perf(cache): tell a stale record from its header instead of parsing it (#633)

The sports plugins cache whole season schedules: 53MB for MLB, 18MB for
NHL, 17MB for NCAA baseball. On a Pi 4, orjson.loads of the MLB file
takes ~1.8s with the GIL held, and every thread in the display service
waits -- the stall watchdog caught the render thread frozen 0.5-1.3s with
the interpreter itself blocked, right on these reads. When a season record
expired, DiskCache.get paid that whole parse only to find the timestamp
too old and throw the result away.

CacheManager.set now writes timestamp and ttl ahead of the data, and
DiskCache.get reads them from the first 256 bytes of the file, applying
the same rule as before (a per-entry ttl wins over max_age; no limit
means never stale). A record that is stale is refused without being
parsed. Files in the old layout, and records from other writers, don't
match the header and are parsed in full as before.

Also: ESPN responses in the background data service and espn_dates are
parsed with orjson when it is installed (src/common/json_body.py). The
stdlib parser behind response.json() takes 3.1s on the MLB season
against orjson's 1.8s, both with the GIL held. espn_dates imports it with
a fallback, since plugins bundle copies of that module for older cores.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/background_data_service.py

@@ -26,6 +26,7 @@ from enum import Enum
 from concurrent.futures import ThreadPoolExecutor
 import pytz
 from src.cache_manager import CacheManager
+from src.common.json_body import response_json
 from src.common.espn_dates import (
     RANGE_RETRY_SECONDS,
     _note_range_rejected,
@@ -389,7 +390,7 @@ class BackgroundDataService:
                         response.raise_for_status()
                 else:
                     response.raise_for_status()
-                    data = response.json()
+                    data = response_json(response)
             
             # Validate data structure
             if not isinstance(data, dict):

src/cache/disk_cache.py

@@ -7,6 +7,7 @@ Handles persistent disk-based caching with atomic writes and error recovery.
 import json
 import math
 import os
+import re
 import stat
 import time
 import tempfile
@@ -98,6 +99,40 @@ def _replace_nonfinite(obj: Any) -> Any:
 # deleted. Both halves are covered by test/test_cache_nonfinite_floats.py.
 
 
+#: Enough of a record to hold its header: ``{"timestamp":<float>,"ttl":<n>,``.
+_HEAD_BYTES = 256
+
+#: A record written with its header first (CacheManager.set does). Anything
+#: else -- older files with "data" first, records from other writers -- does not
+#: match and is parsed in full, as before.
+_HEAD_RE = re.compile(
+    rb'\A\s*\{\s*"timestamp"\s*:\s*(-?[0-9][0-9.eE+-]*)\s*'
+    rb'(?:,\s*"ttl"\s*:\s*(-?[0-9][0-9.eE+-]*))?\s*[,}]'
+)
+
+
+def _stale_from_head(head: bytes, max_age: Optional[int], now: float) -> bool:
+    """True when a record's header alone shows it has expired.
+
+    Mirrors the expiry rule in DiskCache.get: a per-entry ttl wins over the
+    caller's max_age, and no limit at all means never stale. False whenever the
+    header cannot be read, so the full parse decides as it always did.
+    """
+    match = _HEAD_RE.match(head)
+    if not match:
+        return False
+    try:
+        timestamp = float(match.group(1))
+        limit = max_age
+        if match.group(2) is not None:
+            ttl = float(match.group(2))
+            if ttl >= 0:
+                limit = ttl
+    except ValueError:
+        return False
+    return limit is not None and (now - timestamp) > limit
+
+
 if orjson is not None:
     # Encoding the cache record dominated the background fetch worker: on a
     # Pi 4, stdlib json.dumps runs ~12ms per MB and holds the GIL for all of
@@ -266,6 +301,14 @@ class DiskCache:
         try:
             with self._lock:
                 with open(cache_path, 'rb') as f:
+                    # Decide staleness from the header before paying for the
+                    # parse. A stale read is the common case for the biggest
+                    # records (a season schedule is re-fetched when its cache
+                    # expires), and parsing 53MB to throw it away held the GIL
+                    # for ~1.8s -- a visible freeze on the panel.
+                    if _stale_from_head(f.read(_HEAD_BYTES), max_age, time.time()):
+                        return None
+                    f.seek(0)
                     record = _loads(f.read())
             
