FIX: Recovered posts with no user will be taken over by system user (#8834)

lib/guardian/post_guardian.rb

@@ -187,7 +187,7 @@ module PostGuardian
   # Recovery Method
   def can_recover_post?(post)
     if is_staff?
-      post.deleted_at && post.user
+      !!post.deleted_at
     else
       is_my_own?(post) && post.user_deleted && !post.deleted_at
     end

lib/guardian/topic_guardian.rb

@@ -113,7 +113,7 @@ module TopicGuardian
   # Recovery Method
   def can_recover_topic?(topic)
     if is_staff?
-      !!(topic && topic.deleted_at && topic.user)
+      !!(topic && topic.deleted_at)
     else
       topic && can_recover_post?(topic.ordered_posts.first)
     end

lib/post_destroyer.rb

@@ -91,6 +91,7 @@ class PostDestroyer
       user_recovered
     end
     topic = Topic.with_deleted.find @post.topic_id
+    topic.update_column(:user_id, Discourse::SYSTEM_USER_ID) if !topic.user_id
     topic.recover!(@user) if @post.is_first_post?
     topic.update_statistics
     UserActionManager.post_created(@post)
@@ -103,6 +104,7 @@ class PostDestroyer
   end
 
   def staff_recovered
+    @post.update_column(:user_id, Discourse::SYSTEM_USER_ID) if !@post.user_id
     @post.recover!
 
     mark_topic_changed

spec/components/guardian_spec.rb

@@ -1159,7 +1159,7 @@ describe Guardian do
           PostDestroyer.new(moderator, topic.first_post).destroy
           topic.first_post.user.destroy!
 
-          expect(Guardian.new(moderator).can_recover_topic?(topic.reload)).to be_falsey
+          expect(Guardian.new(moderator).can_recover_topic?(topic.reload)).to be_truthy
         end
       end
     end
@@ -1199,7 +1199,7 @@ describe Guardian do
             PostDestroyer.new(moderator, post).destroy
             post.user.destroy!
 
-            expect(Guardian.new(moderator).can_recover_post?(post.reload)).to be_falsey
+            expect(Guardian.new(moderator).can_recover_post?(post.reload)).to be_truthy
           end
         end
       end

spec/components/post_destroyer_spec.rb

@@ -180,6 +180,15 @@ describe PostDestroyer do
       expect(post_action).to be_present
     end
 
+    it "works with topics and posts with no user" do
+      post = Fabricate(:post)
+      UserDestroyer.new(Discourse.system_user).destroy(post.user, delete_posts: true)
+
+      expect { PostDestroyer.new(Fabricate(:admin), post.reload).recover }
+        .to change { post.reload.user_id }.to(Discourse.system_user.id)
+        .and change { post.topic.user_id }.to(Discourse.system_user.id)
+    end
+
     describe "post_count recovery" do
       before do
         post