Prevented duplicate followers from being added to the list of followers

features/accept-follows.feature

@@ -7,3 +7,17 @@ Feature: We automatically accept Follow requests
     Then an "Accept(F)" Activity "A" is created by "Us"
     And Activity "A" is sent to "Alice"
     And "Alice" is in our Followers
+
+  Rule: We can be followed multiple times by the same actor, but we only record them once
+
+    Example: An actor attempts to follow us multiple times
+        Given an Actor "Alice"
+        And a "Follow(Us)" Activity "F1" by "Alice"
+        And a "Follow(Us)" Activity "F2" by "Alice"
+        When "Alice" sends "F1" to the Inbox
+        And "Alice" sends "F2" to the Inbox
+        Then an "Accept(F1)" Activity "A1" is created by "Us"
+        And an "Accept(F2)" Activity "A2" is created by "Us"
+        And Activity "A1" is sent to "Alice"
+        And Activity "A2" is sent to "Alice"
+        And "Alice" is in our Followers once only

features/step_definitions/stepdefs.js

@@ -1,6 +1,7 @@
 import assert from 'assert';
 import Knex from 'knex';
 import { BeforeAll, AfterAll, Before, After, Given, When, Then } from '@cucumber/cucumber';
+import { v4 as uuidv4 } from 'uuid';
 import { WireMock } from 'wiremock-captain';
 
 async function createActivity(activityType, object, actor, remote = true) {
@@ -11,7 +12,7 @@ async function createActivity(activityType, object, actor, remote = true) {
                 'https://w3id.org/security/data-integrity/v1',
             ],
             'type': 'Follow',
-            'id': 'http://wiremock:8080/follow/1',
+            'id': `http://wiremock:8080/follow/${uuidv4()}`,
             'to': 'as:Public',
             'object': object,
             actor: actor,
@@ -318,3 +319,16 @@ Then('{string} is in our Followers', async function (actorName) {
 
     assert(found);
 });
+
+Then('{string} is in our Followers once only', async function (actorName) {
+    const response = await fetch('http://activitypub-testing:8083/.ghost/activitypub/followers/index', {
+        headers: {
+            Accept: 'application/ld+json'
+        }
+    });
+    const followers = await response.json();
+    const actor = this.actors[actorName];
+    const found = followers.orderedItems.filter(item => item === actor.id);
+
+    assert.equal(found.length, 1);
+});

src/dispatchers.ts

@@ -62,14 +62,28 @@ export async function handleFollow(
     if (sender === null || sender.id === null) {
         return;
     }
-    const senderJson = await sender.toJsonLd();
+
+    const currentFollowers = await ctx.data.db.get<string[]>(['followers']) ?? [];
+    let shouldRecordFollower = currentFollowers.includes(sender.id.href) === false;
+
+    // Add follow activity to inbox
     const followJson = await follow.toJsonLd();
+
     ctx.data.globaldb.set([follow.id.href], followJson);
-    ctx.data.globaldb.set([sender.id.href], senderJson);
     await addToList(ctx.data.db, ['inbox'], follow.id.href);
-    await addToList(ctx.data.db, ['followers'], sender.id.href);
-    await addToList(ctx.data.db, ['followers', 'expanded'], senderJson);
 
+    // Record follower in followers list
+    const senderJson = await sender.toJsonLd();
+
+    if (shouldRecordFollower) {
+        await addToList(ctx.data.db, ['followers'], sender.id.href);
+        await addToList(ctx.data.db, ['followers', 'expanded'], senderJson);
+    }
+
+    // Store or update sender in global db
+    ctx.data.globaldb.set([sender.id.href], senderJson);
+
+    // Add accept activity to outbox
     const acceptId = ctx.getObjectUri(Accept, { id: uuidv4() });
     const accept = new Accept({
         id: acceptId,
@@ -80,6 +94,8 @@ export async function handleFollow(
 
     await ctx.data.globaldb.set([accept.id!.href], acceptJson);
     await addToList(ctx.data.db, ['outbox'], accept.id!.href);
+
+    // Send accept activity to sender
     await ctx.sendActivity({ handle: parsed.handle }, sender, accept);
 }