[BUG] Regression when overriding networks

[c-w]

Description

In v2.24.0 there is a regression when using an override file to reset a network to its default state. This used to work all the way up to v2.23.3.

My use-case for this scenario is to use a specific docker network name when running in production (using the docker-compose.yml file) while using the auto-generated network name when running in development (using the docker-compose.override.yml file).

Steps To Reproduce

1. Create docker-compose.yml

services:
  test:
    image: hello-world
    networks:
      test:

networks:
  test:
    name: test
    external: true

2. Create docker-compose.override.yml

networks:
  test:

3. Run docker compose up and observe exit code 15 with error message "cannot override networks.test"

Compose Version

First broken version: v2.24.0
Last good version: v2.23.3

Docker Environment

No response

Anything else?

No response

[ndeloof]

using an override file to reset a network to its default state

this was not expected to "reset" network, but to merge definitions which would be a no-op - the only way to get something removed from compose model is to tag with !reset (https://github.com/compose-spec/compose-spec/blob/master/13-merge.md#reset-value) - so you were relying on a bug.

There's a bug here anyway, let me investigate to fix this, and also check how we could better support your use-case

[c-w]

Thanks for the pointer to !reset. I experimented with it.

As per the docs, I'd assume that the following snippet in docker-compose.override.yml should work, but it throws an error on both v2.24.3 ("service "test" refers to undefined network test: invalid compose project") and v2.23.3 ("networks.test must be a mapping or null"):

networks:
  test: !reset null

The following snippet works with v2.24.3 but with v2.23.3 I get "can't override attribute Name".

networks:
  test:
    name: !reset null
    external: !reset false

[luciangabor]

Just in case the old behavior was a bug, this might help (it achieves the same result with both versions):

networks:                              
  test:
    name: ${COMPOSE_PROJECT_NAME:?}_test
    external: false

(!reset deletes the item, it does not assign a value)

[ndeloof]

I also think the base approach would be for you to use external: false in your override file. Network name can also be set to default value set by project name, but until you run same stack concurrently with distinct project name this probably doesn't matter you inherit the network name.
In the meantime I've proposed compose-spec/compose-go#552 to fix the "cannot override networks.test" error with such an example, as well as more flexibility to replace (not just remove) a value in override file using the ! reset hack

[ndeloof]

See compose-spec/compose-go#552 as a proposed approach to such a use-case