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import type { ExampleRecord } from "./examples-schema";
import { ExamplesError } from "./examples-error";
import type { TutorCapability } from "../course-content/course-content-loader";
import type { ExampleTutorAnnotation } from "../course-content/embedded-tutor-annotation";
export type SourceCacheKey = `examples:source:v1:${RepositorySlug}:${ExamplesRef}`;
export type RevisionCacheKey = `examples:revision:v1:${RepositorySlug}:${FullCommitSha}`;
export function toSourceCacheKey(repository: RepositorySlug, ref: ExamplesRef): SourceCacheKey {
return `examples:source:v1:${repository}:${ref}`;
}
export function toRevisionCacheKey(repository: RepositorySlug, revision: FullCommitSha): RevisionCacheKey {
return `examples:revision:v1:${repository}:${revision}`;
}
export interface SourceCacheEntry {
repository: RepositorySlug;
ref: ExamplesRef;
activeRevision: FullCommitSha;
checkedAt: number;
expiresAt: number;
stale: boolean;
nextRetryAt: number;
lastAccessedAt: number;
}
export interface RevisionCacheEntry {
repository: RepositorySlug;
revision: FullCommitSha;
examples: ExampleRecord[];
tutor?: TutorCapability;
exampleTutorAnnotations?: ReadonlyMap<string, ExampleTutorAnnotation>;
contentBytes: number;
/** Refs of this repository the server resolved to this revision; an example's revision is checked against them. */
resolvedRefs?: ReadonlySet<ExamplesRef>;
lastAccessedAt: number;
}
/** Bounds the provenance kept per snapshot; the oldest resolved ref is forgotten first. */
const MAX_RESOLVED_REFS_PER_REVISION = 16;
export interface ExamplesCacheOptions {
maxSources: number;
maxSnapshots: number;
maxSnapshotBytes: number;
now?: () => number;
/** Sources the LRU never evicts: the operator's default Course, which overrides must not displace. */
protectedSourceKeys?: readonly SourceCacheKey[];
}
interface SharedFlight {
controller: AbortController;
promise: Promise<unknown>;
subscribers: number;
settled: boolean;
}
export class ExamplesCache {
private readonly sources = new Map<SourceCacheKey, SourceCacheEntry>();
private readonly revisions = new Map<RevisionCacheKey, RevisionCacheEntry>();
private readonly sourceFlights = new Map<SourceCacheKey, SharedFlight>();
private readonly revisionFlights = new Map<RevisionCacheKey, SharedFlight>();
private totalSnapshotBytes = 0;
private readonly now: () => number;
private readonly protectedSources: ReadonlySet<SourceCacheKey>;
constructor(private readonly options: ExamplesCacheOptions) {
this.now = options.now ?? Date.now;
this.protectedSources = new Set(options.protectedSourceKeys);
}
getSource(key: SourceCacheKey): SourceCacheEntry | undefined {
const entry = this.sources.get(key);
if (entry) entry.lastAccessedAt = this.now();
return entry;
}
setSource(key: SourceCacheKey, entry: Omit<SourceCacheEntry, "lastAccessedAt">): SourceCacheEntry {
const beforeSources = new Map(this.sources);
const beforeRevisions = new Map(this.revisions);
const beforeBytes = this.totalSnapshotBytes;
const stored = { ...entry, lastAccessedAt: this.now() };
this.sources.set(key, stored);
try {
this.enforceRevisionLimits();
} catch (error) {
this.restore(beforeSources, beforeRevisions, beforeBytes);
throw error;
}
return stored;
}
activateSource(
sourceKey: SourceCacheKey,
source: Omit<SourceCacheEntry, "lastAccessedAt">,
revisionKey: RevisionCacheKey,
revision: Omit<RevisionCacheEntry, "lastAccessedAt">,
): { source: SourceCacheEntry; revision: RevisionCacheEntry } {
const beforeSources = new Map(this.sources);
const beforeRevisions = new Map(this.revisions);
