Generate public url for linked discord attachments

src/web/routes/download-discord.js

@@ -0,0 +1,64 @@
+// @ts-check
+
+const assert = require("assert/strict")
+const {defineEventHandler, getValidatedRouterParams, sendRedirect, createError} = require("h3")
+const {z} = require("zod")
+
+const {discord, as, select} = require("../../passthrough")
+
+const schema = {
+	params: z.object({
+		channel_id: z.string().regex(/^[0-9]+$/),
+		attachment_id: z.string().regex(/^[0-9]+$/),
+		file_name: z.string().regex(/^[-A-Za-z0-9_.,]+$/)
+	})
+}
+
+/** @type {Map<string, Promise<string>>} */
+const cache = new Map()
+
+function hasExpired(url) {
+	const params = new URL(url).searchParams
+	const ex = params.get("ex")
+	assert(ex) // refreshed urls from the discord api always include this parameter
+	return parseInt(ex, 16) < Date.now() / 1000
+}
+
+// purge expired urls from cache every hour
+setInterval(() => {
+	for (const entry of cache.entries()) {
+		if (hasExpired(entry[1])) cache.delete(entry[0])
+	}
+	console.log(`purged discord media cache, it now has ${cache.size} urls`)
+}, 60 * 60 * 1000).unref()
+
+as.router.get(`/download/discordcdn/:channel_id/:attachment_id/:file_name`, defineEventHandler(async event => {
+	const params = await getValidatedRouterParams(event, schema.params.parse)
+
+	const row = select("channel_room", "channel_id", {channel_id: params.channel_id}).get()
+	if (row == null) {
+		throw createError({
+			status: 403,
+			data: `The file you requested isn't permitted by this media proxy.`
+		})
+	}
+
+	const url = `https://cdn.discordapp.com/attachments/${params.channel_id}/${params.attachment_id}/${params.file_name}`
+	let promise = cache.get(url)
+	let refreshed
+	if (promise) {
+		console.log("using existing cache entry")
+		refreshed = await promise
+		if (hasExpired(refreshed)) promise = undefined
+		console.log(promise)
+	}
+	if (!promise) {
+		console.log("refreshing and storing")
+		promise = discord.snow.channel.refreshAttachmentURLs([url]).then(x => x.refreshed_urls[0].refreshed)
+		cache.set(url, promise)
+		refreshed = await promise
+	}
+	assert(refreshed) // will have been assigned by one of the above branches
+
+	return sendRedirect(event, refreshed)
+}))