Merge remote-tracking branch 'origin/main' into claude/fix-cache-write-dedup

# Conflicts:
#	CHANGELOG.md

CHANGELOG.md

@@ -42,6 +42,15 @@ accepts both, but the store flags the old spelling as deprecated
 - A stop request now clears an on-demand error. After a failed request,
   `/display/on-demand/status` kept reporting `status: error` for up to two
   minutes even after a stop.
+- Re-saving unchanged data through `CacheManager.set` no longer rewrites its
+  cache file. The disk cache already skipped a payload identical to the last
+  one written, but `set()` stamps every record with the current time, so the
+  skip never fired and every plugin rewrote its unchanged API data to the SD
+  card on every update cycle. Records are now compared without that
+  timestamp, and a skipped write moves the file's mtime to it instead; reads
+  treat a record as fresh from the later of the two, so it expires exactly
+  when the rewrite would have. Changed data or a changed `ttl` still writes,
+  and so does a file another process has replaced since.
 
 ## 3.7.0
 

src/cache/disk_cache.py

@@ -14,7 +14,7 @@ import tempfile
 import logging
 import threading
 import zlib
-from typing import Dict, Any, Optional, Protocol
+from typing import Dict, Any, Optional, Protocol, Tuple
 from datetime import datetime
 
 from src.common.path_safety import safe_path_component
@@ -111,18 +111,66 @@ _HEAD_RE = re.compile(
 )
 
 
-def _stale_from_head(head: bytes, max_age: Optional[int], now: float) -> bool:
+def _head_timestamp(head: bytes) -> Optional[Tuple[float, int]]:
+    """A header-first record's timestamp and the offset just past it.
+
+    None when the record does not start with a finite numeric timestamp.
+    """
+    match = _HEAD_RE.match(head)
+    if not match:
+        return None
+    try:
+        timestamp = float(match.group(1))
+    except ValueError:
+        return None
+    if not math.isfinite(timestamp):
+        return None
+    return timestamp, match.end(1)
+
+
+# FRESHNESS OF A SKIPPED WRITE
+# ----------------------------
+# CacheManager.set stamps every record with time.time(), so re-saving
+# unchanged data produced a different payload every time and DiskCache.set's
+# identical-payload skip never fired: every plugin rewrote its unchanged API
+# data to the SD card every update cycle. set() now compares header-first
+# records without their timestamp, and on a skip moves the file's mtime to
+# the timestamp the skipped record carried instead of rewriting it. So the
+# file's mtime is when its content was last saved, and a header-first
+# record is as fresh as the later of its embedded timestamp and its mtime.
+#
+# A real write sets the mtime to the embedded timestamp too, so mtime is
+# never later than the timestamp for a record written with an old one on
+# purpose -- only a skip can move it forward.
+
+
+def _refreshed_at(timestamp: float, mtime: float) -> float:
+    """When a header-first record was last saved, embedded time or mtime.
+
+    An mtime within a second of the timestamp is the write that carried it
+    (float rounding, or an older file whose mtime was not set to match),
+    not a skipped rewrite, and leaves the record as written.
+    """
+    return mtime if mtime > timestamp + 1.0 else timestamp
+
+
+def _stale_from_head(head: bytes, max_age: Optional[int], now: float,
+                     refreshed: Optional[float] = None) -> 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.
+    ``refreshed`` is the file's mtime: a skipped rewrite advances it rather
+    than the embedded timestamp (see "FRESHNESS OF A SKIPPED WRITE").
     """
     match = _HEAD_RE.match(head)
     if not match:
         return False
     try:
         timestamp = float(match.group(1))
+        if refreshed is not None:
+            timestamp = max(timestamp, refreshed)
         limit = max_age
         if match.group(2) is not None:
             ttl = float(match.group(2))
@@ -248,11 +296,13 @@ class DiskCache:
         self.cache_dir = cache_dir
         self.logger = logger or logging.getLogger(__name__)
         self._lock = threading.Lock()
-        # key -> adler32 of the last payload successfully written to the
+        # key -> ((length, adler32) of the last content written to the
-        # primary cache path; lets set() skip rewriting identical data
+        # primary cache path, (st_ino, st_size) of the file it left); lets
-        # (per-process only — worst case another process rewrites, never
+        # set() skip rewriting identical data. The file identity catches
-        # a missed write). Guarded by _lock.
+        # another process -- the web interface writes and clears keys too --
-        self._write_digests: Dict[str, int] = {}
+        # having replaced the file since, which would otherwise make the skip
+        # a missed write. Per-process only. Guarded by _lock.
+        self._write_digests: Dict[str, Tuple[Tuple[int, int], Tuple[int, int]]] = {}
     
