ambiguous (cannot
+// tell), never as "no tag". The writer (lib/review/merge-evidence.mjs)
+// refuses the same names.
 
 import { closeSync, constants, fstatSync, lstatSync, openSync, readFileSync, readSync } from 'node:fs';
 import { isAbsolute, join, resolve } from 'node:path';
@@ -28,7 +40,7 @@ const NOT = 'not measured';
 const UNDO = 'Undone by: `git revert -m 1 <merge>`';
 
 // `git merge` options that take a separate value: their value is not the branch.
-const MERGE_VALUED = ['-m', '-F', '-s', '-X', '--message', '--file', '--strategy', '--strategy-option', '--into-name'];
+const MERGE_VALUED = ['-m', '-F', '-s', '-X', '--message', '--file', '--strategy', '--strategy-option', '--into-name', '--cleanup'];
 
 const SHA_RE = /^(?:[0-9a-f]{40}|[0-9a-f]{64})$/;
 const VERDICTS = ['ALLOW', 'WARN', 'BLOCK'];
@@ -77,18 +89,66 @@ function smallRegular(path, max) {
   return readFileSync(path, 'utf8');
 }
 
-/** The head sha of `refs/heads/<branch>`: the loose ref, else `packed-refs`; else null. */
-function headSha(common, branch) {
+const PACKED_MAX = 16 * 1024 * 1024;
+
+/**
+ * `packed-refs` of `common`: '' when there is none, its text, or null when it
+ * is there but not a regular file of at most PACKED_MAX bytes, or unreadable.
+ */
+function packedRefs(common) {
+  const path = join(common, 'packed-refs');
+  let st;
+  try {
+    st = lstatSync(path);
+  } catch (e) {
+    return e && (e.code === 'ENOENT' || e.code === 'ENOTDIR') ? '' : null;
+  }
+  if (!st.isFile() || st.size > PACKED_MAX) return null;
+  try { return readFileSync(path, 'utf8'); } catch { return null; }
+}
+
+/** The ref name of each `packed-refs` line `<sha> <ref>` (a `^` peeled line is none). */
+function packedLines(packed) {
+  const out = [];
+  for (const line of packed.split('\n')) {
+    const m = line.match(/^([0-9a-f]{40}|[0-9a-f]{64}) (.+)$/);
+    if (m) out.push([m[1], m[2]]);
+  }
+  return out;
+}
+
+/**
+ * True when git would read `name` as something other than refs/heads/<name>
+ * (see the header): an entry of any kind at one of the four paths, or a
+ * packed `refs/<name>` or `refs/tags/<name>`. Cannot tell counts as true.
+ */
+function ambiguousName(gitDir, common, name, packed) {
+  const parts = name.split('/');
+  for (const path of [join(gitDir, ...parts), join(common, ...parts), join(common, 'refs', ...parts), join(common, 'refs', 'tags', ...parts)]) {
+    try { lstatSync(path); return true; } catch (e) {
+      // No entry is ENOENT/ENOTDIR; any other error cannot tell.
+      if (!e || (e.code !== 'ENOENT' && e.code !== 'ENOTDIR')) return true;
+    }
+  }
+  if (packed === null) return true;
+  return packedLines(packed).some(([, ref]) => ref === `refs/${name}` || ref === `refs/tags/${name}`);
+}
+
+/**
+ * The head sha of `refs/heads/<branch>`: the loose ref, else `packed-refs`;
+ * null when there is none or the name is ambiguous for git.
+ */
+function headSha(gitDir, common, branch) {
+  const packed = packedRefs(common);
+  if (ambiguousName(gitDir, common, branch, packed)) return null;
   const loose = smallRegular(join(common, 'refs', 'heads', ...branch.split('/')), 4096);
   if (loose !== null) {
     const sha = loose.trim();
     if (SHA_RE.test(sha)) return sha;
   }
-  const packed = smallRegular(join(common, 'packed-refs'), 16 * 1024 * 1024);
-  if (packed === null) return null;
-  for (const line of packed.split('\n')) {
-    const m = line.match(/^([0-9a-f]{40}|[0-9a-f]{64}) (.+)$/);
-    if (m && m[2] === `refs/heads/${branch}`) return m[1];
+  if (!packed) return null;
+  for (const [sha, ref] of packedLines(packed)) {
+    if (ref === `refs/heads/${branch}`) return sha;
   }
   return null;
 }
@@ -165,7 +225,7 @@ export function evidenceLines(repoRoot, mergeArgs) {
     const gitDir = gitDirAt(repoRoot);
     if (!gitDir) return NO_EVIDENCE;
     const common = commonDirOf(gitDir);
-    const sha = headSha(common, branch);
+    const sha = headSha(gitDir, common, branch);
     if (!sha) return NO_EVIDENCE;
     const text = readEvidence(join(common, 'voyage', 'merge-evidence', `${sha}.json`));
     if (text === 'missing') return NO_EVIDENCE;