fix: Step 11 review fixes (a fork branch, a colliding tag or another remote never replaces the base)

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

bin/gstack-diff-scope

@@ -34,15 +34,22 @@ _default_base() {
   echo "main"
 }
 BASE="${1:-$(_default_base)}"
-# A bare branch name reads its remote-tracking twin when there is one: /ship
-# merges against origin/<base>, and a linked worktree or a Capy clone never
-# moves the local branch, so a stale one widens the scope (#3650).
-_NOT_BRANCH='^[0-9a-f]{7,64}$|[~^]|@\{|^[A-Z_]*HEAD$|^refs/'
-if [[ ! "$BASE" =~ $_NOT_BRANCH ]] && git rev-parse --verify -q "refs/remotes/origin/${BASE}^{commit}" >/dev/null 2>&1; then
-  BASE="origin/${BASE}"
-fi
 HEAD_REF=""
 if [ "${2:-}" = "--head" ]; then HEAD_REF="${3:-}"; fi
+# A bare branch name reads origin's tracking branch, by its full ref, when that
+# shares history with HEAD: /ship merges against origin/<base>, and a linked
+# worktree or a Capy clone never moves the local branch, so a stale one widens
+# the scope (#3650). A remote-qualified name is taken as given.
+_NOT_BRANCH='^[0-9a-f]{7,64}$|[~^]|@\{|^[A-Z_]*HEAD$|^refs/'
+_QUALIFIED=false
+for _remote in $(git remote 2>/dev/null); do
+  case "$BASE" in "$_remote"/*) _QUALIFIED=true ;; esac
+done
+if [[ ! "$BASE" =~ $_NOT_BRANCH ]] && [ "$_QUALIFIED" = false ] \
+  && git rev-parse --verify -q "refs/remotes/origin/${BASE}^{commit}" >/dev/null 2>&1 \
+  && git merge-base "refs/remotes/origin/${BASE}" "${HEAD_REF:-HEAD}" >/dev/null 2>&1; then
+  BASE="refs/remotes/origin/${BASE}"
+fi
 
 FRONTEND=false
 BACKEND=false

lib/gate-diff/git.ts

@@ -85,23 +85,31 @@ export function isShallow(cwd: string): boolean {
 
 /**
  * The base as a ref that resolves here and, when `head` is given, shares
- * history with it: for a branch name its remote-tracking branch first
- * (`origin/main` for `main`), the base /ship merges against, else the ref
- * itself. A local `main` that is missing, behind (a linked worktree or a Capy
- * clone never moves it, #3650) or shares no history with HEAD (a fetched-over
- * clone) yields to the twin, so the fix line's fetch into the tracking ref is
- * a fix and not a loop.
+ * history with it. For a bare branch name that is origin's tracking branch
+ * (`refs/remotes/origin/main` for `main`), the base /ship merges against,
+ * else the related local branch, else another remote's tracking branch. A
+ * local `main` that is behind (a linked worktree or a Capy clone never moves
+ * it, #3650), missing or unrelated to HEAD (a fetched-over clone) yields to the
+ * twin, so the fix line's fetch into the tracking ref is a fix and not a loop.
+ * A remote-qualified name (`upstream/main`) is taken as given, and the twin is
+ * read by its full ref, so a fork's branch or a tag cannot stand in for it.
  */
 export function resolveBase(ref: string, cwd: string, head?: string): { ref: string; sha: string; local?: 'missing' | 'unrelated' | 'differs' } | null {
   const related = (sha: string) => !head || mergeBase(sha, head, cwd) !== null;
   const direct = resolvesToCommit(ref, cwd);
-  if (isBranchName(ref)) {
-    for (const remote of remoteNames(cwd)) {
-      const tracking = `${remote}/${ref}`;
-      const sha = resolvesToCommit(tracking, cwd);
-      if (!sha || !related(sha)) continue;
-      return sha === direct ? { ref: tracking, sha } : { ref: tracking, sha, local: !direct ? 'missing' : related(direct) ? 'differs' : 'unrelated' };
-    }
+  const remotes = remoteNames(cwd);
+  if (!isBranchName(ref) || remotes.some(remote => ref.startsWith(`${remote}/`))) return direct ? { ref, sha: direct } : null;
+  const twin = (remote: string) => {
+    const sha = resolvesToCommit(`refs/remotes/${remote}/${ref}`, cwd);
+    if (!sha || !related(sha)) return null;
+    return sha === direct ? { ref: `${remote}/${ref}`, sha } : { ref: `${remote}/${ref}`, sha, local: !direct ? 'missing' as const : related(direct) ? 'differs' as const : 'unrelated' as const };
+  };
+  const origin = remotes.includes('origin') ? twin('origin') : null;
+  if (origin) return origin;
+  if (direct && related(direct)) return { ref, sha: direct };
+  for (const remote of remotes) {
+    const other = twin(remote);
+    if (other) return other;
   }
   return direct ? { ref, sha: direct } : null;
 }

test/diff-scope.test.ts

@@ -362,6 +362,24 @@ describe('a bare base name reads origin/<base> when it exists (#3650)', () => {
     git('update-ref', `refs/remotes/origin/${git('rev-parse', 'release/x')}`, 'HEAD');
     expect(scopeAt(dir, git('rev-parse', 'release/x')).SCOPE_BACKEND).toBe('true');
   });
+
+  test('a colliding tag, a fork branch over an explicit remote base, or an unrelated twin never replaces the base (#3650 review)', () => {
+    const { dir, git } = behindBase('main');
+    const upstreamCommit = git('rev-parse', 'origin/main');
+    git('tag', 'origin/main', 'HEAD');
+    expect([scopeAt(dir, 'main').SCOPE_BACKEND, scopeAt(dir, 'main').SCOPE_DOCS]).toEqual(['false', 'true']);
+    git('remote', 'add', 'upstream', 'file:///nonexistent-upstream');
+    git('update-ref', 'refs/remotes/upstream/main', git('rev-parse', 'main'));
+    git('update-ref', 'refs/remotes/origin/upstream/main', 'HEAD');
+    expect(scopeAt(dir, 'upstream/main').SCOPE_BACKEND).toBe('true');
+    git('tag', '-d', 'origin/main');
+    git('checkout', '-q', '--orphan', 'island');
+    git('commit', '-q', '--allow-empty', '-m', 'unrelated');
+    git('update-ref', 'refs/remotes/origin/main', 'HEAD');
+    git('checkout', '-q', 'feature/test');
+    expect(scopeAt(dir, 'main').SCOPE_BACKEND).toBe('true');
+    expect(upstreamCommit).not.toBe('');
+  });
 });
 
 describe('uncommitted work is visible (#2299)', () => {

test/gate-diff-cli.test.ts

@@ -179,6 +179,33 @@ describe('gstack-gate-diff arguments and exit contract', () => {
     expect(bare.stderr).toContain('gstack-gate-diff: local main is not origin/main; base is origin/main');
   });
 
+  // Value: protects=a ref a contributor controls (a fork's origin/upstream/main branch, a tag named origin/main, another
+  // remote's main) never replaces the base the caller named or a related local base, so it cannot hide a gate edit;
+  // fails_when=resolveBase rewrites remote-qualified names, resolves the twin by its short name, or lets any remote beat a
+  // related local base; why_new=the cases above have one remote and no colliding refs; seam=none; no_claim=nothing about
+  // a remote's HEAD symref.
+  test('a fork branch, a colliding tag or another remote never replaces the base (#3650 review)', () => {
+    const src = repo({ fixture: 'rh1-skip', commit: true });
+    const seed = gitIn(src, ['rev-parse', 'main']).trim();
+    const clone = (remote: string) => {
+      const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'gstack-gate-diff-refs-'));
+      repos.push(dir);
+      gitIn(dir, ['clone', '-q', '-o', remote, '--branch', 'feature', `file://${src}`, '.']);
+      return dir;
+    };
+    const named = clone('upstream');
+    gitIn(named, ['remote', 'add', 'origin', 'file:///nonexistent-fork']);
+    gitIn(named, ['update-ref', 'refs/remotes/origin/upstream/main', 'HEAD']);
+    expect(summary(run(named, ['upstream/main']).stdout).tagged).toBe('RH-1:1');
+    const tagged = clone('origin');
+    gitIn(tagged, ['tag', 'origin/main', 'HEAD']);
+    expect(summary(run(tagged, ['main']).stdout).tagged).toBe('RH-1:1');
+    const fork = clone('zfork');
+    gitIn(fork, ['branch', '-f', 'main', seed]);
+    gitIn(fork, ['update-ref', 'refs/remotes/zfork/main', 'HEAD']);
+    expect(summary(run(fork, ['main']).stdout).tagged).toBe('RH-1:1');
+  });
+
   test('a clone whose only remote is upstream resolves the base to upstream/main, and the fix line fetches from upstream', () => {
     const src = repo({ fixture: 'rh1-skip', commit: true });
     const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'gstack-gate-diff-upstream-'));