fix(issue-45): review round 2 R3 - the .mam fallback obeys the same no-overwrite rules

The fallback wrote <.mam>/herdr-server-config.toml unchecked, so it could
replace a foreign file there, the source itself, or a link to it. It now goes
through output_path like the normal path (source identity incl. aliases,
generated-for-this-source ownership, then a per-source name) and writes
nothing when no name is safe.

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

.agents/skills/lib_py/herdr_config.py

@@ -14,9 +14,10 @@ to false. The user's file is never modified.
     from the server) is followed back to the source recorded in that file.
   * Output: `mam-server-config.toml` next to the source, so relative paths in
     it (herdr resolves them against the config file's directory) keep their
-    meaning. Never the source itself and never a file this module did not
-    write for the same source: then a name with a hash of the source path.
-    `<.mam>/herdr-server-config.toml` only when the directory is not writable.
+    meaning. `<.mam>/herdr-server-config.toml` only when that directory is not
+    writable. Either way never the source itself and never a file this module
+    did not write for the same source: then a name with a hash of the source
+    path, and no write at all when that is taken too.
   * The source is parsed, the key set, and the result serialized; the result
     must parse back to exactly the source plus the key (any TOML spelling of
     the key - header, dotted, inline - works). Then herdr's own `config check`
@@ -188,19 +189,20 @@ def _same_file(a: str, b: str) -> bool:
         return os.path.realpath(a) == os.path.realpath(b)
 
 
-def output_path(source: str) -> str:
-    """Beside the source: OUTPUT_NAME, unless that is the source itself or a
-    file this module did not write for this source; then a per-source name."""
-    d = os.path.dirname(source)
+def output_path(source: str, directory: Optional[str] = None, name: str = OUTPUT_NAME) -> str:
+    """`name` in `directory` (default: beside the source), unless that is the
+    source itself (aliases included) or a file this module did not write for
+    this source; then a per-source name. Every write goes through here."""
+    d = directory or os.path.dirname(source)
     tag = hashlib.sha256(os.path.realpath(source).encode()).hexdigest()[:12]
-    for name in (OUTPUT_NAME, f"mam-server-config-{tag}.toml"):
-        target = os.path.join(d, name)
+    for n in (name, f"{name[:-len('.toml')]}-{tag}.toml"):
+        target = os.path.join(d, n)
         if _same_file(target, source):
             continue
         if os.path.lexists(target) and _generated_source(target) not in ("", source):
             continue
         return target
-    raise OSError(f"no free output name beside {source}")
+    raise OSError(f"no free output name in {d}")
 
 
 def _write_atomic(path: str, text: str, mode: int) -> None:
@@ -268,6 +270,7 @@ def write_server_config(env: Mapping[str, str], fallback_dir: Optional[str] = No
     if fallback_dir:
         target = os.path.join(fallback_dir, FALLBACK_NAME)
         try:
+            target = output_path(source, fallback_dir, FALLBACK_NAME)
             if _write_checked(target, source, body, mode, herdr, warnings):
                 warnings.append(f"wrote {target} instead; relative paths in the herdr config may not resolve")
                 return target, warnings

VERSIONS.md

@@ -55,7 +55,7 @@
 - **실서버 테스트 (`tests/test_issue45_restore.py`, herdr 0.9.1)**: 짧은 일회용 HOME·세션과 가짜 claude 로, MAM bootstrap 경로에서 복원이 꺼짐(사용자 설정에 `[session]` 표가 있을 때 포함)과 같은 레이아웃을 일반 서버로 띄우면 `claude --resume` 이 뜨는 양성 대조, 닫기 조건, create → crash → resume 뒤 pane 하나만 남음을 확인한다. 수정을 끄면 A·B 테스트가 각각 실패함을 확인했다.
 - **단위 (`tests/test_issue45_herdr_config.py`)**: 합치기 12 가지 경우와 실제 `herdr config check`, 원본 경로 규칙, 쓰기·폴백.
 - **관련 스위트**: 새 두 파일 40 passed. issue9·shim contract·issue42·issue24 resume·uuid_target·issue21 prereqs·issue23_24 199 passed / 1 skipped(S11 기준 이동 후). tier1~4·issue21_phase2_loop_a 148 passed. 전체 스위트는 돌리지 않았다.
-- **리뷰**: 1회차 codex NOT PASS(R1 herdr 스키마 검증 없음, R2 점 표기·인라인 표에서 설정 유실, R3 원본과 출력 이름 충돌 시 덮어씀). 811b080 에서 수정.
+- **리뷰**: 1회차 codex NOT PASS(R1 herdr 스키마 검증 없음, R2 점 표기·인라인 표에서 설정 유실, R3 원본과 출력 이름 충돌 시 덮어씀). 811b080 에서 수정. 2회차 codex NOT PASS(R3 의 폴백 경로는 같은 검사를 거치지 않음), 폴백도 같은 이름 선택을 거치도록 수정.
 