     def get_cache_path(self, key: str) -> Optional[str]:
         """
@@ -306,7 +356,12 @@ class DiskCache:
                     # 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()):
+                    head = f.read(_HEAD_BYTES)
+                    stamp = _head_timestamp(head)
+                    fresh_at = None
+                    if stamp is not None:
+                        fresh_at = _refreshed_at(stamp[0], os.fstat(f.fileno()).st_mtime)
+                    if _stale_from_head(head, max_age, time.time(), fresh_at):
                         return None
                     f.seek(0)
                     record = _loads(f.read())
@@ -315,6 +370,11 @@ class DiskCache:
             record_ts = None
             if isinstance(record, dict):
                 record_ts = record.get('timestamp')
+                if fresh_at is not None and fresh_at > stamp[0]:
+                    # A skipped rewrite refreshed this record (see "FRESHNESS
+                    # OF A SKIPPED WRITE"); hand callers the time it was last
+                    # saved, as the rewrite would have.
+                    record['timestamp'] = record_ts = fresh_at
             if record_ts is None:
                 try:
                     record_ts = os.path.getmtime(cache_path)
@@ -403,24 +463,37 @@ class DiskCache:
             self.logger.warning("Cache data for key '%s' not serializable: %s", key, e)
             return
 
-        digest = zlib.adler32(payload)
+        # A header-first record is compared without its timestamp, which
+        # CacheManager.set changes on every call (see "FRESHNESS OF A SKIPPED
+        # WRITE"). The length rides along with adler32, which is weak on its
+        # own for short payloads, and a collision here is a missed write.
+        stamp = _head_timestamp(payload[:_HEAD_BYTES])
+        stamped_at = stamp[0] if stamp is not None else None
+        content = memoryview(payload)[stamp[1]:] if stamp is not None else payload
+        digest = (len(content), zlib.adler32(content))
 
         try:
             # Atomic write to avoid partial/corrupt files
             with self._lock:
-                # Skip the disk entirely when this exact payload was already
+                # Skip the disk entirely when this content was already
                 # written for this key (plugins re-save unchanged API data
                 # every update cycle — each write is real SD-card wear).
-                # Refresh the file mtime so records that rely on it for TTL
+                # Move the file mtime instead, so the record stays as fresh as
-                # (no embedded 'timestamp') don't expire early; a metadata
+                # the rewrite would have left it; a metadata touch is
-                # touch is journal-cheap compared to rewriting the data.
+                # journal-cheap compared to rewriting the data.
-                if self._write_digests.get(key) == digest:
+                known = self._write_digests.get(key)
+                if known is not None and known[0] == digest:
                     try:
-                        os.utime(cache_path, None)
+                        st = os.stat(cache_path)
-                        return
+                        if (st.st_ino, st.st_size) == known[1]:
+                            os.utime(cache_path, None if stamped_at is None
+                                     else (stamped_at, stamped_at))
+                            return
                     except OSError:
-                        # File vanished or perms changed — fall through and write
+                        pass
-                        self._write_digests.pop(key, None)
+                    # File vanished, was replaced by another process, or its
+                    # times cannot be set — fall through and write
+                    self._write_digests.pop(key, None)
 
