Generate public url for linked discord attachments

src/d2m/converters/edit-to-changes.test.js

@@ -99,9 +99,9 @@ test("edit2changes: change file type", async t => {
 	t.deepEqual(eventsToRedact, ["$51f4yqHinwnSbPEQ9dCgoyy4qiIJSX0QYYVUnvwyTCJ"])
 	t.deepEqual(eventsToSend, [{
 		$type: "m.room.message",
-		body: "📝 Uploaded file: https://cdn.discordapp.com/attachments/112760669178241024/1141501302497615912/gaze_into_my_dark_mind.txt (20 MB)",
+		body: "📝 Uploaded file: https://bridge.example.org/download/discordcdn/112760669178241024/1141501302497615912/gaze_into_my_dark_mind.txt (20 MB)",
 		format: "org.matrix.custom.html",
-		formatted_body: "📝 Uploaded file: <a href=\"https://cdn.discordapp.com/attachments/112760669178241024/1141501302497615912/gaze_into_my_dark_mind.txt\">gaze_into_my_dark_mind.txt</a> (20 MB)",
+		formatted_body: "📝 Uploaded file: <a href=\"https://bridge.example.org/download/discordcdn/112760669178241024/1141501302497615912/gaze_into_my_dark_mind.txt\">gaze_into_my_dark_mind.txt</a> (20 MB)",
 		"m.mentions": {},
 		msgtype: "m.text"
 	}])

src/d2m/converters/message-to-event.js

@@ -103,6 +103,7 @@ const embedTitleParser = markdown.markdownEngine.parserFor({
  * @param {DiscordTypes.APIAttachment} attachment
  */
 async function attachmentToEvent(mentions, attachment) {
+	const publicURL = dUtils.getPublicUrlForCdn(attachment.url)
 	const emoji =
 		attachment.content_type?.startsWith("image/jp") ? "📸"
 		: attachment.content_type?.startsWith("image/") ? "🖼️"
@@ -116,9 +117,9 @@ async function attachmentToEvent(mentions, attachment) {
 			$type: "m.room.message",
 			"m.mentions": mentions,
 			msgtype: "m.text",
-			body: `${emoji} Uploaded SPOILER file: ${attachment.url} (${pb(attachment.size)})`,
+			body: `${emoji} Uploaded SPOILER file: ${publicURL} (${pb(attachment.size)})`,
 			format: "org.matrix.custom.html",
-			formatted_body: `<blockquote>${emoji} Uploaded SPOILER file: <a href="${attachment.url}">${attachment.url}</a> (${pb(attachment.size)})</blockquote>`
+			formatted_body: `<blockquote>${emoji} Uploaded SPOILER file: <a href="${publicURL}">${publicURL}</a> (${pb(attachment.size)})</blockquote>`
 		}
 	}
 	// for large files, always link them instead of uploading so I don't use up all the space in the content repo
@@ -127,9 +128,9 @@ async function attachmentToEvent(mentions, attachment) {
 			$type: "m.room.message",
 			"m.mentions": mentions,
 			msgtype: "m.text",
-			body: `${emoji} Uploaded file: ${attachment.url} (${pb(attachment.size)})`,
+			body: `${emoji} Uploaded file: ${publicURL} (${pb(attachment.size)})`,
 			format: "org.matrix.custom.html",
-			formatted_body: `${emoji} Uploaded file: <a href="${attachment.url}">${attachment.filename}</a> (${pb(attachment.size)})`
+			formatted_body: `${emoji} Uploaded file: <a href="${publicURL}">${attachment.filename}</a> (${pb(attachment.size)})`
 		}
 	} else if (attachment.content_type?.startsWith("image/") && attachment.width && attachment.height) {
 		return {

