136 lines
4.3 KiB
TypeScript
136 lines
4.3 KiB
TypeScript
/**
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* Fetches a link's preview through the provider that knows its site, and
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* reuses it for a few minutes. `dump-service.ts` calls it when a link is
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* posted, changed or refreshed, and `GET /api/preview` when the create form
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* previews one.
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*
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* @module
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*/
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import type { RichContent } from "../../shared/dump.ts";
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import type { RichContentProvider } from "../remote/providers/provider.ts";
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import { youtubeProvider } from "../remote/providers/youtube.ts";
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import { bandcampProvider } from "../remote/providers/bandcamp.ts";
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import { soundcloudProvider } from "../remote/providers/soundcloud.ts";
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import { selfProvider } from "../remote/providers/self.ts";
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import { genericProvider } from "../remote/providers/generic.ts";
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/**
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* Lists the providers in priority order: the first that matches a URL reads
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* it. `selfProvider` reads this site's own pages from the database, and
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* `genericProvider` must stay last, since it matches every URL.
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*/
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const providers: RichContentProvider[] = [
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youtubeProvider,
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bandcampProvider,
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soundcloudProvider,
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selfProvider,
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genericProvider,
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];
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/**
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* Returned by {@link tryFetchRichContent} to `GET /api/preview`, which reports
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* `ok` as `reached`, and to `refreshDumpMetadata`, which gives up when it is
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* false.
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*/
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export interface FetchRichContentResult {
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/**
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* False when the page couldn't be reached at all, as opposed to reached and
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* found to carry no metadata. A caller that already holds metadata must not
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* overwrite it with the failure stub.
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*/
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ok: boolean;
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/**
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* On failure, a stub holding the URL and its host name. Missing only when
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* the URL doesn't parse.
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*/
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content?: RichContent;
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}
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/** How long a fetched preview is reused. */
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const RECENT_TTL_MS = 5 * 60 * 1000;
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/** How many previews are kept. The oldest written goes first. */
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const RECENT_MAX = 200;
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/**
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* Makes posting a link fast after the create form previewed it, by keeping the
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* previews fetched in the last few minutes, by URL. Without it, the second
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* fetch of the page was the slow part of posting. Only successes are kept.
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*/
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const recent = new Map<
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string,
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{ at: number; result: Promise<FetchRichContentResult> }
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>();
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/**
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* Forgets every cached preview. The tests' `stubNetwork` calls it, since
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* previews fetched from the previous stubbed network no longer apply.
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*/
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export function clearRichContentCache(): void {
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recent.clear();
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}
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/**
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* Fetches the preview of `url`, and says whether the page could be reached. A
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* preview fetched in the last five minutes is reused unless `fresh` is set, as
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* a metadata refresh does. It never rejects: a failure resolves to a stub.
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*/
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export function tryFetchRichContent(
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url: string,
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{ fresh = false }: { fresh?: boolean } = {},
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): Promise<FetchRichContentResult> {
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const hit = recent.get(url);
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if (!fresh && hit && Date.now() - hit.at < RECENT_TTL_MS) return hit.result;
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recent.delete(url);
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// Stored while in flight too, so a preview and a post racing share a fetch.
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const result = fetchRichContentUncached(url);
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if (recent.size >= RECENT_MAX) {
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const oldest = recent.keys().next().value;
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if (oldest !== undefined) recent.delete(oldest);
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}
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const entry = { at: Date.now(), result };
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recent.set(url, entry);
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result.then(({ ok }) => {
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if (!ok && recent.get(url) === entry) recent.delete(url);
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});
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return result;
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}
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/**
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* Fetches the preview of `url` through the first provider that matches, or
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* makes the failure stub when the provider rejects.
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*/
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async function fetchRichContentUncached(
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url: string,
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): Promise<FetchRichContentResult> {
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try {
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const provider = providers.find((p) => p.matches(url))!;
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return { ok: true, content: await provider.fetch(url) };
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} catch (err) {
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console.error(`[rich-content] Failed to fetch metadata for ${url}:`, err);
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try {
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return {
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ok: false,
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content: {
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type: "generic",
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url,
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siteName: new URL(url).hostname.replace(/^www\./, ""),
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},
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};
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} catch {
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return { ok: false };
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}
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}
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}
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/**
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* Returns the preview of `url`, or a stub holding its host name when the page
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* can't be reached. `createUrlDump` and `updateDump` use it for a new link,
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* which has no earlier preview to keep.
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*/
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export async function fetchRichContent(
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url: string,
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): Promise<RichContent | undefined> {
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return (await tryFetchRichContent(url)).content;
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}
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