LEDMatrix/src/cache/disk_cache.py

"""
Disk Cache

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
import logging
import threading
import zlib
from typing import Dict, Any, Optional, Protocol, Tuple
from datetime import datetime

from src.common.path_safety import safe_path_component

try:  # optional: large speedup on the cache write path, see _dumps below
    import orjson
except ImportError:  # pragma: no cover - exercised on hosts without the wheel
    orjson = None

# How old an abandoned write's temp file must be before the sweep removes it.
# A real write holds its temp file for milliseconds, so an hour is far beyond
# any in-flight write while still clearing the same day's debris. Deliberately
# not tied to the retention policies: those describe how long data stays
# useful, and a half-written file was never useful.
_ORPHAN_TEMP_MAX_AGE_SECONDS = 3600



class CacheStrategyProtocol(Protocol):
    """Protocol for cache strategy objects that categorize cache keys."""
    
    def get_data_type_from_key(self, key: str) -> str:
        """
        Determine the data type from a cache key.
        
        Args:
            key: Cache key
            
        Returns:
            Data type string for strategy lookup
        """
        ...


class DateTimeEncoder(json.JSONEncoder):
    """JSON encoder that handles datetime objects.

    Retained for the stdlib fallback path and for any caller importing it.
    """
    def default(self, obj: Any) -> Any:
        if isinstance(obj, datetime):
            return obj.isoformat()
        return super().default(obj)


def _datetime_default(obj: Any) -> Any:
    """Serialise datetimes exactly as DateTimeEncoder did."""
    if isinstance(obj, datetime):
        return obj.isoformat()
    raise TypeError(f"Object of type {type(obj).__name__} is not JSON serializable")


def _replace_nonfinite(obj: Any) -> Any:
    """Non-finite floats -> None, matching what ``orjson.dumps`` writes.

    Only reached once a strict pass has proved there is something to replace,
    so the ordinary write path never pays for this walk.
    """
    if isinstance(obj, float):
        return obj if math.isfinite(obj) else None
    if isinstance(obj, dict):
        return {k: _replace_nonfinite(v) for k, v in obj.items()}
    if isinstance(obj, (list, tuple)):
        return [_replace_nonfinite(v) for v in obj]
    return obj


# NON-FINITE FLOATS
# -----------------
# JSON has no NaN or Infinity. The stdlib emits them anyway as an extension;
# orjson refuses to and writes null. That divergence is not acceptable in a
# cache whose files outlive the decision of which encoder is installed, so the
# policy here is one behaviour on both paths:
#
#   writing  non-finite floats become null, whichever encoder is in use
#   reading  files already on disk that carry the stdlib's NaN/Infinity
#            tokens stay readable, whichever encoder is in use
#
# Without the write half, installing orjson silently changed cached values.
# Without the read half, installing orjson turned every legacy record holding a
# NaN into a "corrupted cache file" that DiskCache.get logged as an error and
# 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*[,}]'
)


# UNCHANGED RE-SAVES: THE FILE'S MTIME CARRIES THE NEWER TIMESTAMP
# ----------------------------------------------------------------
# Plugins re-save unchanged API data every update cycle, and every one of
# those saves was a full rewrite on the SD card. DiskCache.set skips the write
# when the payload matches the last one it wrote for the key -- but
# CacheManager.set stamps each record with time.time(), so for set() the
# payload never matched and the skip never fired.
#
# The digest now leaves out a header-first record's timestamp, so an unchanged
# set() is skipped. What the skip must not do is make the record look older
# than it is: the timestamp inside the file is from the last real write, and
# a reader in another process (the web interface, with memory_ttl=0) or after
# a restart would call fresh data stale. So the newer timestamp goes where it
# costs no data write -- the file's mtime -- and readers take a record's age
# from the newer of the two. The invariant that makes that safe:
#
#   a file's mtime is the timestamp of the newest record saved for its key
#
#   real write  mtime is set to the record's own timestamp, so a record saved
#               with an old timestamp (data as of some earlier time) cannot
#               borrow freshness from the moment it hit the disk
#   skip        mtime is set to the skipped record's timestamp -- exactly what
#               a rewrite would have stored, without the rewrite
#
# Readers of the on-disk timestamp, all of which go through _effective_timestamp:
# DiskCache.get (the header check and the full parse; it also returns the
# record with 'timestamp' set to the effective value, so CacheManager.get's
# max_age path, the memory tier hydrated from disk, and any plugin reading
# record['timestamp'] all see it). Readers that use mtime alone already see the
# newer value: the retention sweep below, CacheManager.list_cache_files (the
# web UI's cache list). Nothing else opens cache files: web_interface and
# scripts reach them only through CacheManager.
#
# Something other than this class can also move an mtime forward -- a copy
# without -p, an rsync without -t, a `touch`. (backup_manager.py does not
# back up or restore the cache directory, so the in-tree restore cannot.) That
# must not make old data fresh, so the lift is bounded: a reader never takes
# the mtime as more than _MAX_TIMESTAMP_LIFT past the embedded timestamp, and
# set() rewrites the file for real once a skip would need more than that, so
# an honest lift never reaches the bound. A file copied a day after it was
# written therefore reads at most an hour fresher than its contents say, and a
# 30-second live-score record from yesterday stays stale. CacheManager.set
# records written before this change have mtime == write time == embedded
# timestamp, give or take the write itself, and read exactly as before; a
# file an older version wrote or touched later than its embedded timestamp
# says reads at most the same hour fresher, once, until it is next saved.

