fix(pm): judge extra platform gates without packaging in the caller

The historical update takeover runs PM in the OLD venv's interpreter, which
need not ship `packaging`. Carrying lazily installed extras into the first PM
generation made legacy_selection() evaluate every platform gate there, so
every update from an old release failed with "No module named 'packaging'".

When `packaging` is missing, evaluate the marker in PM's runtime (which owns
it), and only judge gates for extras the old venv actually carried. The
takeover fixture now carries a gated extra under -S, reproducing the failure.

pm/_marker_eval.py

@@ -0,0 +1,12 @@
+"""Evaluate one PEP 508 marker inside PM's runtime: `_marker_eval.py MARKER ENV_JSON`.
+
+Prints "1" or "0". Callers whose interpreter has no `packaging` (a historical
+takeover running under an old venv) delegate here; the PM runtime owns it.
+"""
+
+import json
+import sys
+
+from packaging.markers import Marker
+
+print("1" if Marker(sys.argv[1]).evaluate(environment=json.loads(sys.argv[2])) else "0")

pm/extras.py

@@ -142,8 +142,6 @@ def extra_supported(extra: str, *, environment: dict[str, str] | None = None,
     import platform
     import sys
 
-    from packaging.markers import Marker
-
     if environment is None:
         environment = {
             "sys_platform": sys.platform,
@@ -151,6 +149,12 @@ def extra_supported(extra: str, *, environment: dict[str, str] | None = None,
             "platform_machine": platform.machine(),
             "os_name": os.name,
         }
+    try:
+        from packaging.markers import Marker
+    except ImportError:
+        # The historical update takeover runs PM in the OLD venv's
+        # interpreter, which need not ship `packaging`. PM's runtime does.
+        return _evaluate_in_runtime(marker, environment)
     try:
         return bool(Marker(marker).evaluate(environment=environment))
     except Exception:
@@ -159,6 +163,24 @@ def extra_supported(extra: str, *, environment: dict[str, str] | None = None,
         return True
 
 
+def _evaluate_in_runtime(marker: str, environment: dict[str, str]) -> bool:
+    import json
+    import subprocess
+
+    from pm.runtime import runtime_command, runtime_environment
+
+    try:
+        command = runtime_command(Path(__file__).with_name("_marker_eval.py"),
+                                  [marker, json.dumps(environment)])
+        result = subprocess.run(command, env=runtime_environment(), capture_output=True,
+                                text=True, timeout=60)
+    except Exception:
+        return True  # same fallback as a malformed marker: the resolver decides
+    if result.returncode != 0:
+        return True
+    return result.stdout.strip() == "1"
+
+
 def install_hint(extra: str) -> str:
     """The one command users are told to run for a missing extra."""
     return f"hermes pm install --extra {extra}"
@@ -269,7 +291,8 @@ def legacy_selection(project_root: Path) -> list[str]:
         # would install every sibling the user never chose.
         extra for extra in ANCHORS if extra not in {"messaging", "voice", "wake"}
         # PM refuses a gated extra outside its platform even if a hand-synced venv carried it.
-        if extra_supported(extra, importable=lambda _anchor: False)
-        and any(all(_installed_in(tree, anchor) for anchor in _anchors(extra)) for tree in trees)
+        # Installed first: judging a gate may cost a PM-runtime subprocess.
+        if any(all(_installed_in(tree, anchor) for anchor in _anchors(extra)) for tree in trees)
+        and extra_supported(extra, importable=lambda _anchor: False)
     )
     return ["all", *carried]

tests/pm/test_extras.py

@@ -238,3 +238,16 @@ def test_legacy_selection_carries_extras_the_main_era_venv_lazily_installed(monk
     assert "messaging" not in selection
     assert "piper" not in selection
     assert extras.legacy_selection(tmp_path / "no-venv") == ["all"]
+
+
+def test_runtime_marker_evaluation_answers_for_the_given_environment():
+    """The delegate really evaluates the marker (in PM's runtime interpreter)."""
+    import subprocess
+    from pathlib import Path
+
+    helper = Path(extras.__file__).with_name("_marker_eval.py")
+    env = '{"sys_platform": "linux"}'
+    out = [subprocess.run([sys.executable, str(helper), marker, env], capture_output=True,
+                          text=True, timeout=60, check=True).stdout.strip()
+           for marker in ("sys_platform == 'linux'", "sys_platform == 'win32'")]
+    assert out == ["1", "0"]

tests/pm/test_takeover_preparation.py

@@ -30,7 +30,8 @@ def test_fresh_takeover_prepares_generation_and_runs_selected_python(tmp_path):
     _wheel(wheels, "takeover_dep", "1.0")
     (root / "pyproject.toml").write_text(
         '[project]\nname="takeover-fixture"\nversion="1"\nrequires-python=">=3.14"\n'
-        'dependencies=["takeover-dep==1.0"]\n[project.optional-dependencies]\nall=[]\n'
+        'dependencies=["takeover-dep==1.0"]\n[project.optional-dependencies]\nall=[]\nmatrix=[]\n'
+        '[tool.hermes.extras-platforms]\nmatrix="sys_platform == \'no-such-platform\'"\n'
         '[tool.uv]\npackage=false\nno-index=true\n'
         f'find-links=[{json.dumps(wheels.as_posix())}]\n', encoding="utf-8")
     (root / ".git").mkdir()
@@ -41,6 +42,11 @@ def test_fresh_takeover_prepares_generation_and_runs_selected_python(tmp_path):
            if not key.startswith(("HERMES_", "PYTHON", "UV_"))}
     env.update(HOME=str(home), HERMES_HOME=str(home), HERMES_RUNTIME_DIR=str(store), UV_PYTHON_DOWNLOADS="never")
     subprocess.run([uv, "lock", "--offline", "--python", sys.executable], cwd=root, env=env, check=True, capture_output=True)
+    # A main-era venv that carried a gated extra: the takeover interpreter
+    # (-S, so no `packaging`) must still judge the gate and drop it.
+    legacy_site = root / ".venv" / "lib" / "python3.11" / "site-packages"
+    for anchor in ("mautrix", "asyncpg", "aiosqlite", "markdown", "aiohttp_socks"):
+        (legacy_site / anchor).mkdir(parents=True)
     # Only the interpreter and private manager tool are needed by this tiny
     # application. They are real host tools; PM's worker and resolver stay real.
     (root / "pm/lock.json").write_text('{"schema":1,"packages":{}}', encoding="utf-8")
@@ -82,6 +88,8 @@ def test_fresh_takeover_prepares_generation_and_runs_selected_python(tmp_path):
     assert (root / ".hermes/bin/hermes").is_file()
     assert not (root / "venv").exists()
     assert Facts(store / "facts.json").get("dmgbuild") is None
+    venv_fact = json.loads(next((home / "installs").glob("*/facts.json")).read_text())["packages"]["venv"]
+    assert "matrix" not in venv_fact["extras"]
 
     # Repair restores the recorded graph; a checkout update must also advance
     # that graph to the new source inputs before normal bootstrap checks it.