fix(pm): canonicalize default store before runtime identity

pm/environments.py

@@ -120,10 +120,13 @@ def store_root(project_root: Path) -> Path:
             try:
                 data = json.loads(stamp.read_text(encoding="utf-8-sig"))
             except (OSError, ValueError):
-                return get_default_hermes_root() / "tools"
+                break
             value = data.get("runtimeDir") if isinstance(data, dict) else None
-            return Path(value).resolve() if value else get_default_hermes_root() / "tools"
-    return get_default_hermes_root() / "tools"
+            if value:
+                return Path(value).resolve()
+            break
+    # PM hashes the interpreter path: aliases of one store must share an identity.
+    return (get_default_hermes_root() / "tools").resolve()
 
 
 def flush_before_selecting() -> None:

tests/pm/test_store_alias_identity.py

@@ -0,0 +1,96 @@
+"""Regression for #123798: aliases of one store must reuse its PM generation."""
+import json
+from pathlib import Path
+
+import pytest
+
+
+@pytest.mark.parametrize("stamp", [None, {}, [], "malformed", "unreadable"])
+def test_home_store_alias_reuses_selected_runtime(tmp_path, monkeypatch, stamp):
+    from pm import paths, runtime
+
+    project = tmp_path / "project"
+    project.mkdir()
+    (project / "pyproject.toml").write_text("[project]\nname='fixture'\n")
+    (project / "uv.lock").write_text("version = 1\n")
+    stamp_path = project / "install-stamp.json"
+    if stamp is not None:
+        stamp_path.write_text(stamp if isinstance(stamp, str) else json.dumps(stamp))
+    if stamp == "unreadable":
+        read_text = Path.read_text
+
+        def read(path, *args, **kwargs):
+            if path == stamp_path:
+                raise PermissionError("fixture stamp cannot be read")
+            return read_text(path, *args, **kwargs)
+
+        monkeypatch.setattr(Path, "read_text", read)
+    monkeypatch.setattr(paths, "repo_root", lambda: project)
+    monkeypatch.delenv("HERMES_RUNTIME_DIR", raising=False)
+    monkeypatch.delenv("HERMES_INSTALL_ROOT", raising=False)
+    home = tmp_path / "owner"
+    store = home / "tools"
+    store.mkdir(parents=True)
+    alias = tmp_path / "alternate"
+    alias.mkdir()
+    try:
+        (alias / "tools").symlink_to(store, target_is_directory=True)
+    except OSError as exc:
+        pytest.skip(f"directory symlinks unavailable: {exc}")
+    (store / "python").touch()
+    selected = tmp_path / "pm-runtime"
+    generation = selected / "generations" / "existing"
+    generation.mkdir(parents=True)
+    (generation / "pm-runtime.json").write_text("{}")
+    record = {"inputs": runtime._inputs(project, store.resolve() / "python"),
+              "generation": "generations/existing"}
+    (selected / "selected.json").write_text(json.dumps(record))
+    before = (selected / "selected.json").read_bytes()
+    validated = []
+    # Dependency validation is outside this path-identity contract; staging is
+    # forbidden, so no interpreter/dependency installation can mask a mismatch.
+    monkeypatch.setattr(runtime, "_validate", lambda python, env: validated.append(python) or "")
+    monkeypatch.setattr("pm.runtime_stage.stage_runtime", lambda *a, **kw: pytest.fail("rebuilt current runtime"))
+
+    for active in (home, alias, home):
+        monkeypatch.setenv("HERMES_HOME", str(active))
+        result = runtime.prepare_runtime(
+            store / "uv", paths.store_root() / "python", selected,
+            project=project, bootstrap=False,
+        )
+        assert result == runtime._python(generation)
+        assert paths.store_root() == store.resolve()
+        assert (selected / "selected.json").read_bytes() == before
+    assert validated == [runtime._python(generation)] * 3
+    assert list((selected / "generations").iterdir()) == [generation]
+
+
+@pytest.mark.parametrize("source", ["override", "stamp", "payload", "missing-default"])
+def test_store_resolution_keeps_caller_precedence(tmp_path, monkeypatch, source):
+    from pm import environments
+
+    project = tmp_path / "payload" / "project"
+    project.mkdir(parents=True)
+    home = tmp_path / "home"
+    monkeypatch.setenv("HERMES_HOME", str(home))
+    monkeypatch.delenv("HERMES_RUNTIME_DIR", raising=False)
+    monkeypatch.delenv("HERMES_INSTALL_ROOT", raising=False)
+    expected = home / "tools"
+    if source != "missing-default":
+        expected = project.parent / "chosen"
+        expected.mkdir()
+        alias = project.parent / "alias"
+        try:
+            alias.symlink_to(expected, target_is_directory=True)
+        except OSError as exc:
+            pytest.skip(f"directory symlinks unavailable: {exc}")
+        if source == "override":
+            monkeypatch.setenv("HERMES_RUNTIME_DIR", str(alias))
+            (project.parent / "manifest.json").write_text("invalid, overridden")
+        elif source == "stamp":
+            (project / "install-stamp.json").write_text(json.dumps({"runtimeDir": str(alias)}))
+        else:
+            (project.parent / "manifest.json").write_text(json.dumps({"repo": "project", "store": "alias"}))
+    assert environments.store_root(project) == expected.resolve()
+    if source == "missing-default":
+        assert not expected.exists(), "resolving a missing store must not create it"