                 tmp_dir = os.path.dirname(cache_path)
                 # Try to create temp file in cache directory first
@@ -458,7 +531,7 @@ class DiskCache:
                                 # opened it in between was refused.
                                 _share_open_file(tmp_file.fileno(), _shared_group(tmp_dir))
                             os.replace(tmp_path, cache_path)
-                            self._write_digests[key] = digest
+                            self._remember_write(key, cache_path, digest, stamped_at)
                         finally:
                             if os.path.exists(tmp_path):
                                 try:
@@ -471,7 +544,7 @@ class DiskCache:
                             with open(cache_path, 'wb') as cache_file:
                                 cache_file.write(payload)
                                 _share_open_file(cache_file.fileno(), _shared_group(tmp_dir))
-                            self._write_digests[key] = digest
+                            self._remember_write(key, cache_path, digest, stamped_at)
                             self.logger.debug("Wrote cache for %s directly (non-atomic)", key)
                         except (IOError, OSError, PermissionError) as write_error:
                             # If direct write also fails, try fallback location
@@ -520,6 +593,26 @@ class DiskCache:
             )
             return  # Exit gracefully without raising exception
     
+    def _remember_write(self, key: str, cache_path: str,
+                        digest: Tuple[int, int], stamped_at: Optional[float]) -> None:
+        """Record a completed write so an identical set() can skip the disk.
+
+        Caller holds _lock. A header-first record's mtime is set to its
+        timestamp, so only a skipped rewrite ever moves it later (see
+        "FRESHNESS OF A SKIPPED WRITE").
+        """
+        try:
+            if stamped_at is not None:
+                os.utime(cache_path, (stamped_at, stamped_at))
+            st = os.stat(cache_path)
+        except OSError:
+            # Written but not stamped (another user's file, on the direct
+            # write path): mtime is the write time, which _refreshed_at reads
+            # as the write itself. Remember nothing; the next set() writes.
+            self._write_digests.pop(key, None)
+            return
+        self._write_digests[key] = (digest, (st.st_ino, st.st_size))
+
     def clear(self, key: Optional[str] = None) -> None:
         """
         Clear cache entry or all entries.

