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* @file recorder.js
* @description The trace recorder: a bounded buffer plus determinism bookkeeping.
*
* The recorder owns everything a trace needs that the {@link tracer} hook
* surface deliberately does not: node ids, timestamps, the ring buffer, the
* redaction rules, the log of non-deterministic globals, and the running
* judgement of whether this session could be replayed.
*
* That last part is the reason this class exists rather than an array. A trace
* that claims to be deterministic and is not produces a regression test that
* passes for the wrong reason, which is worse than no test. The recorder
* therefore downgrades eagerly and never upgrades.
* @module lib/core/trace/recorder
*/
import { captureValue, wasLossy } from './capture.js';
import { Redactor } from './redact.js';
import { tracer } from './tracer.js';
import {
TRACE_VERSION,
TraceNodeType,
Determinism,
NonDeterminismReason,
createTrace,
} from './schema.js';
/**
* How many nodes a recording keeps before it starts dropping the oldest.
*
* A minute of interaction on a busy list produces a lot of nodes; an unbounded
* array would be a memory leak in a dev server that stays open all day. When
* the buffer wraps, the trace is marked truncated and therefore best-effort,
* because it no longer starts at the beginning of the session.
* @type {number}
*/
export const DEFAULT_MAX_NODES = 5000;
/**
* How deep the serialization scrub walks a node.
*
* Captured values are already bounded by {@link captureValue}, so this only has
* to match that ceiling.
* @type {number}
*/
const SCRUB_MAX_DEPTH = 8;
/**
* Where a recorder is in its lifecycle.
*
* Writes during `setup` are the application constructing itself and are not
* evidence of non-determinism. Writes during `recording` with nothing on the
* causal stack are: something outside the recorded inputs changed state.
* @readonly
* @enum {string}
*/
export const RecorderPhase = {
SETUP: 'setup',
RECORDING: 'recording',
STOPPED: 'stopped',
};
/**
* Generates a short, readable trace id.
* @returns {string} An id such as `trace-4f2a`.
*/
function generateId() {
const suffix = Math.floor(Math.random() * 0xffff)
.toString(16)
.padStart(4, '0');
return `trace-${suffix}`;
}
/**
* Reads a monotonic clock, falling back where `performance` is absent.
* @returns {number} Milliseconds from an arbitrary origin.
*/
function now() {
if (typeof performance !== 'undefined' && typeof performance.now === 'function') {
return performance.now();
}
return Date.now();
}
/**
* Records one session's causal trace.
*/
export class TraceRecorder {
/**
* @param {object} [options] - Recorder options.
* @param {string} [options.id] - An explicit trace id. Generated when omitted.
* @param {number} [options.maxNodes] - Ring-buffer capacity.
* @param {string[]} [options.redact] - Redaction patterns.
* @param {object} [options.meta] - Free-form metadata about the session.
*/
constructor(options = {}) {
/** @type {string} */
this.id = options.id || generateId();
/** @type {number} */
this.maxNodes = options.maxNodes || DEFAULT_MAX_NODES;
/** @type {Redactor} */
this.redactor = new Redactor(options.redact || []);
/** @type {object} */
this.meta = { ...(options.meta || {}) };
/**
* The ring buffer. Kept as a plain array that is spliced in one block when
* it overflows, rather than a circular index: traces are read far more
* often than they overflow, and a linear array keeps every reader simple.
* @type {object[]}
*/
this.nodes = [];
/**
* Open nodes by id, so {@link TraceRecorder#annotate} can find a node the
* buffer may since have dropped.
* @type {Map<number, object>}
*/
this.index = new Map();
/** @type {number} */
this.nextId = 1;
/** @type {number} */
this.seq = 0;
/** @type {number} */
this.dropped = 0;
/** @type {number} */
this.startedAt = now();
/** @type {string} */
this.createdAt = new Date().toISOString();
/** @type {string} */
this.phase = RecorderPhase.SETUP;
/**
* Non-deterministic global values in the order application code observed
* them, so replay can hand back the same sequence.