const beforeBytes = this.totalSnapshotBytes;
const previous = this.revisions.get(revisionKey);
if (previous) this.totalSnapshotBytes -= previous.contentBytes;
const accessedAt = this.now();
const storedRevision = {
...revision,
resolvedRefs: withResolvedRef(previous?.resolvedRefs ?? revision.resolvedRefs, source.ref),
lastAccessedAt: accessedAt,
};
const storedSource = { ...source, lastAccessedAt: accessedAt };
this.revisions.set(revisionKey, storedRevision);
this.totalSnapshotBytes += storedRevision.contentBytes;
this.sources.set(sourceKey, storedSource);
try {
this.enforceRevisionLimits();
} catch (error) {
this.restore(beforeSources, beforeRevisions, beforeBytes);
throw error;
}
return { source: storedSource, revision: storedRevision };
}
markSourceFailure(key: SourceCacheKey, nextRetryAt: number): SourceCacheEntry | undefined {
const entry = this.sources.get(key);
if (!entry) return undefined;
entry.stale = true;
entry.nextRetryAt = nextRetryAt;
entry.lastAccessedAt = this.now();
return entry;
}
getRevision(key: RevisionCacheKey): RevisionCacheEntry | undefined {
const entry = this.revisions.get(key);
if (entry) entry.lastAccessedAt = this.now();
return entry;
}
setRevision(key: RevisionCacheKey, entry: Omit<RevisionCacheEntry, "lastAccessedAt">): RevisionCacheEntry {
const beforeSources = new Map(this.sources);
const beforeRevisions = new Map(this.revisions);
const beforeBytes = this.totalSnapshotBytes;
const previous = this.revisions.get(key);
Iif (previous) this.totalSnapshotBytes -= previous.contentBytes;
const stored = { ...entry, lastAccessedAt: this.now() };
this.revisions.set(key, stored);
this.totalSnapshotBytes += stored.contentBytes;
try {
this.enforceRevisionLimits();
} catch (error) {
this.restore(beforeSources, beforeRevisions, beforeBytes);
throw error;
}
return stored;
}
async withSourceSingleflight<T>(
key: SourceCacheKey,
load: (signal: AbortSignal) => Promise<T>,
requestSignal?: AbortSignal,
): Promise<T> {
Iif (requestSignal?.aborted) throw requestSignal.reason ?? new DOMException("Request aborted", "AbortError");
const current = this.sourceFlights.get(key);
if (!current || current.controller.signal.aborted) this.admitSource(key);
return this.withSingleflight(this.sourceFlights, key, load, requestSignal);
}
async withRevisionSingleflight<T>(
key: RevisionCacheKey,
load: (signal: AbortSignal) => Promise<T>,
requestSignal?: AbortSignal,
): Promise<T> {
return this.withSingleflight(this.revisionFlights, key, load, requestSignal);
}
getStats() {
return {
sources: this.sources.size,
revisions: this.revisions.size,
snapshotBytes: this.totalSnapshotBytes,
sourceFlights: this.sourceFlights.size,
revisionFlights: this.revisionFlights.size,
};
}
private admitSource(key: SourceCacheKey): void {
if (this.sources.has(key) || this.sourceFlights.has(key)) return;
const sourceCount = () => new Set([...this.sources.keys(), ...this.sourceFlights.keys()]).size;
while (sourceCount() >= this.options.maxSources) {
if (!this.evictOldestSource()) {
throw new ExamplesError("LOAD_CAPACITY_EXCEEDED", "External examples source capacity is exhausted");
}
}
}
private withSingleflight<K extends string, T>(
flights: Map<K, SharedFlight>,
key: K,
load: (signal: AbortSignal) => Promise<T>,
requestSignal?: AbortSignal,
): Promise<T> {
Iif (requestSignal?.aborted) return Promise.reject(requestSignal.reason ?? new DOMException("Request aborted", "AbortError"));
let flight = flights.get(key) as SharedFlight & { promise: Promise<T> } | undefined;
Iif (flight?.controller.signal.aborted) {
if (flights.get(key) === flight) flights.delete(key);