             # Determine record timestamp (prefer embedded, else file mtime)

src/cache_manager.py

@@ -522,8 +522,9 @@ class CacheManager:
     def update_cache(self, data_type: str, data: Dict[str, Any]) -> bool:
         """Update cache with new data."""
         cache_data = {
+            # Header first; see DiskCache's stale check.
+            'timestamp': time.time(),
             'data': data,
-            'timestamp': time.time()
         }
         return self.save_cache(data_type, cache_data)
 
@@ -556,12 +557,15 @@ class CacheManager:
                  from the key and is only a fallback for entries that did not
                  say. Omit it to keep that inferred behaviour.
         """
-        cache_data = {
-            'data': data,
-            'timestamp': time.time()
-        }
+        # timestamp and ttl before data, so they are the first bytes on disk:
+        # DiskCache.get reads them from the head of the file and can call a
+        # record stale without parsing it. That matters for the big ones -- a
+        # whole MLB season is 53MB and ~1.8s of orjson.loads with the GIL held,
+        # paid in full only to learn the record had expired.
+        cache_data: Dict[str, Any] = {'timestamp': time.time()}
         if ttl is not None:
             cache_data['ttl'] = ttl
+        cache_data['data'] = data
         self.save_cache(key, cache_data)
 
     @deprecated("3.7.0")

src/common/espn_dates.py

@@ -39,6 +39,14 @@ from datetime import date, timedelta
 from functools import partial
 from typing import Any, Dict, List, Optional, Tuple
 
+try:
+    from src.common.json_body import response_json
+except ImportError:
+    # Plugins bundle copies of this module for older cores, which predate
+    # json_body; the stdlib parse is what those cores always used.
+    def response_json(response: Any) -> Any:
+        return response.json()
+
 # Above this, ESPN returns a truncated list instead of an error. See module
 # docstring: 500 is the largest value measured to return complete data.
 ESPN_MAX_LIMIT = 500
@@ -194,7 +202,7 @@ def _fetch_one_chunk(
             timeout=timeout,
         )
         response.raise_for_status()
-        return response.json()
+        return response_json(response)
     except Exception as exc:  # noqa: BLE001 - see docstring
         if logger:
             logger.warning("ESPN chunk %s failed, skipping it: %s", chunk, exc)
@@ -371,4 +379,4 @@ def fetch_espn_scoreboard(
         if data is not None:
             return data
     response.raise_for_status()
-    return response.json()
+    return response_json(response)

src/common/json_body.py

@@ -0,0 +1,29 @@
+"""Parse an HTTP response body as JSON, with orjson when it is installed.
+
+``requests``' ``response.json()`` uses the stdlib parser. For the payloads the
+sports plugins fetch -- a season schedule is tens of MB -- that runs ~1.7x
+slower than orjson on a Pi 4 (3.1s against 1.8s for the 53MB MLB season), and
+both hold the GIL for the whole parse, which freezes the display for as long.
+Nothing else changes: the result is the same Python objects.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+try:
+    import orjson
+except ImportError:  # optional dependency; see docs/SCROLL_PERFORMANCE.md
+    orjson = None
+
+
+def response_json(response: Any) -> Any:
+    """``response.json()``, parsed by orjson when available."""
+    body = getattr(response, "content", None)
+    if orjson is None or not isinstance(body, (bytes, bytearray)):
+        return response.json()
+    try:
+        return orjson.loads(body)
+    except orjson.JSONDecodeError:
+        # Let requests raise its usual error, with its usual message.
+        return response.json()