* @type {{now: number[], random: number[]}}
*/
this.globals = { now: [], random: [] };
/** @type {string} */
this.determinism = Determinism.DETERMINISTIC;
/** @type {Map<string, string>} */
this.reasons = new Map();
}
/**
* Switches from application startup to recording user interaction.
*
* Until this is called, state writes with no recorded cause are treated as
* the application initialising itself rather than as non-determinism.
* @returns {TraceRecorder} This recorder.
*/
arm() {
this.phase = RecorderPhase.RECORDING;
return this;
}
/**
* Ends the recording.
* @returns {TraceRecorder} This recorder.
*/
stop() {
this.phase = RecorderPhase.STOPPED;
return this;
}
/**
* Appends a node.
* @param {string} type - A {@link TraceNodeType}.
* @param {object} data - Type-specific fields.
* @param {number|null} parent - The causal parent id.
* @returns {object} The stored node.
*/
push(type, data, parent) {
const node = {
id: this.nextId++,
parent: parent === undefined ? null : parent,
seq: ++this.seq,
t: Math.round((now() - this.startedAt) * 1000) / 1000,
type,
...data,
};
this.nodes.push(node);
this.index.set(node.id, node);
if (type === TraceNodeType.WRITE && parent === null && this.phase === RecorderPhase.RECORDING) {
this.markNonDeterministic(
NonDeterminismReason.UNATTRIBUTED_WRITE,
`${data.path || 'unknown path'} changed with no recorded input to explain it`,
);
}
if (this.nodes.length > this.maxNodes) {
this.#evict();
}
return node;
}
/**
* Drops the oldest tenth of the buffer.
*
* Evicting in blocks rather than one node at a time keeps the amortised cost
* of a splice off the hot path once a long session settles into steady state.
* @private
*/
#evict() {
const removeCount = Math.max(1, Math.floor(this.maxNodes / 10));
const removed = this.nodes.splice(0, removeCount);
for (const node of removed) {
this.index.delete(node.id);
}
this.dropped += removed.length;
this.markNonDeterministic(
NonDeterminismReason.TRUNCATED,
`${this.dropped} node(s) dropped; the buffer holds ${this.maxNodes}`,
);
}
/**
* Merges fields into a node that is still in the buffer.
* @param {number} id - The node id.
* @param {object} fields - Fields to merge.
*/
annotate(id, fields) {
const node = this.index.get(id);
if (node) {
Object.assign(node, fields);
}
}
/**
* Captures a value under this recorder's redaction and bounds.
* @param {any} value - The value.
* @param {string} [path] - The property path, for redaction matching.
* @returns {any} A JSON-representable capture.
*/
capture(value, path = '') {
const captured = captureValue(value, { path, redactor: this.redactor });
if (wasLossy()) {
this.markNonDeterministic(
NonDeterminismReason.UNSERIALIZABLE_VALUE,
path ? `at ${path}` : 'a recorded value could not be represented in JSON',
);
}
return captured;
}
/**
* Records a non-deterministic global value the sandbox handed to application
* code, so replay can hand back the same one.
* @param {'now'|'random'} kind - Which source produced it.
* @param {number} value - The value.
* @returns {number} The same value, so callers can record inline.
*/
recordGlobal(kind, value) {
const log = this.globals[kind];
if (log) {
log.push(value);
}
return value;
}
/**
* Downgrades the trace to best-effort.
*
* Reasons are de-duplicated by kind: a session that made a thousand
* unattributed writes should say so once, with an example.
* @param {string} reason - A {@link NonDeterminismReason}.
* @param {string} [detail] - Context for the developer.
*/
markNonDeterministic(reason, detail = '') {
this.determinism = Determinism.BEST_EFFORT;
if (!this.reasons.has(reason)) {
this.reasons.set(reason, detail);
}
}
/**
* Whether this recording is currently believed to be replayable.
*
* "Believed" is doing real work in that sentence: this reflects only the
* escapes the runtime could observe. Replay verifies the claim independently
* by comparing what it observes against what was recorded.
