Assigns bug in forwarded routes

A single-file Phoenix application which demonstrates a bug when using the Phoenix.router.forward/4 macro with a LiveView.

Summary

When forwarding a route to another router, values assigned to a socket in a live_session on_mount handler are discarded when the socket is connected. The values are present when the LiveView is first mounted.

Detailed explanation

The repository contains two routers. The first router (Example.Router) simply forwards all requests for the /forwarded route to the second router (Example.ForwardedRouter).

Example.ForwardedRouter defines a single live route, inside a live_session block. The live_session block sets the on_mount option to {Example.MountHandler, :default}.

The Example.MountHandler.on_mount/4 function sets the value of socket.assigns.test_key to "Test value".

The Example.HomeLive.mount/3 function sets the value of socket.assigns.test_key to "Fallback value" if it is not already assigned (using assign_new).

When loading the /forwarded route in the browser, you will briefly see the text "Test value". This is replaced by "Fallback value" once the socket is connected.

It is important to note that the bug described above only manifests under the following specific circumstances:

1. The route is forwarded from the main router to a second router. If you use a single router, everything works.
2. The forwarded route is not /. If you forward the / route, everything works.
3. The second router defines the on_mount handler as a live_session option. If the LiveView itself defines the on_mount handler, everything works.