fix(core): harden text-measurement caches; surface snapshot failures

Deep-dive findings, all three latent on every 24/7 install:

- font_manager.metrics_cache and display_manager._text_width_cache were
  unbounded dicts keyed by (text, id(font)). Two problems: keys embed
  the measured TEXT, so ever-changing strings (a clock, a live score, a
  ticker) grow them without limit; and id()-keying without holding a
  reference means a garbage-collected font's id can be recycled by a
  DIFFERENT font, silently returning wrong widths/metrics (classic
  plugins create fonts per render, so this is reachable). Both caches
  are now LRU-bounded (1024) and pin the font in the entry so its id
  stays valid. metrics_cache also keyed on the text itself instead of
  hash(text), removing a collision path.

- _write_snapshot_if_due logged failures at DEBUG — invisible at the
  default level. The snapshot's mtime is the web UI's display mirror
  AND its hardware-liveness proxy, so a quiet failure freezes the
  mirror and makes health checks lie (seen in the field: a stale
  root-owned /tmp file froze it for a day). Failures now WARN, rate-
  limited to once per 5 minutes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FqzC1nzTWL4kaqgMaQZFam

src/display_manager.py

@@ -33,7 +33,8 @@ else:
 from contextlib import contextmanager
 from PIL import Image, ImageDraw, ImageFont
 import time
-from typing import Dict, Any, List, Optional
+from collections import OrderedDict
+from typing import Dict, Any, List, Optional, Tuple
 import logging
 import math
 import freetype
@@ -180,14 +181,25 @@ class DisplayManager:
         # the logical image is blitted to the matrix unchanged.
         self._double_sided = None  # dict {copies, axis, logical_width, logical_height} or None
         self._physical_image = None  # full-chain buffer reused each frame when tiling
-        # Text-width measurement cache: (text, id(font)) -> pixel_width
+        # Text-width measurement cache: (text, id(font)) -> (width, font_ref)
         # Avoids re-measuring the same string+font on every display() call.
+        # LRU-bounded: keys embed the TEXT, so changing strings (a clock, a
+        # live score) would otherwise grow it forever on a 24/7 service.
+        # Entries hold a strong reference to the font so its id() can't be
+        # recycled by a different font object — an id-keyed cache without
+        # the reference can return the WRONG width after garbage collection.
         # Cleared on _load_fonts() so stale entries don't survive a font reload.
-        self._text_width_cache: Dict[tuple, int] = {}
+        self._text_width_cache: "OrderedDict[tuple, Tuple[int, Any]]" = OrderedDict()
+        self._TEXT_WIDTH_CACHE_MAX = 1024
         # Snapshot settings for web preview integration (service writes, web reads)
         self._snapshot_path = "/tmp/led_matrix_preview.png"  # nosec B108 - fixed path intentional; web UI reads same path
         self._snapshot_min_interval_sec = 0.2  # max ~5 fps
         self._last_snapshot_ts = 0.0
+        # Snapshot failures are logged as warnings, rate-limited so a
+        # persistent failure (e.g. an unwritable file) can't spam the log —
+        # but is never silent: the snapshot's mtime doubles as the web UI's
+        # hardware-liveness signal, so a quiet failure makes health checks lie.
+        self._snapshot_fail_log_ts = 0.0
         
         # Scrolling state tracking for graceful updates
         self._scrolling_state = {
@@ -699,12 +711,15 @@ class DisplayManager:
 
         Results are cached by (text, font identity) so plugins that measure
         the same string every frame (e.g. to centre a score) pay only one
-        measurement per unique (text, font) pair.
+        measurement per unique (text, font) pair. The entry keeps the font
+        alive so its id() can't be recycled, and the cache is LRU-bounded so
+        ever-changing text (clocks, tickers) can't grow it without limit.
         """
         cache_key = (text, id(font))
         cached = self._text_width_cache.get(cache_key)
         if cached is not None:
-            return cached
+            self._text_width_cache.move_to_end(cache_key)
+            return cached[0]
 
         try:
             if isinstance(font, freetype.Face):
@@ -719,7 +734,9 @@ class DisplayManager:
             logger.error("Error getting text width: %s", e)
             return 0
 
