Resolve Matrix room aliases to Discord channels

src/m2d/converters/event-to-message.js

@@ -330,9 +330,9 @@ async function uploadEndOfMessageSpriteSheet(content, attachments, pendingFiles,
  */
 async function handleRoomOrMessageLinks(input, di) {
 	let offset = 0
-	for (const match of [...input.matchAll(/("?https:\/\/matrix.to\/#\/(![^"/, ?)]+)(?:\/(\$[^"/ ?)]+))?(?:\?[^",:!? )]*?)?)(">|[,<\n )]|$)/g)]) {
+	for (const match of [...input.matchAll(/("?https:\/\/matrix.to\/#\/((?:#|%23|!)[^"/, ?)]+)(?:\/(\$[^"/ ?)]+))?(?:\?[^",:!? )]*?)?)(">|[,<\n )]|$)/g)]) {
 		assert(typeof match.index === "number")
-		const [_, attributeValue, roomID, eventID, endMarker] = match
+		let [_, attributeValue, roomID, eventID, endMarker] = match
 		let result
 
 		const resultType = endMarker === '">' ? "html" : "plain"
@@ -350,6 +350,16 @@ async function handleRoomOrMessageLinks(input, di) {
 		// Don't process links that are part of the reply fallback, they'll be removed entirely by turndown
 		if (input.slice(match.index + match[0].length + offset).startsWith("In reply to")) continue
 
+		// Resolve room alias to room ID if needed
+		roomID = decodeURIComponent(roomID)
+		if (roomID[0] === "#") {
+			try {
+				roomID = await di.api.getAlias(roomID)
+			} catch (e) {
+				continue // Room alias is unresolvable, so it can't be converted
+			}
+		}
+
 		const channelID = select("channel_room", "channel_id", {room_id: roomID}).pluck().get()
 		if (!channelID) continue
 		if (!eventID) {

src/m2d/converters/event-to-message.test.js

@@ -2957,6 +2957,133 @@ test("event2message: mentioning bridged rooms works (plaintext body)", async t =
 	)
 })
 