flight = undefined;
}
if (!flight) {
const controller = new AbortController();
let created!: SharedFlight & { promise: Promise<T> };
let resolveFlight!: (value: T) => void;
let rejectFlight!: (error: unknown) => void;
const promise = new Promise<T>((resolve, reject) => {
resolveFlight = resolve;
rejectFlight = reject;
});
const settle = () => {
created.settled = true;
Eif (flights.get(key) === created) flights.delete(key);
};
created = { controller, promise, subscribers: 0, settled: false };
flight = created;
flights.set(key, created);
void Promise.resolve()
.then(() => load(controller.signal))
.then(
(value) => {
settle();
resolveFlight(value);
},
(error: unknown) => {
settle();
rejectFlight(error);
},
);
}
return this.subscribeToFlight(flight, requestSignal);
}
private subscribeToFlight<T>(
flight: SharedFlight & { promise: Promise<T> },
requestSignal?: AbortSignal,
): Promise<T> {
Iif (requestSignal?.aborted) return Promise.reject(requestSignal.reason ?? new DOMException("Request aborted", "AbortError"));
flight.subscribers += 1;
return new Promise<T>((resolve, reject) => {
let left = false;
const leave = () => {
if (left) return;
left = true;
requestSignal?.removeEventListener("abort", onAbort);
flight.subscribers -= 1;
if (flight.subscribers === 0 && !flight.settled) flight.controller.abort();
};
const onAbort = () => {
leave();
reject(requestSignal?.reason ?? new DOMException("Request aborted", "AbortError"));
};
requestSignal?.addEventListener("abort", onAbort, { once: true });
Iif (requestSignal?.aborted) {
onAbort();
return;
}
flight.promise.then(
(value) => {
leave();
resolve(value as T);
},
(error: unknown) => {
leave();
reject(error);
},
);
});
}
private enforceRevisionLimits(): void {
while (this.revisions.size > this.options.maxSnapshots || this.totalSnapshotBytes > this.options.maxSnapshotBytes) {
Eif (this.evictOldestUnpinnedRevision()) continue;
if (this.evictOldestSource()) continue;
throw new ExamplesError("LOAD_CAPACITY_EXCEEDED", "External examples snapshot cache is exhausted");
}
}
private evictOldestSource(): boolean {
const oldest = [...this.sources.entries()]
.filter(([key]) => !this.sourceFlights.has(key) && !this.protectedSources.has(key))
.sort(([, left], [, right]) => left.lastAccessedAt - right.lastAccessedAt)[0];
if (!oldest) return false;
this.sources.delete(oldest[0]);
return true;
}
private evictOldestUnpinnedRevision(): boolean {
const pinned = new Set(
[...this.sources.values()].map((entry) => toRevisionCacheKey(entry.repository, entry.activeRevision)),
);
const oldest = [...this.revisions.entries()]
.filter(([key]) => !pinned.has(key) && !this.revisionFlights.has(key))
.sort(([, left], [, right]) => left.lastAccessedAt - right.lastAccessedAt)[0];
Iif (!oldest) return false;
this.revisions.delete(oldest[0]);
this.totalSnapshotBytes -= oldest[1].contentBytes;
return true;
}
private restore(
sources: Map<SourceCacheKey, SourceCacheEntry>,
revisions: Map<RevisionCacheKey, RevisionCacheEntry>,
totalSnapshotBytes: number,
): void {
this.sources.clear();
for (const [key, entry] of sources) this.sources.set(key, entry);
this.revisions.clear();
for (const [key, entry] of revisions) this.revisions.set(key, entry);
this.totalSnapshotBytes = totalSnapshotBytes;
}
}
function withResolvedRef(refs: ReadonlySet<ExamplesRef> | undefined, ref: ExamplesRef): ReadonlySet<ExamplesRef> {
const next = new Set(refs);
next.delete(ref);
next.add(ref);
while (next.size > MAX_RESOLVED_REFS_PER_REVISION) next.delete(next.values().next().value as ExamplesRef);
return next;
}
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