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* @file globals.js
* @description Deterministic substitution for the sandbox's global whitelist.
*
* This is the hinge the whole replay story turns on.
*
* Avenx evaluates every template expression, computed property and action body
* as source text under `with(sandbox)`, and the sandbox proxy's `has` trap
* claims every key. So *every* identifier those expressions resolve — `Date`,
* `Math`, `state`, anything — passes through one `get` trap, and that trap
* hands out globals from a single line. Substituting a recording clock there
* costs nothing and cannot be bypassed from inside a template. Substituting a
* replaying clock in the same place is what makes a recorded session
* reproducible.
*
* Frameworks that compile expressions to closures have no equivalent point:
* by the time the code runs, `Date` is a free variable resolved against the
* real global scope, and the only way back in is to rewrite the program.
*
* ## What this does not cover
*
* Bridge modules and anything they import are ordinary ES modules. They are
* not sandboxed, so a `Date.now()` in a bridge action reaches the real clock
* and is invisible here. That is a genuine hole, and it is why recording does
* not get the final say on whether a trace is deterministic — replay verifies
* the claim by comparing observations. See `replay.js`.
* @module lib/core/trace/globals
*/
import { tracer } from './tracer.js';
import { TraceNodeType } from './schema.js';
/**
* Causal parents that have already been marked as reading a given
* non-deterministic global, so a loop reading `Date.now()` a thousand times
* produces one marker rather than a thousand.
* @type {Set<string>}
*/
const markedReads = new Set();
/**
* Records that application code observed a non-deterministic global.
*
* The values themselves go into the recorder's compact log; this marker exists
* so the causal graph knows *where* the read happened. Contract analysis then
* asks whether any ancestor of that point declared itself `deterministic` —
* which is how a build-time contract becomes a claim checked against what the
* code actually did. See `trace/contracts.js`.
* @param {'now'|'random'} kind - Which source was read.
*/
function markGlobalRead(kind) {
if (!tracer.sink) {
return;
}
const parent = tracer.current();
const key = `${parent}:${kind}`;
if (markedReads.has(key)) {
return;
}
markedReads.add(key);
tracer.record(TraceNodeType.GLOBAL, {
source: kind === 'now' ? 'Date.now' : 'Math.random',
kind,
});
}
/**
* Globals currently substituted for sandbox lookups, keyed by identifier.
*
* Empty in normal operation, so {@link resolveSandboxGlobal} degenerates to a
* `Map.size` check and a property read.
* @type {Map<string, any>}
*/
const overrides = new Map();
/**
* Resolves a whitelisted global for the expression sandbox.
*
* The sandbox calls this instead of reading `globalThis[key]` directly, so a
* recording or replaying session can substitute a deterministic implementation
* without patching the page's real globals — which would leak into unrelated
* scripts and outlive the recording.
* @param {string} key - The whitelisted global identifier.
* @returns {any} The value template code should see.
*/
export function resolveSandboxGlobal(key) {
if (overrides.size > 0 && overrides.has(key)) {
return overrides.get(key);
}
return globalThis[key];
}
/**
* Whether any global is currently substituted.
* @returns {boolean}
*/
export function hasGlobalOverrides() {
return overrides.size > 0;
}
/**
* Removes every substitution, restoring the real globals.
*/
export function clearGlobalOverrides() {
overrides.clear();
markedReads.clear();
}
/**
* Builds a `Date` stand-in that reports a caller-supplied clock.
*
* `prototype` is aliased to the real `Date.prototype` and construction
* delegates to the real constructor, so instances are genuine `Date` objects
* and `x instanceof Date` keeps working in application code.
* @param {function(): number} readClock - Supplies the epoch milliseconds for a "now" reading.
* @returns {Function} A drop-in replacement for `Date`.
*/
function buildDate(readClock) {
const RealDate = Date;
/**
* A `Date` whose zero-argument form is supplied by the trace clock.
* @param {...any} args - Standard `Date` arguments.
* @returns {Date} A real Date instance.
*/
function TracedDate(...args) {
if (!new.target) {
return new RealDate(readClock()).toString();
}
if (args.length === 0) {
return new RealDate(readClock());
}
return new RealDate(...args);
}
TracedDate.prototype = RealDate.prototype;
TracedDate.now = () => readClock();
TracedDate.parse = RealDate.parse.bind(RealDate);
TracedDate.UTC = RealDate.UTC.bind(RealDate);
return TracedDate;
}
/**
* Builds a `Math` stand-in whose `random` is supplied by the trace.
*
* Created with `Object.create(Math)` so every other member — `max`, `floor`,
* `PI` — resolves through the prototype chain to the real implementation
* rather than being copied, which would silently drop anything a future engine
* adds.
* @param {function(): number} readRandom - Supplies the next random value.
* @returns {object} A drop-in replacement for `Math`.
*/
function buildMath(readRandom) {
return Object.create(Math, {
random: { value: () => readRandom(), enumerable: false, configurable: true },
});
}
/**
* Substitutes globals that log every non-deterministic value they hand out.
*
* The values themselves are real — a recording session must behave exactly as
* an untraced one would — but each is appended to the recorder's global log so
* replay can hand the same sequence back.
* @param {object} recorder - The recorder to log into.
*/
export function installRecordingGlobals(recorder) {
markedReads.clear();
overrides.set(
'Date',
buildDate(() => {
markGlobalRead('now');
return recorder.recordGlobal('now', Date.now());
}),
);
overrides.set(
'Math',
buildMath(() => {
markGlobalRead('random');
return recorder.recordGlobal('random', Math.random());
}),
);
}
/**
* Substitutes globals that replay a recorded sequence of values.
*
* When the recorded sequence is exhausted the replay has consumed more
* non-determinism than the recording produced, which means it has already
* diverged. That is reported through `onExhausted` rather than papered over by
* looping or by falling back to the real clock, either of which would let a
* diverged replay finish looking successful.
* @param {object} globals - The `globals` block of a trace.
* @param {number[]} [globals.now] - Recorded `Date.now()` readings, in order.
* @param {number[]} [globals.random] - Recorded `Math.random()` readings, in order.
* @param {function(string): void} onExhausted - Called with the source name when a log runs out.
* @returns {{cursors: {now: number, random: number}}} Live read cursors, for assertions.
*/
export function installReplayGlobals(globals, onExhausted) {
const nowLog = (globals && globals.now) || [];
const randomLog = (globals && globals.random) || [];
const cursors = { now: 0, random: 0 };
/**
* Reads the next recorded clock value.
* @returns {number} Epoch milliseconds.
*/
const readClock = () => {
if (cursors.now >= nowLog.length) {
onExhausted('Date.now');
// Repeating the final recorded reading keeps replay running so the
// divergence report can be complete, rather than throwing at the first
// extra read and hiding everything after it.
return nowLog.length > 0 ? nowLog[nowLog.length - 1] : 0;
}
return nowLog[cursors.now++];
};
/**
* Reads the next recorded random value.
* @returns {number} A value in [0, 1).
*/
const readRandom = () => {
if (cursors.random >= randomLog.length) {
onExhausted('Math.random');
return randomLog.length > 0 ? randomLog[randomLog.length - 1] : 0;
}
return randomLog[cursors.random++];
};
overrides.set('Date', buildDate(readClock));
overrides.set('Math', buildMath(readRandom));
return { cursors };
}
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