test/test_cache_stale_header.py

@@ -0,0 +1,118 @@
+"""A stale cache record is recognised from its header, without parsing it.
+
+The sports plugins cache whole season schedules -- 53MB for MLB, 18MB for NHL.
+When one expired, DiskCache.get parsed all of it (~1.8s of orjson.loads on a
+Pi 4, GIL held, the whole display frozen) only to find the timestamp too old
+and throw the result away. CacheManager.set now writes timestamp and ttl ahead
+of the data, and DiskCache.get reads them from the first bytes of the file.
+"""
+
+import json
+import time
+from types import SimpleNamespace
+
+import pytest
+
+from src.cache import disk_cache as disk_cache_module
+from src.cache.disk_cache import DiskCache, _stale_from_head
+from src.common import json_body
+
+
+@pytest.fixture
+def disk(tmp_path):
+    return DiskCache(cache_dir=str(tmp_path))
+
+
+@pytest.fixture
+def parses(monkeypatch):
+    """Count full parses of cache files."""
+    calls = []
+    real = disk_cache_module._loads
+
+    def counting(raw):
+        calls.append(len(raw))
+        return real(raw)
+
+    monkeypatch.setattr(disk_cache_module, "_loads", counting)
+    return calls
+
+
+def _header_first(age=0.0, ttl=None, events=100):
+    record = {"timestamp": time.time() - age}
+    if ttl is not None:
+        record["ttl"] = ttl
+    record["data"] = {"events": [{"id": n, "name": "x" * 50} for n in range(events)]}
+    return record
+
+
+def test_cache_manager_writes_the_header_first(monkeypatch):
+    from src.cache_manager import CacheManager
+    written = {}
+    manager = CacheManager.__new__(CacheManager)
+    monkeypatch.setattr(manager, "save_cache",
+                        lambda key, record: written.update({key: record}),
+                        raising=False)
+    CacheManager.set(manager, "k", {"events": []}, ttl=60)
+    assert list(written["k"]) == ["timestamp", "ttl", "data"]
+    CacheManager.set(manager, "k", {"events": []})
+    assert list(written["k"]) == ["timestamp", "data"]
+
+
+def test_a_stale_record_is_not_parsed(disk, parses):
+    disk.set("season", _header_first(age=600))
+    assert disk.get("season", max_age=300) is None
+    assert parses == []
+
+
+def test_a_fresh_record_is_parsed_and_returned(disk, parses):
+    disk.set("season", _header_first(age=10))
+    record = disk.get("season", max_age=300)
+    assert record["data"]["events"][0]["id"] == 0
+    assert len(parses) == 1
+
+
+def test_the_entry_ttl_wins_over_max_age(disk, parses):
+    disk.set("long", _header_first(age=600, ttl=3600))
+    assert disk.get("long", max_age=300) is not None     # ttl says fresh
+    disk.set("short", _header_first(age=60, ttl=30))
+    parses.clear()
+    assert disk.get("short", max_age=300) is None        # ttl says stale
+    assert parses == []
+
+
+def test_no_limit_means_never_stale(disk):
+    disk.set("forever", _header_first(age=10 ** 7))
+    assert disk.get("forever", max_age=None) is not None
+
+
+def test_older_files_with_data_first_still_work(disk, parses):
+    # Records written before the header moved: parsed in full, as before.
+    disk.set("legacy_fresh", {"data": {"v": 1}, "timestamp": time.time()})
+    disk.set("legacy_stale", {"data": {"v": 1}, "timestamp": time.time() - 600})
+    assert disk.get("legacy_fresh", max_age=300)["data"] == {"v": 1}
+    assert disk.get("legacy_stale", max_age=300) is None
+    assert len(parses) == 2
+
+
+@pytest.mark.parametrize("head, stale", [
+    (b'{"timestamp":100.0,"data":{}}', True),
+    (b'{"timestamp": 100.0, "ttl": 1000, "data": {}}', False),   # stdlib spacing
+    (b'{"timestamp":1e2,"ttl":5,"data":1}', True),
+    (b'{"timestamp":100.0}', True),
+    (b'{"data":{},"timestamp":100.0}', False),                  # unknown layout
+    (b'{"timestamp":"100.0","data":{}}', False),                # string: parse it
+    (b'', False),
+])
+def test_reading_the_header(head, stale):
+    assert _stale_from_head(head, 300, now=1000.0) is stale
+
+
+def test_response_json_prefers_orjson_and_falls_back():
+    payload = {"events": [1, 2, 3]}
+    response = SimpleNamespace(content=json.dumps(payload).encode(),
+                               json=lambda: pytest.fail("used the slow path"))
+    if json_body.orjson is None:
+        pytest.skip("orjson not installed")
+    assert json_body.response_json(response) == payload
+    # A response object without bytes content (a test double) still works.
+    assert json_body.response_json(SimpleNamespace(json=lambda: payload)) == payload