#: Longest a skipped write may stand in for a real one, and so the furthest a
#: file's mtime is ever trusted past the record's own timestamp. Unchanged data
#: is rewritten at least this often, at most once an hour per key instead of
#: once per update cycle.
_MAX_TIMESTAMP_LIFT = 3600.0


def _record_timestamp(value: Any) -> Optional[float]:
    """A record's timestamp as a finite float, or None if it has no usable one."""
    if isinstance(value, bool) or not isinstance(value, (int, float)):
        return None
    value = float(value)
    return value if math.isfinite(value) else None


def _effective_timestamp(embedded: float, mtime: Optional[float]) -> float:
    """When a record was last saved: its timestamp, or the file's mtime if a
    later unchanged save moved that forward -- never by more than
    _MAX_TIMESTAMP_LIFT. See "UNCHANGED RE-SAVES" above."""
    if mtime is None:
        return embedded
    return max(embedded, min(mtime, embedded + _MAX_TIMESTAMP_LIFT))


def _stale_from_head(head: bytes, max_age: Optional[int], now: float,
                     mtime: 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. ``mtime``
    is the file's, which may carry a newer save than the header does.
    """
    match = _HEAD_RE.match(head)
    if not match:
        return False
    try:
        timestamp = _effective_timestamp(float(match.group(1)), mtime)
        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
    # it, which stalls the render thread mid-scroll. orjson measures ~7x
    # faster on the same payloads (11.9ms -> 1.6ms for 985KB). Decoding gains
    # far less (~1.3x on large payloads) because the cost there is building
    # the Python objects, not scanning the text, but it is still free to take.
    #
    # OPT_NON_STR_KEYS: stdlib json coerces int/float dict keys to strings;
    # orjson raises without this, and cache records do carry numeric keys.
    # OPT_PASSTHROUGH_DATETIME: orjson would otherwise emit its own RFC 3339
    # form for datetimes instead of calling default(). Routing them through
    # _datetime_default keeps byte-for-byte parity with the records already
    # on disk.
    _DUMPS_OPTS = orjson.OPT_NON_STR_KEYS | orjson.OPT_PASSTHROUGH_DATETIME

    def _dumps(data: Any) -> bytes:
        return orjson.dumps(data, default=_datetime_default, option=_DUMPS_OPTS)

    def _loads(raw: bytes) -> Any:
        try:
            return orjson.loads(raw)
        except orjson.JSONDecodeError:
            # Legacy record written by the stdlib path, carrying NaN or
            # Infinity. Genuinely malformed files raise again from here, as
            # json.JSONDecodeError, which is what DiskCache.get expects.
            return json.loads(raw)
else:
    def _dumps(data: Any) -> bytes:
        try:
            return json.dumps(data, cls=DateTimeEncoder,
                              allow_nan=False).encode("utf-8")
        except ValueError:
            # allow_nan=False is what detects the non-finite values; the walk
            # runs only now that we know there is one to replace.
            return json.dumps(_replace_nonfinite(data), cls=DateTimeEncoder,
                              allow_nan=False).encode("utf-8")

    def _loads(raw: bytes) -> Any:
        return json.loads(raw)


