[BUG] Background callbacks not working with pages

[brianleslie]

Describe your context
Dash Callback Example 4: Progress Bar from https://dash.plotly.com/background-callbacks

dash                      2.6.1                                                                                         
dash-bootstrap-components 1.2.1                                                                                        
dash-core-components      2.0.0                                                                                         
dash-daq                  0.5.0                                                                                         
dash-html-components      2.0.0                                                                                        
 dash-table                5.0.0

- OS: Windows
- Browser Chrome
- Version 104.0.5112.81

Describe the bug

using background callbacks does not work with pages.

not using pages the code runs correctly

app.py

import os
import dash
from dash import DiskcacheManager, CeleryManager, html, page_container

# code to test without pages
# import pages.page as page

if 'REDIS_URL' in os.environ:
    # Use Redis & Celery if REDIS_URL set as an env variable
    from celery import Celery
    celery_app = Celery(__name__, broker=os.environ['REDIS_URL'], backend=os.environ['REDIS_URL'])
    background_callback_manager = CeleryManager(celery_app)

else:
    # Diskcache for non-production apps when developing locally
    import diskcache
    cache = diskcache.Cache("./cache")
    background_callback_manager = DiskcacheManager(cache)

app = dash.Dash(__name__,
                use_pages=True,
                # code to test without pages
                # use_pages=False,
                background_callback_manager=background_callback_manager)

app.layout = html.Div(
    page_container

    # code to test without pages
    # page.layout()
)

if __name__ == "__main__":
    app.run_server(debug=True)

pages/page.py

import dash
from dash import Input, Output, html
import time

dash.register_page(__name__, path='/page')


def layout():
    return html.Div([
        html.Div(
            [
                html.P(id="paragraph_id", children=["Button not clicked"]),
                html.Progress(id="progress_bar", value="0"),
            ]
        ),
        html.Button(id="button_id", children="Run Job!"),
        html.Button(id="cancel_button_id", children="Cancel Running Job!"),
    ])


@dash.callback(
    output=Output("paragraph_id", "children"),
    inputs=Input("button_id", "n_clicks"),
    background=True,
    running=[
        (Output("button_id", "disabled"), True, False),
        (Output("cancel_button_id", "disabled"), False, True),
        (
                Output("paragraph_id", "style"),
                {"visibility": "hidden"},
                {"visibility": "visible"},
        ),
        (
                Output("progress_bar", "style"),
                {"visibility": "visible"},
                {"visibility": "hidden"},
        ),
    ],
    cancel=Input("cancel_button_id", "n_clicks"),
    progress=[Output("progress_bar", "value"), Output("progress_bar", "max")],
    prevent_initial_call=True
)
def update_progress(set_progress, n_clicks):
    total = 5
    for i in range(total + 1):
        set_progress((str(i), str(total)))
        time.sleep(1)

    return f"Clicked {n_clicks} times"

Expected behavior

Ability to use background callbacks AND pages

Screenshots

[T4rk1n]

Seems like the pickling calls register_page when unpickling the function definition on the workers, adding a context var to check if inside a background callback could fix this.

[roykoand]

@T4rk1n You've done a great work on solving this issue! It helped a lot :)
Do you have any documentation on ignoring a register page?
Is it correct that as of now the only solution is dash.testing.ignore_register_page context manager?