FIX: User merge should not fail when primary email address is missing

The merge process might move all email addresses of the source user to the target user. Destroying the source user failed in that case.

app/services/user_merger.rb

@@ -347,7 +347,11 @@ class UserMerger
 
   def delete_source_user
     @source_user.reload
-    @source_user.update_attribute(:admin, false)
+    @source_user.update_attributes(
+      admin: false,
+      email: "#{@source_user.username}_#{SecureRandom.hex}@no-email.invalid"
+    )
+
     UserDestroyer.new(Discourse.system_user).destroy(@source_user)
   end
 

spec/services/user_merger_spec.rb

@@ -953,6 +953,15 @@ describe UserMerger do
     expect(User.find_by_username(source_user.username)).to be_nil
   end
 
+  it "deletes the source user even when it is a member of a group that grants a trust level" do
+    group = Fabricate(:group, grant_trust_level: 3)
+    group.bulk_add([source_user.id, target_user.id])
+
+    merge_users!
+
+    expect(User.find_by_username(source_user.username)).to be_nil
+  end
+
   it "deletes external auth infos of source user" do
     FacebookUserInfo.create(user_id: source_user.id, facebook_user_id: "example")
     GithubUserInfo.create(user_id: source_user.id, screen_name: "example", github_user_id: "examplel123123")