# SHARING CACHE FILES BETWEEN THE TWO SERVICES
# --------------------------------------------
# The display service runs as root and the web interface as the installing
# user, and the web interface reads records only the display writes
# (display_current_state, display_on_demand_state, plugin_metrics_snapshot). Files are
# written 0660, so the web interface can read one only through its group.
#
# The installers rely on the directory's setgid bit to set that group. That is
# not something the cache can count on: systemd's CacheDirectory=, which
# ledmatrix-web.service carried until Sept 2026, re-owns the directory and
# everything in it to the web user and its primary group whenever the
# directory's owner does not match, and the setgid layout never survives that.
# From then on every file root creates is root:root 0660, unreadable by the web
# interface. Measured on one rig: 365 such files, and the web UI's display
# status, on-demand state and plugin health all silently empty.
#
# So a cache file takes its group from the directory explicitly, whether or
# not setgid is set. Only a group-writable directory counts as shared: that
# group can already replace any file in it, so reading them grants nothing new.
#
# Everything here works on an open descriptor, never a path. The directory is
# writable by the web user, so between a path check and a path operation that
# user could put a symlink in the file's place, and root would then chown and
# chmod whatever it points at.

_CACHE_FILE_MODE = 0o660


def _shared_group(directory: str) -> Optional[int]:
    """The group a cache file in ``directory`` should carry, if it is shared."""
    try:
        st = os.stat(directory)
    except OSError:
        return None
    if not st.st_mode & stat.S_IWGRP:
        return None
    return st.st_gid


def _share_open_file(fd: int, group: Optional[int]) -> None:
    """Make an open cache file readable by the other service. Best effort."""
    fchmod = getattr(os, 'fchmod', None)  # absent on Windows before 3.13
    if fchmod is not None:
        try:
            fchmod(fd, _CACHE_FILE_MODE)
        except OSError:
            pass
    fchown = getattr(os, 'fchown', None)  # absent on Windows
    if fchown is None or group is None:
        return
    try:
        if os.fstat(fd).st_gid != group:
            fchown(fd, -1, group)
    except OSError:
        # Not a member of the directory's group and not root: nothing to do,
        # and the file keeps the group it was created with.
        pass


class DiskCache:
    """Manages persistent disk-based cache."""
    
    def __init__(self, cache_dir: Optional[str], logger: Optional[logging.Logger] = None) -> None:
        """
        Initialize disk cache.
        
        Args:
            cache_dir: Directory for cache files (None = disabled)
            logger: Optional logger instance
        """
        self.cache_dir = cache_dir
        self.logger = logger or logging.getLogger(__name__)
        self._lock = threading.Lock()
        # key -> what set() last put at the primary cache path: the adler32 of
        # the payload (less a header-first timestamp), the timestamp the file
        # holds (None for records without one), and the file's inode and size.
        # Lets set() skip rewriting identical data. Per-process only, and the
        # inode/size check means another process's write is never mistaken
        # for ours -- worst case a redundant write, never a missed one.
        # Guarded by _lock.
        self._write_digests: Dict[str, Tuple[int, Optional[float], int, int]] = {}
    
    def get_cache_path(self, key: str) -> Optional[str]:
        """
        Get the path for a cache file.

        The key becomes a filename, so it has to be one. Keys reach this
        method from the web API -- POST /api/v3/cache/delete passes the
        request body's ``key`` straight through CacheManager.clear_cache to
        os.remove -- and a key of ``../../../../etc/whatever`` named a file
        well outside the cache directory. Every real key is the stem of a
        file already sitting flat in cache_dir (that is how list_cache_files
        derives them), so rejecting anything with a path component turns
        away only inputs that could never have been written here.

        Args:
            key: Cache key

        Returns:
            Path to cache file, or None if cache is disabled or the key is
            not a usable filename
        """
        if not self.cache_dir:
            return None
        safe_key = safe_path_component(key)
        if safe_key is None:
            self.logger.warning("Rejected unsafe cache key %r", key)
            return None
        return os.path.join(self.cache_dir, f"{safe_key}.json")
    
    def get(self, key: str, max_age: Optional[int] = 300) -> Optional[Dict[str, Any]]:
        """
        Get data from disk cache.

        Args:
            key: Cache key
            max_age: Maximum age in seconds; None disables age-based expiry
                (the record never counts as stale). Mirrors MemoryCache.get.

        Returns:
            Cached data or None if not found or expired
        """
        cache_path = self.get_cache_path(key)
        if not cache_path or not os.path.exists(cache_path):
            return None
        
        try:
            with self._lock:
                with open(cache_path, 'rb') as f:
                    # The open file's mtime, not the path's: the file a skipped
                    # write touched is the one being read.
                    mtime = os.fstat(f.fileno()).st_mtime
                    # 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(), mtime):
                        return None
                    f.seek(0)
                    record = _loads(f.read())

            # Determine record timestamp: the embedded one, moved forward by a
            # later unchanged save if there was one (see "UNCHANGED RE-SAVES"),
            # else the file mtime.
            record_ts = None
            if isinstance(record, dict):
                record_ts = record.get('timestamp')
            if record_ts is None:
                record_ts = mtime
            else:
                embedded_ts = _record_timestamp(record_ts)
                if embedded_ts is None:
                    try:
                        record_ts = float(record_ts)
                    except (TypeError, ValueError):
                        record_ts = None
                else:
                    record_ts = _effective_timestamp(embedded_ts, mtime)
                    if record_ts != embedded_ts:
                        # Hand the record back as a rewrite would have left it,
                        # so callers that age it themselves agree with us.
                        record['timestamp'] = record_ts

            now = time.time()

            # An explicit per-entry ttl wins over the caller's max_age. The
            # caller that wrote the record knows what its data is; max_age is
            # inferred from substrings in the key ("live", "odds", "stock") and
            # is only a fallback for records that never said. Until now the ttl
            # was stored and ignored, so `set(key, data, ttl=...)` did nothing
            # at all -- 48 plugin call sites and 4 in the core were writing a
            # number no read path consulted.
            effective_max_age = max_age
            if isinstance(record, dict):
                stored_ttl = record.get('ttl')
                if isinstance(stored_ttl, (int, float)) and not isinstance(stored_ttl, bool) \
                        and stored_ttl >= 0:
                    effective_max_age = stored_ttl
            max_age = effective_max_age

            # max_age=None means "never expires" (mirrors MemoryCache and the
            # cache_manager docstring). Guard it explicitly — otherwise the
            # comparison below raises TypeError and the record is treated as a
            # miss, which silently breaks callers that persist long-lived state
            # via get(key, max_age=None) (e.g. plugin health/metrics that must
            # survive restarts and be read cross-process).
            if record_ts is None or max_age is None or (now - record_ts) <= max_age:
                return record
            else:
                # Stale on disk; keep file for potential diagnostics but treat as miss
                return None
                
        except json.JSONDecodeError as e:
            self.logger.error("Error parsing cache file for %s at %s: %s", key, cache_path, e, exc_info=True)
            # If the file is corrupted, remove it
            try:
                os.remove(cache_path)
                self.logger.info("Removed corrupted cache file: %s", cache_path)
            except OSError as remove_error:
                self.logger.warning("Could not remove corrupted cache file %s: %s", cache_path, remove_error)
            return None
        except PermissionError as e:
            # Permission errors are recoverable - cache just won't be available
            self.logger.warning("Permission denied loading cache for %s from %s: %s. Cache unavailable for this key.", key, cache_path, e)
            return None
        except (IOError, OSError) as e:
            self.logger.error("Error loading cache for %s from %s: %s", key, cache_path, e, exc_info=True)
            return None
        except Exception as e:
            self.logger.error("Unexpected error loading cache for %s from %s: %s", key, cache_path, e, exc_info=True)
            return None
    
    def set(self, key: str, data: Dict[str, Any]) -> None:
        """
        Save data to disk cache with atomic write.
        
        This method gracefully handles permission errors. If the cache directory
        is not writable, it will log a warning and return silently rather than
        raising an exception. This allows the application to continue functioning
        even when running as a non-root user without write access to system cache
        directories.
        
        Args:
            key: Cache key
            data: Data to cache
        """
        cache_path = self.get_cache_path(key)
        if not cache_path:
            return

        # Serialize once, compact (no indent): the payload is reused by every
        # write path below, and cache files are machine-read only — indenting
        # them just multiplied the bytes written to the SD card.
        try:
            payload = _dumps(data)
        except (TypeError, ValueError) as e:
            self.logger.warning("Cache data for key '%s' not serializable: %s", key, e)
            return

        timestamp = _record_timestamp(data.get('timestamp')) if isinstance(data, dict) else None
        # A header-first record (CacheManager.set's layout) is compared without
        # its timestamp, which differs on every save; see "UNCHANGED RE-SAVES".
        # Any other layout is compared whole, as before.
        head = _HEAD_RE.match(payload) if timestamp is not None else None
        digest = zlib.adler32(memoryview(payload)[head.end(1):] if head else payload)

        try:
            # Atomic write to avoid partial/corrupt files
            with self._lock:
                # Skip the disk entirely when this exact payload was already
                # written for this key (plugins re-save unchanged API data
                # every update cycle — each write is real SD-card wear).
                # A metadata touch is journal-cheap compared to rewriting
                # the data.
                if self._skip_unchanged(key, cache_path, digest, timestamp):
                    return

                tmp_dir = os.path.dirname(cache_path)
                # Try to create temp file in cache directory first
                # If that fails due to permissions, fall back to direct write
                tmp_path = None
                fd = None
                try:
                    # First try the cache directory
                    if os.access(tmp_dir, os.W_OK):
                        try:
                            fd, tmp_path = tempfile.mkstemp(prefix=f".{os.path.basename(cache_path)}.", dir=tmp_dir)
                        except (IOError, OSError, PermissionError):
                            # If temp file creation fails, try direct write as fallback
                            self.logger.warning("Could not create temp file in %s, using direct write for %s", tmp_dir, key)
                            tmp_path = None
                            fd = None
                    else:
                        # Directory not writable, use direct write
                        self.logger.warning("Cache directory %s not writable, using direct write for %s", tmp_dir, key)
                        tmp_path = None
                        fd = None
                    
                    if tmp_path and fd is not None:
                        # Atomic write with temp file. No fsync: os.replace
                        # already guarantees readers never see a torn file,
                        # and cache data is re-fetchable — forcing a disk
                        # flush per write was the single biggest SD-card
                        # wear source (dozens of fsyncs/min on API-heavy
                        # installs) for data that can be re-downloaded.
                        try:
                            with os.fdopen(fd, 'wb') as tmp_file:
                                tmp_file.write(payload)
                                # Before the rename, not after: mkstemp
                                # creates the file 0600, and a reader that
                                # opened it in between was refused.
                                _share_open_file(tmp_file.fileno(), _shared_group(tmp_dir))
                            os.replace(tmp_path, cache_path)
                            self._remember_write(key, cache_path, digest, timestamp)
                        finally:
                            if os.path.exists(tmp_path):
                                try:
                                    os.remove(tmp_path)
                                except OSError:
                                    pass
                    else:
                        # Fallback: direct write (not atomic, but better than failing)
                        try:
                            with open(cache_path, 'wb') as cache_file:
                                cache_file.write(payload)
                                _share_open_file(cache_file.fileno(), _shared_group(tmp_dir))
                            self._remember_write(key, cache_path, digest, timestamp)
                            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
                            self.logger.warning("Direct write failed for key '%s' to %s: %s", key, cache_path, write_error)
                            raise  # Re-raise to trigger fallback logic
                except (IOError, OSError, PermissionError):
                    # Attempt one-time fallback write to user's home cache directory
                    try:
                        # Try user's home cache directory as fallback
                        home_dir = os.path.expanduser('~')
                        fallback_dir = os.path.join(home_dir, '.ledmatrix_cache')
                        # Ensure fallback directory exists
                        try:
                            os.makedirs(fallback_dir, exist_ok=True)
                        except (OSError, PermissionError):
                            pass
                        
                        if os.path.isdir(fallback_dir) and os.access(fallback_dir, os.W_OK):
                            # NOTE: no digest record here — the fallback file
                            # is a different path, so future sets must keep
                            # retrying the primary location.
                            fallback_path = os.path.join(fallback_dir, os.path.basename(cache_path))
                            with open(fallback_path, 'wb') as tmp_file:
                                tmp_file.write(payload)
                                _share_open_file(tmp_file.fileno(), _shared_group(fallback_dir))
                            self.logger.debug("Cache wrote to fallback location: %s", fallback_path)
                            return  # Successfully wrote to fallback, exit gracefully
                    except (IOError, OSError, PermissionError) as e2:
                        self.logger.debug("Fallback cache write also failed for key '%s': %s", key, e2)
                    