-        self._text_width_cache[cache_key] = width
+        self._text_width_cache[cache_key] = (width, font)
+        while len(self._text_width_cache) > self._TEXT_WIDTH_CACHE_MAX:
+            self._text_width_cache.popitem(last=False)
         return width
 
     def get_font_height(self, font):
@@ -1164,5 +1181,15 @@ class DisplayManager:
                 pass
             self._last_snapshot_ts = now
         except Exception as e:
-            # Snapshot failures should never break display; log at debug to avoid noise
+            # Snapshot failures must never break display — but they must not
+            # be silent either: the snapshot's mtime is the web UI's display
+            # mirror AND its hardware-liveness proxy, so a quietly failing
+            # write freezes the mirror and makes health checks lie (seen in
+            # the field: a stale root-owned /tmp file froze it for a day).
+            # Warn at most once per 5 minutes to avoid log spam.
+            if (now - self._snapshot_fail_log_ts) > 300:
+                self._snapshot_fail_log_ts = now
+                logger.warning("Snapshot write failing (web preview/health "
+                               "mirror is stale): %s", e)
+            else:
                 logger.debug(f"Snapshot write skipped: {e}")

src/font_manager.py

@@ -35,6 +35,7 @@ import urllib.request
 import zipfile
 import tempfile
 import time
+from collections import OrderedDict
 from pathlib import Path
 from PIL import ImageFont
 from typing import Dict, Tuple, Optional, Union, Any, List
@@ -58,7 +59,13 @@ class FontManager:
         # Font discovery and catalog
         self.font_catalog: Dict[str, str] = {}  # family_name -> file_path
         self.font_cache: Dict[str, Union[ImageFont.FreeTypeFont, freetype.Face]] = {}  # (family, size) -> font
-        self.metrics_cache: Dict[str, Tuple[int, int, int]] = {}  # (text, font_id) -> (width, height, baseline)
+        # (text, id(font)) -> ((width, height, baseline), font_ref).
+        # LRU-bounded — keys embed the measured TEXT, so changing strings
+        # (clocks, live scores) would otherwise grow it forever. Entries
+        # keep the font alive so its id() can't be recycled by a different
+        # font object (which would silently return wrong metrics).
+        self.metrics_cache: "OrderedDict[Any, Tuple[Tuple[int, int, int], Any]]" = OrderedDict()
+        self._METRICS_CACHE_MAX = 1024
 
         # Plugin font management
         self.plugin_fonts: Dict[str, Dict[str, Any]] = {}  # plugin_id -> font_manifest
@@ -507,10 +514,14 @@ class FontManager:
         Returns:
             Tuple of (width, height, baseline_offset)
         """
-        cache_key = f"{hash(text)}_{id(font)}"
+        # Key on the text itself (hash(text) could collide) + font identity;
+        # the entry below keeps the font referenced so the id stays valid.
+        cache_key = (text, id(font))
 
-        if cache_key in self.metrics_cache:
+        cached = self.metrics_cache.get(cache_key)
-            return self.metrics_cache[cache_key]
+        if cached is not None:
+            self.metrics_cache.move_to_end(cache_key)
+            return cached[0]
 
         try:
             if isinstance(font, freetype.Face):
@@ -547,7 +558,9 @@ class FontManager:
             baseline = 10
 
         result = (width, height, baseline)
-        self.metrics_cache[cache_key] = result
+        self.metrics_cache[cache_key] = (result, font)
+        while len(self.metrics_cache) > self._METRICS_CACHE_MAX:
+            self.metrics_cache.popitem(last=False)
         return result
 
     def get_font_height(self, font: Union[ImageFont.FreeTypeFont, freetype.Face]) -> int: