shadow: fail closed instead of raising on unreadable lock/fact inputs

The enforce loop has no per-tick guard, so an OSError/ValueError from
paused_by or read_now would crash-loop the daemon and stop enforced
billing. Such a tick now bills shadow time unpaused (Q8) and logs a warning.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Dsh4Znwh2KnC7q6vbGpX1N

steam_backlog_enforcer/_shadow.py

@@ -24,6 +24,7 @@ Overlapping sources bill the wall clock once and split it evenly, so
 from __future__ import annotations
 
 from dataclasses import dataclass, replace
+import logging
 from pathlib import Path
 from typing import TYPE_CHECKING, Final
 
@@ -44,6 +45,8 @@ CLAIMS_DIR: Final = Path("/run/user/1000/gatelock/claims")
 _MAX_AGE_SECONDS: Final = 15
 FAILCLOSED_KEY: Final = "failclosed:watcher"
 
+logger = logging.getLogger(__name__)
+
 
 @dataclass(frozen=True)
 class ShadowVerdict:
@@ -90,12 +93,18 @@ def observe_shadow(owners: dict[int, str], *, stamp: float) -> ShadowVerdict:
         stamp: Epoch seconds of this tick, the freshness reference.
 
     Returns:
-        The tick's shadow verdict.
+        The tick's shadow verdict; the fail-closed one if an input raised.
     """
-    paused = paused_by(CLAIMS_DIR)
-    if paused is not None:
-        return ShadowVerdict(paused_by=paused, keys=())
-    fact = read_now(NOW_PATH, stamp, max_age=_MAX_AGE_SECONDS)
+    # The enforce loop has no per-tick guard, so a raise here would crash-loop
+    # the daemon and stop *enforced* billing. An unreadable input bills, unpaused.
+    try:
+        paused = paused_by(CLAIMS_DIR)
+        if paused is not None:
+            return ShadowVerdict(paused_by=paused, keys=())
+        fact = read_now(NOW_PATH, stamp, max_age=_MAX_AGE_SECONDS)
+    except (OSError, ValueError) as exc:
+        logger.warning("Shadow inputs unreadable (%s); failing closed.", exc)
+        fact = None
     return ShadowVerdict(paused_by=None, keys=shadow_keys(owners, fact))
 
 

steam_backlog_enforcer/tests/test_shadow.py

@@ -85,6 +85,19 @@ class TestObserveShadow:
         assert verdict.keys == ("domain:youtube.com",)
         read.assert_called_once_with(_shadow.NOW_PATH, T0, max_age=15)
 
+    @pytest.mark.parametrize("where", ["paused_by", "read_now"])
+    def test_a_failing_input_fails_closed_instead_of_raising(self, where: str) -> None:
+        """The enforce loop has no per-tick guard: a raise would crash-loop it."""
+        with (
+            patch.object(_shadow, "paused_by", return_value=None),
+            patch.object(_shadow, "read_now", return_value=None),
+            patch.object(_shadow, where, side_effect=PermissionError("denied")),
+        ):
+            assert observe_shadow({1: "app:440"}, stamp=T0) == ShadowVerdict(
+                paused_by=None, keys=("app:440",)
+            )
+            assert observe_shadow({}, stamp=T0).keys == (FAILCLOSED_KEY,)
+
     def test_root_paths_are_the_desktop_users(self) -> None:
         assert str(_shadow.NOW_PATH) == "/run/user/1000/leisure-watch/now.json"
         assert str(_shadow.CLAIMS_DIR) == "/run/user/1000/gatelock/claims"