                    # If all write attempts failed, log warning but don't raise exception
                    # Cache is a performance optimization, not critical for operation
                    self.logger.warning(
                        "Could not write cache for key '%s' to %s (permission denied). "
                        "Cache will be unavailable for this key, but application will continue.",
                        key, cache_path
                    )
                    return  # Exit gracefully without raising exception
        
        except Exception as e:
            # For any other unexpected errors, log but don't crash
            self.logger.warning(
                "Unexpected error saving cache for key '%s' to %s: %s. "
                "Application will continue without caching for this key.",
                key, cache_path, e, exc_info=True
            )
            return  # Exit gracefully without raising exception
    
    def _skip_unchanged(self, key: str, cache_path: str, digest: int,
                        timestamp: Optional[float]) -> bool:
        """Stand in for a write of an unchanged record by touching the file.

        True when the file already holds this record bar its timestamp and the
        touch landed; False means write it. The touch sets mtime to the
        record's timestamp -- what a rewrite would have stored -- or to now
        for a record without one, whose age readers already take from mtime.
        Caller holds _lock.
        """
        last = self._write_digests.get(key)
        if last is None or last[0] != digest:
            return False
        _, written_ts, ino, size = last
        if (timestamp is None) != (written_ts is None):
            return False
        if timestamp is not None and written_ts is not None:
            # Never backwards (a rewrite would make the record older), and
            # never further than readers will trust the mtime: past that the
            # record is rewritten, so its own timestamp catches up.
            if not written_ts <= timestamp <= written_ts + _MAX_TIMESTAMP_LIFT:
                return False
        try:
            st = os.stat(cache_path)
            if (st.st_ino, st.st_size) != (ino, size):
                # Replaced since our write (another process, a restore):
                # its contents are not the ones the digest describes.
                self._write_digests.pop(key, None)
                return False
            # Setting an explicit time needs the file's owner; a file someone
            # else wrote fails here and is rewritten (as our own file) instead.
            os.utime(cache_path, None if timestamp is None else (timestamp, timestamp))
            return True
        except OSError:
            # File vanished or perms changed — fall through and write
            self._write_digests.pop(key, None)
            return False

    def _remember_write(self, key: str, cache_path: str, digest: int,
                        timestamp: Optional[float]) -> None:
        """After a real write: pin mtime to the record's timestamp and note
        what was written, so the next unchanged save can be skipped.

        Pinning keeps a record saved with an older timestamp from looking as
        fresh as the moment it was written (see "UNCHANGED RE-SAVES"); for
        CacheManager.set's records the two differ only by the write itself.
        A timestamp in the future is left alone, mtime already being older.
        Never raises: the data is on disk, and anything failing here only
        costs the next save its skip. Caller holds _lock.
        """
        self._write_digests.pop(key, None)
        try:
            if timestamp is not None and timestamp <= time.time():
                os.utime(cache_path, (timestamp, timestamp))
            st = os.stat(cache_path)
        except OSError as e:
            self.logger.debug("Could not pin mtime of %s: %s", cache_path, e)
            return
        self._write_digests[key] = (digest, timestamp, st.st_ino, st.st_size)

    def clear(self, key: Optional[str] = None) -> None:
        """
        Clear cache entry or all entries.
        
        Args:
            key: Specific key to clear, or None to clear all
        """
        if not self.cache_dir:
            return
        
        with self._lock:
            if key:
                self._write_digests.pop(key, None)
                cache_path = self.get_cache_path(key)
                if cache_path and os.path.exists(cache_path):
                    try:
                        os.remove(cache_path)
                    except OSError as e:
                        self.logger.warning("Could not remove cache file %s: %s", cache_path, e)
            else:
                # Clear all cache files
                self._write_digests.clear()
                if os.path.exists(self.cache_dir):
                    for filename in os.listdir(self.cache_dir):
                        if filename.endswith('.json'):
                            try:
                                os.remove(os.path.join(self.cache_dir, filename))
                            except OSError as e:
                                self.logger.warning("Could not remove cache file %s: %s", filename, e)
    
    def get_cache_dir(self) -> Optional[str]:
        """Get the cache directory path."""
        return self.cache_dir

    def share_existing_files(self) -> int:
        """Give cache files already on disk the group set() now gives new ones.

        set() fixes every file it writes from now on; this repairs the ones an
        older version left behind as root:root, which the web interface cannot
        read until each key happens to be rewritten -- and some, like a
        plugin's metrics, may not be for a long time. Meant to run once per
        process, off the startup path.

        Only this process's own regular files are touched, and each one through
        a descriptor opened with O_NOFOLLOW and checked for a single link: the
        directory is writable by the web user, and a root process must not be
        steered into changing a file outside it.

        Returns:
            Number of files whose group or mode was changed.
        """
        fchown = getattr(os, 'fchown', None)
        geteuid = getattr(os, 'geteuid', None)
        nofollow = getattr(os, 'O_NOFOLLOW', None)
        if not self.cache_dir or fchown is None or geteuid is None or nofollow is None:
            return 0
        group = _shared_group(self.cache_dir)
        if group is None:
            return 0
        euid = geteuid()

        changed = 0
        try:
            entries = list(os.scandir(self.cache_dir))
        except OSError as e:
            self.logger.debug("Could not scan %s to share cache files: %s", self.cache_dir, e)
            return 0
        for entry in entries:
            if not entry.name.endswith('.json'):
                continue
            try:
                st = entry.stat(follow_symlinks=False)
            except OSError:
                continue
            if (not stat.S_ISREG(st.st_mode) or st.st_uid != euid
                    or (st.st_gid == group and stat.S_IMODE(st.st_mode) == _CACHE_FILE_MODE)):
                continue
            try:
                fd = os.open(entry.path, os.O_RDONLY | nofollow | getattr(os, 'O_NONBLOCK', 0))
            except OSError:
                continue
            try:
                st = os.fstat(fd)
                if not stat.S_ISREG(st.st_mode) or st.st_uid != euid or st.st_nlink != 1:
                    continue
                _share_open_file(fd, group)
                st = os.fstat(fd)
                if st.st_gid == group and stat.S_IMODE(st.st_mode) == _CACHE_FILE_MODE:
                    changed += 1
            except OSError:
                continue
            finally:
                os.close(fd)
        if changed:
            self.logger.info(
                "Made %d cache file(s) in %s readable by the directory's group "
                "(gid %d) so the web interface can read them",
                changed, self.cache_dir, group)
        return changed
    
    @staticmethod
    def _is_orphaned_temp(filename: str) -> bool:
        """Whether a name is one of set()'s temp files rather than real data.

        Matches only what this class creates: mkstemp with a prefix of
        ".<cache filename>." , so ".weather.json.a1b2c3d4". The shape is
        checked rather than just the leading dot, because this predicate
        deletes things -- a stray dotfile someone left in the cache directory
        is not ours to remove, and a completed ".json" never is either.
        """
        if not filename.startswith('.') or filename.endswith('.json'):
            return False
        head, sep, suffix = filename.rpartition('.json.')
        # head is the key (non-empty after the leading dot), suffix is
        # mkstemp's random component.
        return bool(sep) and len(head) > 1 and bool(suffix)

    def cleanup_expired_files(self, cache_strategy: CacheStrategyProtocol, retention_policies: Dict[str, int]) -> Dict[str, Any]:
        """
        Clean up expired cache files based on retention policies.
        
        Args:
            cache_strategy: Object implementing CacheStrategyProtocol for categorizing files
            retention_policies: Dict mapping data types to retention days
            
        Returns:
            Dictionary with cleanup statistics:
                - files_scanned: Total files checked
                - files_deleted: Files removed
                - space_freed_bytes: Bytes freed
                - errors: Number of errors encountered
        """
        if not self.cache_dir or not os.path.exists(self.cache_dir):
            self.logger.warning("Cache directory not available for cleanup")
            return {'files_scanned': 0, 'files_deleted': 0, 'space_freed_bytes': 0, 'errors': 0}
        
        stats = {
            'files_scanned': 0,
            'files_deleted': 0,
            'space_freed_bytes': 0,
            'errors': 0
        }
        
        current_time = time.time()
        
        try:
            # Collect files to process outside the lock to avoid blocking cache operations
            # Only hold lock during directory listing to get snapshot of files
            try:
                with self._lock:
                    # Get snapshot of files while holding lock briefly
                    entries = os.listdir(self.cache_dir)
            except OSError as list_error:
                self.logger.error("Error listing cache directory %s: %s", self.cache_dir, list_error, exc_info=True)
                stats['errors'] += 1
                return stats

            filenames = [f for f in entries if f.endswith('.json')]

