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fix(core): harden text-measurement caches; surface snapshot failures (#400)

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* 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

* test: sync mock cache manager signature with CacheManager.get

test_circuit_breaker has been failing on main: plugin_health passes
memory_ttl= to cache_manager.get(), and the conftest mock's signature
was never updated — the same component/double drift class as the
monitored_update bug (#392).

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

---------

Co-authored-by: Chuck <chuck@example.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
This commit is contained in:
Chuck
2026-07-13 08:59:41 -04:00
committed by GitHub
co-authored by Claude Fable 5 Chuck
parent 6edd80d9f3
commit 9e3b5f366e
3 changed files with 58 additions and 14 deletions
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src/display_manager.py
+35 -8
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@@ -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
logger.debug(f"Snapshot write skipped: {e}")
# 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}")