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Fix dogpile cache-overwrite race in CacheHelper::remember() (#91)

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remember() had no protection against a slow cache-miss recomputation
overwriting a fresher write. If Request A started computing stats just
before a ticket mutation + invalidateCache(), and Request B started
just after (correctly computing fresh, post-mutation data), A could
finish (using stale pre-mutation data) after B and overwrite B's fresh
cache entry — extending staleness by up to another full TTL.

Added a per-prefix invalidation epoch: delete() bumps it, and
remember() snapshots it before running the callback and only writes
if the epoch hasn't changed since — otherwise a newer invalidation
happened mid-computation and the result being written is already
stale, so it's dropped (the caller still gets its own result; only the
cache write is skipped).

Verified with two real concurrent PHP processes racing against the
same cache key (a slow "Request A" callback vs. a fast "Request B"
that invalidates then recomputes): the cache ends up holding B's fresh
value, not A's late stale overwrite.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MGDKHiU5RJdo3dqQUDow3X
This commit is contained in:
jared
2026-09-08 12:25:04 -04:00
co-authored by Claude Sonnet 5
parent 4fade1a9d3
commit 6553c0227d
1 changed files with 36 additions and 1 deletions
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helpers/CacheHelper.php
+36 -1
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@@ -121,6 +121,33 @@ class CacheHelper
return $written;
}
/**
* Read the current invalidation epoch for a prefix (0 if never bumped).
* Used by remember() to detect an invalidation that happened while a
* cache-miss recomputation was in flight.
*/
private static function getEpoch(string $prefix): int
{
$safePrefix = preg_replace('/[^a-zA-Z0-9_]/', '_', $prefix);
$file = self::getCacheDir() . '/' . $safePrefix . '.epoch';
$val = @file_get_contents($file);
return $val !== false ? (int)$val : 0;
}
/**
* Bump a prefix's invalidation epoch. Called whenever anything under the
* prefix is invalidated.
*/
private static function bumpEpoch(string $prefix): void
{
$safePrefix = preg_replace('/[^a-zA-Z0-9_]/', '_', $prefix);
$file = self::getCacheDir() . '/' . $safePrefix . '.epoch';
$next = self::getEpoch($prefix) + 1;
if (@file_put_contents($file, (string)$next, LOCK_EX) !== false) {
@chmod($file, 0600);
}
}
/**
* Delete cached data
*
@@ -130,6 +157,8 @@ class CacheHelper
*/
public static function delete(string $prefix, $identifier = null): bool
{
self::bumpEpoch($prefix);
if ($identifier !== null) {
$key = self::makeKey($prefix, $identifier);
unset(self::$memoryCache[$key]);
@@ -192,8 +221,14 @@ class CacheHelper
$data = self::get($prefix, $identifier, $ttl);
if ($data === null) {
// Snapshot the epoch before running the (possibly slow) callback so
// a concurrent invalidation mid-computation can be detected below —
// otherwise this request's stale pre-invalidation result could
// overwrite a newer request's fresher write, extending staleness by
// up to another full TTL.
$epochBefore = self::getEpoch($prefix);
$data = $callback();
if ($data !== null) {
if ($data !== null && self::getEpoch($prefix) === $epochBefore) {
self::set($prefix, $identifier, $data);
}
}