 ### 🚀 `v4.8.4` — `.claude/skills` 링크 검사가 두 로컬 배치를 받고 깨진 링크를 잡는다 (2026-10-04, PR #47)
 

tests/test_issue45_herdr_config.py

@@ -170,11 +170,79 @@ def test_unwritable_config_dir_falls_back_to_mam_dir(tmp_path):
         assert path == str(mam / hc.FALLBACK_NAME)
         assert any("relative paths" in w for w in warnings), warnings
         assert tomllib.loads(Path(path).read_text())["ui"]["confirm_close"] is False
+        if HERDR:
+            assert hc.herdr_rejects(path, HERDR) is None
         assert hc.write_server_config(_env(tmp_path))[0] is None  # no fallback given
     finally:
         cfg_dir.chmod(0o755)
 
 
+# --- the fallback obeys the same rules (review round 2, R3) ----------------
+
+def _tag(source: Path) -> str:
+    import hashlib
+    return hashlib.sha256(os.path.realpath(source).encode()).hexdigest()[:12]
+
+
+@pytest.mark.skipif(os.geteuid() == 0, reason="root ignores directory permissions")
+def test_a_foreign_fallback_file_is_kept(tmp_path):
+    cfg_dir = tmp_path / "cfg"
+    cfg_dir.mkdir()
+    source = cfg_dir / "config.toml"
+    source.write_text("[ui]\nconfirm_close = false\n")
+    cfg_dir.chmod(0o555)
+    mam = tmp_path / ".mam"
+    mam.mkdir()
+    foreign = mam / hc.FALLBACK_NAME
+    foreign.write_bytes(b"# user-owned\n[ui]\nconfirm_close = true\n")
+    try:
+        path, warnings = hc.write_server_config({"HERDR_CONFIG_PATH": str(source)}, str(mam), HERDR)
+    finally:
+        cfg_dir.chmod(0o755)
+    assert foreign.read_bytes() == b"# user-owned\n[ui]\nconfirm_close = true\n"
+    assert path == str(mam / f"herdr-server-config-{_tag(source)}.toml"), warnings
+    assert tomllib.loads(Path(path).read_text())["ui"]["confirm_close"] is False
+
+
+@pytest.mark.parametrize("alias", [False, True], ids=["same-path", "symlink-alias"])
+def test_the_fallback_is_never_the_source(tmp_path, alias):
+    mam = tmp_path / ".mam"
+    mam.mkdir()
+    body = b"[ui]\nconfirm_close = false\n"
+    if alias:
+        source = tmp_path / "real.toml"
+        source.write_bytes(body)
+        (mam / hc.FALLBACK_NAME).symlink_to(source)
+        source_dir = tmp_path
+    else:
+        source = mam / hc.FALLBACK_NAME
+        source.write_bytes(body)
+        source_dir = mam
+    # both names beside the source are someone else's, so the fallback is used
+    for name in (hc.OUTPUT_NAME, f"mam-server-config-{_tag(source)}.toml"):
+        (source_dir / name).write_bytes(b"# foreign\n")
+    path, _ = hc.write_server_config({"HERDR_CONFIG_PATH": str(source)}, str(mam), HERDR)
+    assert source.read_bytes() == body
+    if alias:  # os.replace would swap out the user's link itself
+        assert (mam / hc.FALLBACK_NAME).is_symlink()
+    assert path and not os.path.samefile(path, source)
+    assert tomllib.loads(Path(path).read_text())["ui"]["confirm_close"] is False
+
+
+def test_no_write_when_every_name_is_taken(tmp_path):
+    mam = tmp_path / ".mam"
+    mam.mkdir()
+    source = tmp_path / "config.toml"
+    source.write_text("[ui]\nconfirm_close = false\n")
+    taken = [tmp_path / hc.OUTPUT_NAME, tmp_path / f"mam-server-config-{_tag(source)}.toml",
+             mam / hc.FALLBACK_NAME, mam / f"herdr-server-config-{_tag(source)}.toml"]
+    for f in taken:
+        f.write_bytes(b"# foreign\n")
+    path, warnings = hc.write_server_config({"HERDR_CONFIG_PATH": str(source)}, str(mam))
+    assert path is None and any("no free output name" in w for w in warnings), warnings
+    assert all(f.read_bytes() == b"# foreign\n" for f in taken)
+
+
 def test_cli_prints_the_path(tmp_path):
     env = dict(os.environ, HOME=str(tmp_path), PYTHONPATH=str(REPO_ROOT / ".agents" / "skills"))
     env.pop("HERDR_CONFIG_PATH", None)