Added initial reply action

ref https://linear.app/tryghost/issue/AP-306

We force a mention for the author of the object being replied to in
order to make sure they get a notification in mastodon, and we also
make sure that the activity is sent directly to the author, in case
they are not one of our followers.

The endpoint responds with the created activity so that the client can
update its local cache with the new object.

src/app.ts

@@ -63,7 +63,8 @@ import {
     inboxHandler,
     postPublishedWebhook,
     siteChangedWebhook,
-    getActivities
+    getActivities,
+    replyAction,
 } from './handlers';
 
 if (process.env.SENTRY_DSN) {
@@ -291,6 +292,7 @@ app.post('/.ghost/activitypub/webhooks/site/changed', siteChangedWebhook);
 app.post('/.ghost/activitypub/actions/follow/:handle', followAction);
 app.post('/.ghost/activitypub/actions/like/:id', likeAction);
 app.post('/.ghost/activitypub/actions/unlike/:id', unlikeAction);
+app.post('/.ghost/activitypub/actions/reply/:id', replyAction);
 
 /** Federation wire up */
 

src/handlers.ts

@@ -12,6 +12,7 @@ import {
     Update,
     Actor,
     PUBLIC_COLLECTION,
+    Mention,
 } from '@fedify/fedify';
 import { Buffer } from 'node:buffer';
 import { Context, Next } from 'hono';
@@ -208,6 +209,110 @@ export async function likeAction(
     });
 }
 
+
+const ReplyActionSchema = z.object({
+    content: z.string()
+});
+
+export async function replyAction(
+    ctx: Context<{ Variables: HonoContextVariables }>,
+) {
+    const id = ctx.req.param('id');
+
+    const data = ReplyActionSchema.parse(
+        await ctx.req.json() as unknown
+    );
+
+    const apCtx = fedify.createContext(ctx.req.raw as Request, {
+        db: ctx.get('db'),
+        globaldb: ctx.get('globaldb'),
+    });
+
+    const objectToReplyTo = await apCtx.lookupObject(id);
+    if (!objectToReplyTo) {
+        return new Response(null, {
+            status: 404,
+        });
+    }
+
+    const actor = await apCtx.getActor(ACTOR_DEFAULT_HANDLE);
+
+    let attributionActor: Actor | null = null;
+    if (objectToReplyTo.attributionId) {
+        attributionActor = await lookupActor(apCtx, objectToReplyTo.attributionId.href);
+    }
+
+    if (!attributionActor) {
+        return new Response(null, {
+            status: 400,
+        });
+    }
+
+    const to = PUBLIC_COLLECTION;
+    const cc = [attributionActor, apCtx.getFollowersUri(ACTOR_DEFAULT_HANDLE)];
+
+    const conversation = objectToReplyTo.replyTargetId || objectToReplyTo.id!;
+    const mentions = [new Mention({
+        href: attributionActor.id,
+        name: attributionActor.name,
+    })];
+
+    const replyId = apCtx.getObjectUri(Note, {
+        id: uuidv4(),
+    });
+
+    const reply = new Note({
+        id: replyId,
+        attribution: actor,
+        replyTarget: objectToReplyTo,
+        content: data.content,
+        summary: null,
+        published: Temporal.Now.instant(),
+        contexts: [conversation],
+        tags: mentions,
+        to: to,
+        ccs: cc,
+    });
+
+    const createId = apCtx.getObjectUri(Create, {
+        id: uuidv4(),
+    });
+
+    const create = new Create({
+        id: createId,
+        actor: actor,
+        object: reply,
+        to: to,
+        ccs: cc,
+    });
+
+    const activityJson = await create.toJsonLd();
+
+    await ctx
+        .get('globaldb')
+        .set([create.id!.href], activityJson);
+    await ctx
+        .get('globaldb')
+        .set([reply.id!.href], await reply.toJsonLd());
+
+    await addToList(ctx.get('db'), ['outbox'], create.id!.href);
+
+    apCtx.sendActivity({ handle: ACTOR_DEFAULT_HANDLE }, attributionActor, create, {
+        preferSharedInbox: true,
+    });
+
+    await apCtx.sendActivity({ handle: ACTOR_DEFAULT_HANDLE }, 'followers', create, {
+        preferSharedInbox: true,
+    });
+
+    return new Response(JSON.stringify(activityJson), {
+        headers: {
+            'Content-Type': 'application/activity+json',
+        },
+        status: 200,
+    });
+}
+
 export async function followAction(
     ctx: Context<{ Variables: HonoContextVariables }>,
 ) {