404-solution / includes / diagnostics / DetachAbAttemptCounter.php

DetachAbAttemptCounter.php in 404 Solution trunk, at includes/diagnostics/DetachAbAttemptCounter.php

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1	<?php
2
3	if (!defined('ABSPATH')) {
4	exit;
5	}
6
7	/**
8	* The persisted attempt counter for one detach A/B workload scope.
9	*
10	* Split out of ABJ_404_Solution_DetachAbExperiment, which decides WHICH mode an
11	* attempt slot receives and had no business also owning where the slot number
12	* is stored. The experiment is policy; this is the data access behind it.
13	*
14	* The store is a transient and the read-modify-write is deliberately not
15	* atomic. This is a bounded diagnostic sequence, not a security limit: two
16	* tabs racing can land on the same slot, which costs one unusable pair in the
17	* evidence, and that is cheaper than coupling diagnostics policy to the
18	* database layer. The race is recorded here as a known property rather than
19	* left for a reader to infer from the absence of a lock.
20	*/
21	final class ABJ_404_Solution_DetachAbAttemptCounter {
22
23	/** No slot could be reserved. Callers answer inert rather than guessing one. */
24	const NO_SLOT = -1;
25
26	/** How long an idle scope keeps its place in the sequence. */
27	const DEFAULT_TTL_SECONDS = 3600;
28
29	/**
30	* Reserve and return this scope's next attempt slot.
31	*
32	* Returns NO_SLOT when the scope cannot carry a counter, when the transient
33	* API is unavailable, or when the reservation could not be persisted.
34	*
35	* That last case used to return the slot anyway. A failed write leaves the
36	* stored counter where it was, so every later request in the session read
37	* the same value and was handed attempt zero again: the sequence could
38	* never reach MAX_ATTEMPTS, never revert to `default`, and the experiment's
39	* promise that a probe never permanently degrades an admin session was
40	* broken by precisely the infrastructure failure the plugin is built to
41	* degrade past. An unreserved slot is not a slot.
42	*/
43	public static function reserveNextSlot(ABJ_404_Solution_DetachAbScope $scope): int {
44	if ($scope->isSessionless()
45	\|\| !function_exists('get_transient')
46	\|\| !function_exists('set_transient')) {
47	return self::NO_SLOT;
48	}
49
50	$key = $scope->transientKey();
51	$stored = ABJ_404_Solution_DetachAbResolutionTracer::traceTransientOperation(
52	'get_transient',
53	$key,
54	static function () use ($key) {
55	return get_transient($key);
56	}
57	);
58	$slot = self::storedSlot($stored);
59
60	// allow-cache-empty: locally computed attempt counter (always a valid
61	// non-negative int), not a fetched query result.
62	$persisted = ABJ_404_Solution_DetachAbResolutionTracer::traceTransientOperation(
63	'set_transient',
64	$key,
65	static function () use ($key, $slot) {
66	// allow-cache-empty: the locally computed attempt counter is a
67	// valid non-negative integer, never a fetched result.
68	return set_transient($key, $slot + 1, self::DEFAULT_TTL_SECONDS);
69	}
70	);
71
72	return $persisted ? $slot : self::NO_SLOT;
73	}
74
75	/**
76	* Interpret a stored counter, accepting only a canonical non-negative integer.
77	*
78	* is_numeric() plus an int cast accepted '2.9' and '1e3' and turned them
79	* into 2 and 1000, so a corrupted or foreign value under this key was
80	* promoted into a real-looking slot and journalled as A/B coordinates --
81	* fabricated evidence, which is worse for a diagnostic than no evidence.
82	* Anything that is not a counter starts the sequence over; the write that
83	* follows immediately replaces it with a canonical value, so a garbled
84	* store self-heals on the next request instead of resetting forever.
85	*
86	* ABJ_404_Solution_ExactInteger is that rule, already generalised: its own
87	* docblock names this experiment's attempt ordinal as one of the two places
88	* that discovered the defect independently. This counter is the third, and
89	* hand-rolling a fourth narrow pattern here is how there would be a fifth.
90	*
91	* @param mixed $stored
92	*/
93	private static function storedSlot($stored): int {
94	return ABJ_404_Solution_ExactInteger::readOr($stored, 0, 0);
95	}
96	}
97