* @returns {boolean}
*/
get isDeterministic() {
return this.determinism === Determinism.DETERMINISTIC;
}
/**
* How many distinct components appear in the trace.
* @returns {number}
*/
get componentCount() {
const seen = new Set();
for (const node of this.nodes) {
if (node.component) {
seen.add(node.component);
}
}
return seen.size;
}
/**
* Produces the serializable trace.
* @returns {object} A trace matching {@link TRACE_VERSION}.
*/
toJSON() {
const trace = createTrace(this.meta);
trace.traceVersion = TRACE_VERSION;
trace.id = this.id;
trace.createdAt = this.createdAt;
trace.dropped = this.dropped;
trace.globals = { now: [...this.globals.now], random: [...this.globals.random] };
trace.redactions = [...this.redactor.patterns];
trace.redacted = this.redactor.applied;
trace.determinism = {
status: this.determinism,
reasons: [...this.reasons.entries()].map(([reason, detail]) => ({ reason, detail })),
};
// Withheld values are scrubbed here rather than when a node is recorded: an
// action's body and a call's arguments are captured before the writes that
// identify a value as a secret, so at record time it is not yet known to be
// one. Serialization is the moment the trace leaves this process, which
// makes it the right last line of defence.
trace.nodes = this.redactor.withheldValues.size > 0 ? this.nodes.map((node) => this.#scrubNode(node)) : this.nodes;
return trace;
}
/**
* Returns a copy of a node with withheld values removed from its source text.
* @param {object} node - The node.
* @returns {object} The node, or a scrubbed copy of it.
* @private
*/
#scrubNode(node) {
return this.#scrubValue(node, SCRUB_MAX_DEPTH);
}
/**
* Replaces withheld values anywhere inside a captured structure.
*
* Path rules are the primary mechanism and this is the backstop: a value the
* rules withheld in one place must not survive somewhere the rules did not
* anticipate — an action's verbatim source, or the arguments of a call that
* was recorded before the write that identified the value as a secret.
*
* The original object is returned untouched when nothing changed, so a trace
* with no matches is not needlessly copied.
* @param {any} value - The value to scrub.
* @param {number} depth - Remaining depth budget.
* @returns {any} The value, or a scrubbed copy.
* @private
*/
#scrubValue(value, depth) {
if (typeof value === 'string') {
return this.redactor.scrub(value);
}
if (depth <= 0 || value === null || typeof value !== 'object') {
return value;
}
if (Array.isArray(value)) {
let copy = value;
for (let i = 0; i < value.length; i++) {
const scrubbed = this.#scrubValue(value[i], depth - 1);
if (scrubbed !== value[i]) {
if (copy === value) {
copy = [...value];
}
copy[i] = scrubbed;
}
}
return copy;
}
let copy = value;
for (const key of Object.keys(value)) {
const scrubbed = this.#scrubValue(value[key], depth - 1);
if (scrubbed !== value[key]) {
if (copy === value) {
copy = { ...value };
}
copy[key] = scrubbed;
}
}
return copy;
}
/**
* Serializes the trace to a JSON string.
* @param {number} [indent] - Indentation passed to `JSON.stringify`.
* @returns {string}
*/
serialize(indent = 0) {
return JSON.stringify(this.toJSON(), null, indent);
}
}
/**
* Starts recording, replacing any recording already in progress.
* @param {object} [options] - Options forwarded to {@link TraceRecorder}.
* @returns {TraceRecorder} The active recorder.
*/
export function startRecording(options = {}) {
const recorder = new TraceRecorder(options);
tracer.attach(recorder);
return recorder;
}
/**
* Stops the active recording.
* @returns {object|null} The finished trace, or null if nothing was recording.
*/
export function stopRecording() {
const recorder = tracer.detach();
if (!recorder) {
return null;
}
recorder.stop();
return recorder.toJSON();
}
/**
* The recorder currently attached to the tracer, if any.
* @returns {TraceRecorder|null}
*/
export function activeRecorder() {
return tracer.sink;
}
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