+test("event2message: mentioning bridged rooms by alias works", async t => {
+	let called = 0
+	t.deepEqual(
+		await eventToMessage({
+			content: {
+				msgtype: "m.text",
+				body: "wrong body",
+				format: "org.matrix.custom.html",
+				formatted_body: `I'm just <a href="https://matrix.to/#/%23worm-farm%3Acadence.moe?via=cadence.moe">worm-farm</a> testing channel mentions`
+			},
+			event_id: "$g07oYSZFWBkxohNEfywldwgcWj1hbhDzQ1sBAKvqOOU",
+			origin_server_ts: 1688301929913,
+			room_id: "!kLRqKKUQXcibIMtOpl:cadence.moe",
+			sender: "@cadence:cadence.moe",
+			type: "m.room.message",
+			unsigned: {
+				age: 405299
+			}
+		}, {}, {
+			api: {
+				async getAlias(alias) {
+					called++
+					t.equal(alias, "#worm-farm:cadence.moe")
+					return "!BnKuBPCvyfOkhcUjEu:cadence.moe"
+				}
+			}
+		}),
+		{
+			ensureJoined: [],
+			messagesToDelete: [],
+			messagesToEdit: [],
+			messagesToSend: [{
+				username: "cadence [they]",
+				content: "I'm just <#1100319550446252084> testing channel mentions",
+				avatar_url: undefined,
+				allowed_mentions: {
+					parse: ["users", "roles"]
+				}
+			}]
+		}
+	)
+	t.equal(called, 1)
+})
+
+test("event2message: mentioning bridged rooms by alias works (plaintext body)", async t => {
+	let called = 0
+	t.deepEqual(
+		await eventToMessage({
+			content: {
+				msgtype: "m.text",
+				body: `I'm just https://matrix.to/#/#worm-farm:cadence.moe?via=cadence.moe testing channel mentions`
+			},
+			event_id: "$g07oYSZFWBkxohNEfywldwgcWj1hbhDzQ1sBAKvqOOU",
+			origin_server_ts: 1688301929913,
+			room_id: "!kLRqKKUQXcibIMtOpl:cadence.moe",
+			sender: "@cadence:cadence.moe",
+			type: "m.room.message",
+			unsigned: {
+				age: 405299
+			}
+		}, {}, {
+			api: {
+				async getAlias(alias) {
+					called++
+					t.equal(alias, "#worm-farm:cadence.moe")
+					return "!BnKuBPCvyfOkhcUjEu:cadence.moe"
+				}
+			}
+		}),
+		{
+			ensureJoined: [],
+			messagesToDelete: [],
+			messagesToEdit: [],
+			messagesToSend: [{
+				username: "cadence [they]",
+				content: "I'm just <#1100319550446252084> testing channel mentions",
+				avatar_url: undefined,
+				allowed_mentions: {
+					parse: ["users", "roles"]
+				}
+			}]
+		}
+	)
+	t.equal(called, 1)
+})
+
+test("event2message: mentioning bridged rooms by alias skips the link when alias is unresolvable", async t => {
+	let called = 0
+	t.deepEqual(
+		await eventToMessage({
+			content: {
+				msgtype: "m.text",
+				body: `I'm just https://matrix.to/#/#worm-farm:cadence.moe?via=cadence.moe and https://matrix.to/#/!BnKuBPCvyfOkhcUjEu:cadence.moe?via=cadence.moe testing channel mentions`
+			},
+			event_id: "$g07oYSZFWBkxohNEfywldwgcWj1hbhDzQ1sBAKvqOOU",
+			origin_server_ts: 1688301929913,
+			room_id: "!kLRqKKUQXcibIMtOpl:cadence.moe",
+			sender: "@cadence:cadence.moe",
+			type: "m.room.message",
+			unsigned: {
+				age: 405299
+			}
+		}, {}, {
+			api: {
+				async getAlias(alias) {
+					called++
+					throw new MatrixServerError("Alias doesn't exist or something")
+				}
+			}
+		}),
+		{
+			ensureJoined: [],
+			messagesToDelete: [],
+			messagesToEdit: [],
+			messagesToSend: [{
+				username: "cadence [they]",
+				content: "I'm just <https://matrix.to/#/#worm-farm:cadence.moe?via=cadence.moe> and <#1100319550446252084> testing channel mentions",
+				avatar_url: undefined,
+				allowed_mentions: {
+					parse: ["users", "roles"]
+				}
+			}]
+		}
+	)
+	t.equal(called, 1)
+})
+
 test("event2message: mentioning known bridged events works (plaintext body)", async t => {
 	t.deepEqual(
 		await eventToMessage({

src/matrix/api.js

@@ -367,6 +367,16 @@ async function ackEvent(event, mxid) {
 	await sendReadReceipt(event.room_id, event.event_id, mxid)
 }
 
+/**
+ * Resolve a room alias to a room ID.
+ * @param {string} alias
+ */
+async function getAlias(alias) {
+	/** @type {Ty.R.ResolvedRoom} */
+	const root = await mreq.mreq("GET", `/client/v3/directory/room/${encodeURIComponent(alias)}`)
+	return root.room_id
+}
+
 module.exports.path = path
 module.exports.register = register
 module.exports.createRoom = createRoom
@@ -395,3 +405,4 @@ module.exports.ping = ping
 module.exports.getMedia = getMedia
 module.exports.sendReadReceipt = sendReadReceipt
 module.exports.ackEvent = ackEvent
+module.exports.getAlias = getAlias

src/types.d.ts

@@ -336,6 +336,11 @@ export namespace R {
 		room_id: string
 		room_type?: string
 	}
+
+	export type ResolvedRoom = {
+		room_id: string
+		servers: string[]
+	}
 }
 
 export type Pagination<T> = {