Flarum is missing authorization in discussion replies

@@ -79,7 +79,7 @@ public function handle(StartDiscussion $command)
         // We will do this by running the PostReply command.
         try {
             $post = $this->bus->dispatch(
-                new PostReply($discussion->id, $actor, $data, $ipAddress)
+                new PostReply($discussion->id, $actor, $data, $ipAddress, true)
             );
         } catch (Exception $e) {
             $discussion->delete();

@@ -74,7 +74,7 @@ public function handle(PostReply $command)
 
         // If this is the first post in the discussion, it's technically not a
         // "reply", so we won't check for that permission.
-        if ($discussion->first_post_id !== null) {
+        if (! $command->isFirstPost) {
             $actor->assertCan('reply', $discussion);
         }
 

@@ -41,17 +41,23 @@ class PostReply
      */
     public $ipAddress;
 
+    /**
+     * @var bool
+     */
+    public $isFirstPost;
+
     /**
      * @param int $discussionId The ID of the discussion to post the reply to.
      * @param User $actor The user who is performing the action.
      * @param array $data The attributes to assign to the new post.
      * @param string $ipAddress The IP address of the actor.
      */
-    public function __construct($discussionId, User $actor, array $data, $ipAddress = null)
+    public function __construct($discussionId, User $actor, array $data, $ipAddress = null, bool $isFirstPost = false)
     {
         $this->discussionId = $discussionId;
         $this->actor = $actor;
         $this->data = $data;
         $this->ipAddress = $ipAddress;
+        $this->isFirstPost = $isFirstPost;
     }
 }

@@ -10,6 +10,7 @@
 namespace Flarum\Tests\integration\api\posts;
 
 use Carbon\Carbon;
+use Flarum\Group\Group;
 use Flarum\Testing\integration\RetrievesAuthorizedUsers;
 use Flarum\Testing\integration\TestCase;
 
@@ -26,36 +27,70 @@ protected function setUp(): void
 
         $this->prepareDatabase([
             'discussions' => [
-                ['id' => 1, 'title' => __CLASS__, 'created_at' => Carbon::now()->toDateTimeString(), 'user_id' => 2],
+                ['id' => 1, 'title' => __CLASS__, 'created_at' => Carbon::now()->toDateTimeString(), 'user_id' => 2, 'first_post_id' => 1],
+                // Discussion with deleted first post.
+                ['id' => 2, 'title' => __CLASS__, 'created_at' => Carbon::now()->toDateTimeString(), 'user_id' => 2, 'first_post_id' => null],
+            ],
+            'posts' => [
+                ['id' => 1, 'discussion_id' => 1, 'number' => 1, 'created_at' => Carbon::now()->subDay()->toDateTimeString(), 'user_id' => 2, 'type' => 'comment', 'content' => '<t></t>'],
             ],
             'users' => [
                 $this->normalUser(),
-            ]
+                ['id' => 3, 'username' => 'restricted', 'email' => 'restricted@machine.local', 'is_email_confirmed' => 1],
+            ],
+            'groups' => [
+                ['id' => 40, 'name_singular' => 'tess', 'name_plural' => 'tess'],
+            ],
+            'group_user' => [
+                ['group_id' => 40, 'user_id' => 3],
+            ],
+            'group_permission' => [
+                ['group_id' => 40, 'permission' => 'discussion.reply'],
+            ],
         ]);
     }
 
     /**
+     * @dataProvider discussionRepliesPrvider
      * @test
      */
-    public function can_create_reply()
+    public function can_create_reply_if_allowed(int $actorId, int $discussionId, int $responseStatus)
     {
+        // Reset permissions for normal users group.
+        $this->database()
+            ->table('group_permission')
+            ->where('permission', 'discussion.reply')
+            ->where('group_id', Group::MEMBER_ID)
+            ->delete();
+
         $response = $this->send(
             $this->request('POST', '/api/posts', [
-                'authenticatedAs' => 2,
+                'authenticatedAs' => $actorId,
                 'json' => [
                     'data' => [
                         'attributes' => [
                             'content' => 'reply with predetermined content for automated testing - too-obscure',
                         ],
                         'relationships' => [
-                            'discussion' => ['data' => ['id' => 1]],
+                            'discussion' => ['data' => ['id' => $discussionId]],
                         ],
                     ],
                 ],
             ])
         );
 
-        $this->assertEquals(201, $response->getStatusCode());
+        $this->assertEquals($responseStatus, $response->getStatusCode());
+    }
+
+    public function discussionRepliesPrvider(): array
+    {
+        return [
+            // [$actorId, $discussionId, $responseStatus]
+            'can_create_reply_with_ability' => [3, 1, 201],
+            'cannot_create_reply_without_ability' => [2, 1, 403],
+            'can_create_reply_with_ability_when_first_post_is_deleted' => [3, 2, 201],
+            'cannot_create_reply_without_ability_when_first_post_is_deleted' => [2, 2, 403],
+        ];
     }
 
     /**