fix: persist actor URL to MongoDB so follow survives server restarts

resolveActorUrl() relied on an in-memory idToUrl cache that is cleared
on every server restart. Accounts discovered via search or profile lookup
but not yet in followers/following/timeline would fail with 404 when
attempting to follow after a restart.

Fix: add ap_actor_cache MongoDB collection. resolveRemoteAccount() now
persists { _id: hash(actorUrl), actorUrl } on every successful remote
resolution (fire-and-forget, non-fatal). resolveActorUrl() checks this
collection after the in-memory cache, before falling through to DB scans.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.js

@@ -1203,6 +1203,8 @@ export default class ActivityPubEndpoint {
     // Filters and filter keywords
     Indiekit.addCollection("ap_filters");
     Indiekit.addCollection("ap_filter_keywords");
+    // Persistent actor URL cache (survives server restarts; used by follow/unfollow)
+    Indiekit.addCollection("ap_actor_cache");
 
     // Plugin settings (single document, admin UI at /admin/settings)
     Indiekit.addCollection("ap_settings");
@@ -1253,6 +1255,8 @@ export default class ActivityPubEndpoint {
       // Filters and filter keywords
       ap_filters: indiekitCollections.get("ap_filters"),
       ap_filter_keywords: indiekitCollections.get("ap_filter_keywords"),
+      // Persistent actor URL cache (survives server restarts)
+      ap_actor_cache: indiekitCollections.get("ap_actor_cache"),
       get posts() {
         return indiekitCollections.get("posts");
       },

lib/mastodon/helpers/resolve-account.js

@@ -6,15 +6,17 @@
  */
 import { serializeAccount } from "../entities/account.js";
 import { cacheAccountStats } from "./account-cache.js";
+import { remoteActorId } from "./id-mapping.js";
 import { lookupWithSecurity } from "../../lookup-helpers.js";
 
 /**
  * @param {string} acct - Account identifier (user@domain or URL)
  * @param {object} pluginOptions - Plugin options with federation, handle, publicationUrl
  * @param {string} baseUrl - Server base URL
+ * @param {object|null} collections - MongoDB collections (optional; if provided, persists actorUrl to ap_actor_cache)
  * @returns {Promise<object|null>} Serialized Mastodon Account or null
  */
-export async function resolveRemoteAccount(acct, pluginOptions, baseUrl) {
+export async function resolveRemoteAccount(acct, pluginOptions, baseUrl, collections = null) {
   const { federation, handle, publicationUrl } = pluginOptions;
   if (!federation) return null;
 
@@ -138,6 +140,16 @@ export async function resolveRemoteAccount(acct, pluginOptions, baseUrl) {
       avatarUrl: avatarUrl || undefined,
     });
 
+    // Persist actor URL mapping to MongoDB so follow/unfollow survives server restarts
+    if (collections?.ap_actor_cache && actorUrl) {
+      const hashId = remoteActorId(actorUrl);
+      collections.ap_actor_cache.updateOne(
+        { _id: hashId },
+        { $set: { actorUrl, updatedAt: new Date() } },
+        { upsert: true },
+      ).catch(() => {}); // fire-and-forget, non-fatal
+    }
+
     return account;
   } catch (error) {
     console.warn(`[Mastodon API] Remote account resolution failed for ${acct}:`, error.message);

lib/mastodon/routes/accounts.js

@@ -143,7 +143,7 @@ router.get("/api/v1/accounts/lookup", async (req, res, next) => {
     }
 
     // Resolve remotely via federation (WebFinger + actor fetch)
-    const resolved = await resolveRemoteAccount(bareAcct, pluginOptions, baseUrl);
+    const resolved = await resolveRemoteAccount(bareAcct, pluginOptions, baseUrl, collections);
     if (resolved) {
       return res.json(resolved);
     }
@@ -375,6 +375,7 @@ router.get("/api/v1/accounts/:id", tokenRequired, scopeRequired("read", "read:ac
         actorUrl,
         pluginOptions,
         baseUrl,
+        collections,
       );
       if (remoteAccount) {
         return res.json(remoteAccount);
@@ -816,6 +817,12 @@ async function resolveActorUrl(id, collections) {
   const cachedUrl = getActorUrlFromId(id);
   if (cachedUrl) return cachedUrl;
 
+  // Check persistent actor cache (survives server restarts)
+  if (collections.ap_actor_cache) {
+    const cached = await collections.ap_actor_cache.findOne({ _id: id });
+    if (cached?.actorUrl) return cached.actorUrl;
+  }
+
   // Check followers
   const followers = await collections.ap_followers.find({}).toArray();
   for (const f of followers) {

lib/mastodon/routes/search.js

@@ -84,7 +84,7 @@ router.get("/api/v2/search", tokenRequired, scopeRequired("read", "read:search")
 
       // If no local results and resolve=true, try remote lookup
       if (results.accounts.length === 0 && resolve && query.includes("@")) {
-        const resolved = await resolveRemoteAccount(query, pluginOptions, baseUrl);
+        const resolved = await resolveRemoteAccount(query, pluginOptions, baseUrl, collections);
         if (resolved) {
           results.accounts.push(resolved);
         }