test/test_cache_write_dedup.py

@@ -0,0 +1,158 @@
+"""Re-saving unchanged data through CacheManager.set does not rewrite the file.
+
+Regression under test: DiskCache.set skipped the disk when a payload matched
+the last one written for the key, but CacheManager.set stamps every record
+with time.time(), so no two payloads ever matched and every plugin rewrote its
+unchanged API data to the SD card on every update cycle. The skip now ignores
+the timestamp, and a skipped write moves the file's mtime instead -- so the
+entry must stay exactly as fresh as the rewrite would have left it.
+"""
+
+import os
+import time
+from unittest.mock import patch
+
+import pytest
+
+from src.cache import disk_cache as disk_cache_module
+from src.cache.disk_cache import DiskCache
+from src.cache_manager import CacheManager
+
+class Clock:
+    def __init__(self):
+        self.now = time.time()
+
+    def __call__(self):
+        return self.now
+
+
+@pytest.fixture
+def clock(monkeypatch):
+    fake = Clock()
+    monkeypatch.setattr(time, "time", fake)
+    return fake
+
+
+@pytest.fixture
+def writes(monkeypatch):
+    """Paths DiskCache.set actually replaced (its atomic write path)."""
+    replaced = []
+    real = os.replace
+
+    def counting(src, dst, *args, **kwargs):
+        replaced.append(os.path.basename(dst))
+        return real(src, dst, *args, **kwargs)
+
+    monkeypatch.setattr(disk_cache_module.os, "replace", counting)
+    return replaced
+
+
+def _manager(cache_dir):
+    with patch('src.cache_manager.CacheManager._get_writable_cache_dir',
+               return_value=str(cache_dir)):
+        manager = CacheManager()
+    manager.stop_cleanup_thread()
+    return manager
+
+
+@pytest.fixture
+def cm(tmp_path):
+    return _manager(tmp_path)
+
+
+DATA = {"events": [{"id": n, "name": "x" * 20} for n in range(50)]}
+
+
+def test_resaving_unchanged_data_does_not_rewrite_the_file(cm, clock, writes):
+    cm.set("scores", DATA)
+    path = cm._get_cache_path("scores")
+    first = os.stat(path)
+    with open(path, "rb") as f:
+        first_bytes = f.read()
+
+    for _ in range(5):
+        clock.now += 60
+        cm.set("scores", DATA)
+
+    assert writes == ["scores.json"]
+    after = os.stat(path)
+    assert after.st_ino == first.st_ino
+    with open(path, "rb") as f:
+        assert f.read() == first_bytes
+    # The file records when its content was last saved.
+    assert after.st_mtime == pytest.approx(clock.now, abs=1e-3)
+
+
+def test_skipped_writes_keep_the_entry_fresh(cm, clock, tmp_path):
+    cm.set("scores", DATA)
+    for _ in range(4):          # re-saved unchanged every 200s
+        clock.now += 200
+        cm.set("scores", DATA)
+    # 800s past the only real write, well beyond max_age=300 of it.
+    assert cm.get("scores", max_age=300) == DATA
+
+    # A reader with no memory tier -- the web interface, or this service
+    # after a restart -- sees the same freshness from disk.
+    other = _manager(tmp_path)
+    record = other.get_cached_data("scores", max_age=300)
+    assert record is not None and record["data"] == DATA
+    assert record["timestamp"] == pytest.approx(clock.now, abs=1e-3)
+    assert DiskCache(str(tmp_path)).get("scores", max_age=300) is not None
+
+    # Freshness is the last save, not forever.
+    clock.now += 301
+    assert DiskCache(str(tmp_path)).get("scores", max_age=300) is None
+    assert _manager(tmp_path).get("scores", max_age=300) is None
+
+
+def test_skipped_writes_keep_a_ttl_entry_fresh(cm, clock, tmp_path):
+    cm.set("odds", DATA, ttl=120)
+    for _ in range(3):
+        clock.now += 100
+        cm.set("odds", DATA, ttl=120)
+    assert _manager(tmp_path).get("odds", max_age=10) == DATA
+
+
+def test_changed_data_writes(cm, clock, writes, tmp_path):
+    cm.set("scores", DATA)
+    clock.now += 60
+    cm.set("scores", {"events": []})
+    assert writes == ["scores.json", "scores.json"]
+    assert _manager(tmp_path).get("scores", max_age=300) == {"events": []}
+
+
+def test_changed_ttl_writes(cm, clock, writes, tmp_path):
+    cm.set("scores", DATA, ttl=60)
+    clock.now += 10
+    cm.set("scores", DATA, ttl=600)
+    clock.now += 10
+    cm.set("scores", DATA)
+    assert writes == ["scores.json"] * 3
+    record = _manager(tmp_path).get_cached_data("scores", max_age=300)
+    assert "ttl" not in record
+
+
+def test_a_record_written_old_stays_old(tmp_path, clock):
+    # Only a skip may move freshness forward: a record saved with an old
+    # timestamp on purpose is not made fresh by the write's own mtime.
+    disk = DiskCache(str(tmp_path))
+    disk.set("k", {"timestamp": clock.now - 600, "data": DATA})
+    assert disk.get("k", max_age=300) is None
+
+
+def test_a_file_replaced_by_another_process_is_rewritten(tmp_path, clock):
+    ours, theirs = DiskCache(str(tmp_path)), DiskCache(str(tmp_path))
+    ours.set("k", {"timestamp": clock.now, "data": "ours"})
+    clock.now += 10
+    theirs.set("k", {"timestamp": clock.now, "data": "theirs"})
+    clock.now += 10
+    ours.set("k", {"timestamp": clock.now, "data": "ours"})
+    assert DiskCache(str(tmp_path)).get("k", max_age=300)["data"] == "ours"
+
+
+def test_a_cleared_file_is_rewritten(cm, clock, tmp_path):
+    cm.set("scores", DATA)
+    os.remove(cm._get_cache_path("scores"))   # e.g. the web UI's delete
+    clock.now += 10
+    cm.set("scores", DATA)
+    assert _manager(tmp_path).get("scores", max_age=300) == DATA