`@render.download` not namespacing properly, used to work in deprecated `@session.download`

[nsiicm0]

Hi all,
I faced an issue in one of our applications, where when using @render.download the generated URLs are not properly namespaced (actually the download URL appears to be living in "root" added at all).
This is usually fine, however, in our application, I am trying to modularize the download capabilities and embed it on multiple subpages (up to 10 or more times), hence, it will become an issue, since every embedding will produce a different download.

I am using shiny version 1.1.0.

Here is an example app illustrating my issue:

app.py

import shiny
from shiny import App, Inputs, Outputs, Session, reactive, render, ui, module
from starlette.applications import Starlette
from starlette.routing import Route, Mount

from download import download_ui, renderer_download_server, session_download_server

version = shiny.__version__

app_ui = ui.page_navbar(  
    ui.nav_panel("Home", ui.TagList([
        download_ui('download_renderer')
        , ui.tags.br()
        , download_ui('download_session')
    ])), 
    title=f"Download Bug App - Shiny Version {version}",  
    id="page",  
)  

def server(input, output, session):
    renderer_download_server('download_renderer')
    session_download_server('download_session')

shiny_app = App(app_ui, server, debug=True)
shiny_app.sanitize_errors = False

routes = [
    Mount('/', app=shiny_app)
]

app = Starlette(routes=routes, debug=True)

download.py

from shiny import module, ui, render, event, reactive

@module.ui
def download_ui():
    return ui.download_button('dwnld', "Download")

@module.server
def renderer_download_server(input, output, session):
    @render.download(filename=lambda: f"test.txt")
    def dwnld():
        ui.notification_show(f'Download using renderer started | {session.ns=}')
        
@module.server
def session_download_server(input, output, session):
    @session.download(filename=lambda: f"test.txt")
    def dwnld():
        ui.notification_show(f'Download using session started | {session.ns=}')

This app will produce two buttons. When hovering over the first button (that uses the @render.download functionality) then the link will look like session/<id>/download/dwnld?w=, however, the second button (@session.download) will produce a namespaced subpath that looks like session/<id>/download/download_session-dwnld?w= (see screenshot below of the DOM).

This namespaced variation is important when implementing multiple download buttons that are being generated with a module. Otherwise a click on any of the generated module download buttons will trigger the "last" registered download handler function.
To illustrate this, see this modified example:

## app.py
app_ui = ui.page_navbar(  
    ui.nav_panel("Home", ui.TagList([
        download_ui('download_renderer')
        , ui.tags.br()
        , download_ui('download_session')
        , ui.tags.br()
        , download_ui('download_renderer2')
    ])), 
    title=f"Download Bug App - Shiny Version {version}",  
    id="page",  
)  
def server(input, output, session):
    renderer_download_server('download_renderer', value='Foo')
    session_download_server('download_session', value='Bar')
    renderer_download_server('download_renderer2', value='Baz')

## download.py
@module.server
def renderer_download_server(input, output, session, value):
    @render.download(filename=lambda: f"test.txt")
    def dwnld():
        ui.notification_show(f'Download using renderer started {value=} | {session.ns=}')
        
@module.server
def session_download_server(input, output, session, value):
    @session.download(filename=lambda: f"test.txt")
    def dwnld():
        ui.notification_show(f'Download using session started {value=} | {session.ns=}')

This should output (notification) "Foo" on the first button, "Bar" on the second button, and "Baz" on the third button. However, when clicking on the first button I get:

Additionaly, as you can see, the session namespace is a entirely different one (the namespace of the third button). This flaw can be quite dangerous, as it messes up everything.

On top of this, the @render.download does not allow me to specify an "id" as it was the case with @session.download, which can be used to override the automatic linking of a download handler function to a download button.

As a workaround I managed to use @session.download in combination with the id= parameter with a pinch of ResolvedId magic, however, this inconsistency and also the deprecation warning in @session.download gives me quite some headaches for the future.
See the following simplified snippet of what I mean by this previous sentence:

from shiny._namespaces import ResolvedId
parent = 'test' # provided by calling module

@module.ui
def patched_download_ui():
    return ui.download_button(ResolvedId(f'{parent}-dwnld'), "Patched Download")

@module.server
def patched_download_server(input, output, session):
    @session.download(id=ResolvedId(f'{parent}-dwnld'), filename=lambda: f"test.txt")
    def dwnld():
        ...

With this ResolvedId method, I am fully in control which methods I want to tie to which button.

How to resolve this?

I think it would be best to...

• ... make sure that @render.download properly namespaces the button/subpaths similar to @session.download
• ... introduce the id= parameter in @render.download

What do you guys think?