src/d2m/converters/message-to-event.test.js

@@ -305,9 +305,9 @@ test("message2event: spoiler attachment", async t => {
 		$type: "m.room.message",
 		"m.mentions": {},
 		msgtype: "m.text",
-		body: "📄 Uploaded SPOILER file: https://cdn.discordapp.com/attachments/1100319550446252084/1147465564307079258/SPOILER_69-GNDP-CADENCE.nfs.gci (74 KB)",
+		body: "📄 Uploaded SPOILER file: https://bridge.example.org/download/discordcdn/1100319550446252084/1147465564307079258/SPOILER_69-GNDP-CADENCE.nfs.gci (74 KB)",
 		format: "org.matrix.custom.html",
-		formatted_body: "<blockquote>📄 Uploaded SPOILER file: <a href=\"https://cdn.discordapp.com/attachments/1100319550446252084/1147465564307079258/SPOILER_69-GNDP-CADENCE.nfs.gci\">https://cdn.discordapp.com/attachments/1100319550446252084/1147465564307079258/SPOILER_69-GNDP-CADENCE.nfs.gci</a> (74 KB)</blockquote>"
+		formatted_body: "<blockquote>📄 Uploaded SPOILER file: <a href=\"https://bridge.example.org/download/discordcdn/1100319550446252084/1147465564307079258/SPOILER_69-GNDP-CADENCE.nfs.gci\">https://bridge.example.org/download/discordcdn/1100319550446252084/1147465564307079258/SPOILER_69-GNDP-CADENCE.nfs.gci</a> (74 KB)</blockquote>"
 	}])
 })
 
@@ -788,7 +788,7 @@ test("message2event: very large attachment is linked instead of being uploaded",
 		content: "hey",
 		attachments: [{
 			filename: "hey.jpg",
-			url: "https://discord.com/404/hey.jpg",
+			url: "https://cdn.discordapp.com/attachments/123/456/789.mega",
 			content_type: "application/i-made-it-up",
 			size: 100e6
 		}]
@@ -802,9 +802,9 @@ test("message2event: very large attachment is linked instead of being uploaded",
 		$type: "m.room.message",
 		"m.mentions": {},
 		msgtype: "m.text",
-		body: "📄 Uploaded file: https://discord.com/404/hey.jpg (100 MB)",
+		body: "📄 Uploaded file: https://bridge.example.org/download/discordcdn/123/456/789.mega (100 MB)",
 		format: "org.matrix.custom.html",
-		formatted_body: '📄 Uploaded file: <a href="https://discord.com/404/hey.jpg">hey.jpg</a> (100 MB)'
+		formatted_body: '📄 Uploaded file: <a href="https://bridge.example.org/download/discordcdn/123/456/789.mega">hey.jpg</a> (100 MB)'
 	}])
 })
 

src/discord/utils.js

@@ -3,6 +3,8 @@
 const DiscordTypes = require("discord-api-types/v10")
 const assert = require("assert").strict
 
+const {reg} = require("../matrix/read-registration")
+
 const EPOCH = 1420070400000
 
 /**
@@ -117,6 +119,13 @@ function timestampToSnowflakeInexact(timestamp) {
 	return String((timestamp - EPOCH) * 2**22)
 }
 
+/** @param {string} url */
+function getPublicUrlForCdn(url) {
+	const match = url.match(`https://cdn.discordapp.com/attachments/([0-9]+)/([0-9]+)/([-A-Za-z0-9_.,]+)`)
+	if (!match) return url
+	return `${reg.ooye.bridge_origin}/download/discordcdn/${match[1]}/${match[2]}/${match[3]}`
+}
+
 module.exports.getPermissions = getPermissions
 module.exports.hasPermission = hasPermission
 module.exports.hasSomePermissions = hasSomePermissions
@@ -125,3 +134,4 @@ module.exports.isWebhookMessage = isWebhookMessage
 module.exports.isEphemeralMessage = isEphemeralMessage
 module.exports.snowflakeToTimestampExact = snowflakeToTimestampExact
 module.exports.timestampToSnowflakeInexact = timestampToSnowflakeInexact
+module.exports.getPublicUrlForCdn = getPublicUrlForCdn

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)
+}))

src/web/server.js

@@ -3,3 +3,4 @@
 const {sync, as} = require("../passthrough")
 
 sync.require("./routes/download-matrix")
+sync.require("./routes/download-discord")