SECURITY: Handle concurrent invite accepts

Raise an error on concurrent invite accept attempts.

app/controllers/invites_controller.rb

@@ -278,7 +278,10 @@ class InvitesController < ApplicationController
         end
 
         user = invite.redeem(**attrs)
-      rescue ActiveRecord::RecordInvalid, ActiveRecord::RecordNotSaved, Invite::UserExists => e
+      rescue ActiveRecord::RecordInvalid,
+             ActiveRecord::RecordNotSaved,
+             ActiveRecord::LockWaitTimeout,
+             Invite::UserExists => e
         return render json: failed_json.merge(message: e.message), status: 412
       end
 

app/models/invite_redeemer.rb

@@ -243,7 +243,10 @@ class InviteRedeemer
     @invited_user_record = InvitedUser.create!(invite_id: invite.id, redeemed_at: Time.zone.now)
 
     if @invited_user_record.present?
-      Invite.increment_counter(:redemption_count, invite.id)
+      invite.with_lock("FOR UPDATE NOWAIT") do
+        Invite.increment_counter(:redemption_count, invite.id)
+        invite.save!
+      end
       delete_duplicate_invites
     end
 

spec/requests/invites_controller_spec.rb

@@ -984,6 +984,27 @@ RSpec.describe InvitesController do
         Fabricate(:invite, email: nil, emailed_status: Invite.emailed_status_types[:not_required])
       end
 
+      it "does not create multiple users for a single use invite" do
+        user_count = User.count
+
+        2
+          .times
+          .map do
+            Thread.new do
+              put "/invites/show/#{invite.invite_key}.json",
+                  params: {
+                    email: "test@example.com",
+                    password: "verystrongpassword",
+                  }
+            end
+          end
+          .each(&:join)
+
+        expect(invite.reload.max_redemptions_allowed).to eq(1)
+        expect(invite.reload.redemption_count).to eq(1)
+        expect(User.count).to eq(user_count + 1)
+      end
+
       it "sends an activation email and does not activate the user" do
         expect {
           put "/invites/show/#{invite.invite_key}.json",