src/d2m/actions/create-room.js

@@ -439,12 +439,12 @@ async function _syncRoom(channelID, shouldActuallySync) {
 	return roomID
 }
 
-/** Ensures the room exists. If it doesn't, creates the room with an accurate initial state. Please check that a channel_room entry exists or guild autocreate = 1 before calling this. */
+/** Ensures the room exists. If it doesn't, creates the room with an accurate initial state. Before calling this, please make sure that: a channel_room entry exists, guild autocreate = 1, or you're operating on a thread.*/
 function ensureRoom(channelID) {
 	return _syncRoom(channelID, false)
 }
 
-/** Actually syncs. Gets all room state from the homeserver in order to diff, and uploads the icon to mxc if it has changed. Please check that a channel_room entry exists or guild autocreate = 1 before calling this. */
+/** Actually syncs. Gets all room state from the homeserver in order to diff, and uploads the icon to mxc if it has changed. Before calling this, please make sure that: a channel_room entry exists, guild autocreate = 1, or you're operating on a thread.*/
 function syncRoom(channelID) {
 	return _syncRoom(channelID, true)
 }

src/d2m/event-dispatcher.js

@@ -212,7 +212,7 @@ module.exports = {
 		const channelID = thread.parent_id || undefined
 		const parentRoomID = select("channel_room", "room_id", {channel_id: channelID}).pluck().get()
 		if (!parentRoomID) return // Not interested in a thread if we aren't interested in its wider channel (won't autocreate)
-		const threadRoomID = await createRoom.syncRoom(thread.id) // Create room (will share the same inflight as the initial message to the thread)
+		const threadRoomID = await createRoom.ensureRoom(thread.id)
 		await announceThread.announceThread(parentRoomID, threadRoomID, thread)
 	},
 

src/m2d/event-dispatcher.js

@@ -214,7 +214,7 @@ async event => {
 	if (event.content["m.relates_to"]?.rel_type === "m.thread") {
 		const bridgedTo = utils.getThreadRoomFromThreadEvent(event.content["m.relates_to"].event_id)
 		if (bridgedTo) event.room_id = bridgedTo;
-		else throw new Error("Detected that a message was sent from a Matrix thread, yet there doesn't seem to be a related Discord thread!")
+		else await bridgeThread(event);
 	}
 
 	const messageResponses = await sendEvent.sendEvent(event)
@@ -233,6 +233,24 @@ async event => {
 	await api.ackEvent(event)
 }))
 
+/**
+ * @param {Ty.Event.Outer_M_Room_Message | Ty.Event.Outer_M_Room_Message_File} event Its room_id will mutate to the target thread, if such thread gets created.
+ */
+async function bridgeThread(event) {
+	/** @type {string} */ // @ts-ignore
+	const channelID = select("channel_room", "channel_id", {room_id: event.room_id}).pluck().get();
+	const channel = discord.channels.get(channelID)
+	const guildID = channel?.["guild_id"]
+	if (!guildID) return; //Room not bridged? We don't care. It's a Matrix-naive room, let Matrix users have standard Matrix-native threads there.
+
+	const eventID = event.content["m.relates_to"]?.event_id
+	if (!eventID) throw new Error("There was an event sent inside SOME Matrix thread, but it lacked any information as to what thread it actually was!"); //An „ugly error” is justified because if something like this DOES happen, then that means that it should be reported to us, as there is some broken client out there that we should account for.
+	const messageID = select("event_message", "message_id", {event_id: eventID}).pluck().get()
+	if (!messageID) return; //Message not bridged? Too bad! Discord users will just see normal replies, and Matrix uses won't get a thread-room.We COULD technically create a "headless" thread on Discord side and bridge it to a new thread-room, but that comes with a whole host of complications on its own (notably: what do we do if the message gets bridged later (by reaction emoji), and maybe gets its own thread; and: getThreadRoomFromThreadEvent will have to be much more complex than a simple DB call (probably a whole new DB table would have to be created, just to hold these Matrix-branched-but-headless-on-Discord threads) because the simple „MX event --(db)--> Discord Message --(Discord spec)--> Discord thread” relation would no longer hold true), which may not be worth it, as an unbridged message in a bridged channel is already an edge-case and it seems somewhat pointless to introduce and account for a whole bunch of edgier-cases that handling this edge-case "properly" would bring.
+	
+	event.room_id = await createRoom.ensureRoom((await discord.snow.channel.createThreadWithMessage(channelID, messageID, {name: "TODO: name-gen"})).id)
+}
+
 sync.addTemporaryListener(as, "type:m.sticker", guard("m.sticker",
 /**
  * @param {Ty.Event.Outer_M_Sticker} event it is a m.sticker because that's what this listener is filtering for