            # Sweep temp files abandoned by a write that never finished. set()
            # removes its own in a finally, so these are the ones where the
            # process died between mkstemp and os.replace -- a SIGKILL, a lost
            # restart race, a power cut. Nothing ever collected them: they are
            # named ".<key>.json.<random>", and the scan above only matches
            # names ending in .json, so they accumulated indefinitely. Measured
            # on a live rig: 76 files, 1,050 MB, 81% of the whole cache
            # directory, the oldest six months old.
            stats['orphan_temp_files_deleted'] = 0
            for filename in (f for f in entries if self._is_orphaned_temp(f)):
                # Counted as scanned like any other candidate, so files_deleted
                # can never exceed files_scanned and the summary line reads
                # honestly ("77/8864", not "77/0").
                stats['files_scanned'] += 1
                path = os.path.join(self.cache_dir, filename)
                try:
                    # An in-flight write lives for milliseconds, so anything
                    # this old is certainly abandoned rather than in progress.
                    if (current_time - os.path.getmtime(path)) <= _ORPHAN_TEMP_MAX_AGE_SECONDS:
                        continue
                    with self._lock:
                        size = os.path.getsize(path)
                        os.remove(path)
                    stats['files_deleted'] += 1
                    stats['orphan_temp_files_deleted'] += 1
                    stats['space_freed_bytes'] += size
                except FileNotFoundError:
                    continue          # another sweep got there first
                except OSError as e:
                    stats['errors'] += 1
                    self.logger.warning("Error deleting orphaned temp file %s: %s", filename, e)

            if stats['orphan_temp_files_deleted']:
                self.logger.info(
                    "Removed %d abandoned cache temp file(s)",
                    stats['orphan_temp_files_deleted'])
            
            # Process files outside the lock to avoid blocking get/set operations
            for filename in filenames:
                stats['files_scanned'] += 1
                file_path = os.path.join(self.cache_dir, filename)
                
                try:
                    # Get file age (outside lock - stat operations are generally atomic)
                    file_mtime = os.path.getmtime(file_path)
                    file_age_days = (current_time - file_mtime) / 86400  # Convert to days
                    
                    # Extract cache key from filename (remove .json extension)
                    cache_key = filename[:-5]
                    
                    # Determine data type and retention policy
                    data_type = cache_strategy.get_data_type_from_key(cache_key)
                    retention_days = retention_policies.get(data_type, retention_policies.get('default', 30))
                    
                    # Delete if older than retention period
                    # Only hold lock during actual file deletion to ensure atomicity
                    if file_age_days > retention_days:
                        try:
                            # Hold lock only during delete operation (get size and remove atomically)
                            with self._lock:
                                # Double-check file still exists (may have been deleted by another process)
                                if os.path.exists(file_path):
                                    try:
                                        file_size = os.path.getsize(file_path)
                                        os.remove(file_path)
                                        # Only increment stats if removal succeeded
                                        stats['files_deleted'] += 1
                                        stats['space_freed_bytes'] += file_size
                                        self.logger.debug(
                                            "Deleted expired cache file: %s (age: %.1f days, type: %s, retention: %d days)",
                                            filename, file_age_days, data_type, retention_days
                                        )
                                    except FileNotFoundError:
                                        # File was deleted by another process between exists check and remove
                                        # This is a benign race condition, silently continue
                                        pass
                                else:
                                    # File was deleted by another process before lock was acquired
                                    # This is a benign race condition, silently continue
                                    pass
                        except FileNotFoundError:
                            # File was already deleted by another process, skip it
                            # This is a benign race condition, silently continue
                            continue
                        except OSError as e:
                            # Other file system errors, log but don't fail the entire cleanup
                            stats['errors'] += 1
                            self.logger.warning("Error deleting cache file %s: %s", filename, e)
                            continue
                
                except FileNotFoundError:
                    # File was deleted by another process between listing and processing
                    # This is a benign race condition, silently continue
                    continue
                except OSError as e:
                    stats['errors'] += 1
                    self.logger.warning("Error processing cache file %s: %s", filename, e)
                    continue
                except Exception as e:
                    stats['errors'] += 1
                    self.logger.error("Unexpected error processing cache file %s: %s", filename, e, exc_info=True)
                    continue
        
        except OSError as e:
            self.logger.error("Error listing cache directory %s: %s", self.cache_dir, e, exc_info=True)
            stats['errors'] += 1
        
        return stats