Look the endpoint up past the query cache, and keep a subscription a race refused

The lookup under the write lock repeated the one made before
validation, so the query cache could answer it with that earlier miss
even after another connection had registered the endpoint. It runs
uncached now.

Enabling notifications unsubscribed the browser on any refusal, so a
session signed out between authenticating and saving lost the browser's
subscription too, and the next session had nothing to post. Only a
refused endpoint unsubscribes it now.

From #371.

(cherry picked from commit aa93269ceb)

test/controllers/users/push_subscriptions_controller_test.rb

@@ -139,3 +139,33 @@ class Users::PushSubscriptionsControllerTest < ActionDispatch::IntegrationTest
       { "endpoint" => "https://fcm.googleapis.com/fcm/send/abc123", "p256dh_key" => "123", "auth_key" => "456" }
     end
 end
+
+class Users::PushSubscriptionsControllerForeignWriteTest < ActionDispatch::IntegrationTest
+  self.use_transactional_tests = false
+
+  setup do
+    sign_in :david
+    stub_web_push_dns_resolution
+  end
+
+  test "a subscription another connection registers while this one validates isn't registered twice" do
+    params = { "endpoint" => "https://fcm.googleapis.com/fcm/send/foreign", "p256dh_key" => "123", "auth_key" => "456" }
+    Push::Subscription.any_instance.stubs(:valid?).with { foreign_insert(params); true }.returns(true)
+
+    post user_push_subscriptions_url, params: { push_subscription: params }
+
+    assert_response :ok
+    assert_equal 1, Push::Subscription.where(endpoint: params["endpoint"]).count
+  end
+
+  private
+    def foreign_insert(params)
+      unless @inserted
+        SQLite3::Database.new(ActiveRecord::Base.connection_db_config.database) do |database|
+          database.execute "INSERT INTO push_subscriptions (user_id, endpoint, p256dh_key, auth_key, created_at, updated_at) " \
+            "VALUES (?, ?, ?, ?, datetime('now'), datetime('now'))", [ users(:david).id, *params.values ]
+        end
+        @inserted = true
+      end
+    end
+end