[BUG] config --no-interpolate ineffective in a corner case

[luciangabor]

Description

Please see the steps below

Steps To Reproduce

1. Create the compose.yaml

services:
  service:
    network_mode: none
    image: bash
    entrypoint:
      - bash
      - -vc
    command:
      - echo "FOO:$${FOO:-default}, BAR:$${BAR:-default}"
    environment:
      FOO: ${FOO:-}
      BAR:

2. Experiment with setting or not the two variables

$ docker compose config --no-interpolate
name: interpolate
services:
  service:
    command:
      - echo "FOO:$${FOO:-default}, BAR:$${BAR:-default}"
    entrypoint:
      - bash
      - -vc
    environment:
      BAR: null
      FOO: ${FOO:-}
    image: bash
    network_mode: none

$ env FOO=FOO BAR=BAR docker compose config --no-interpolate
name: interpolate
services:
  service:
    command:
      - echo "FOO:$${FOO:-default}, BAR:$${BAR:-default}"
    entrypoint:
      - bash
      - -vc
    environment:
      BAR: BAR
      FOO: ${FOO:-}
    image: bash
    network_mode: none

Observe that BAR is "interpolated" despite the flag, while both ways to pass a homonym variables work more or less the same:

$ docker compose --progress none up --no-log-prefix 
Attaching to service-1
echo "FOO:${FOO:-default}, BAR:${BAR:-default}"
FOO:default, BAR:default
service-1 exited with code 0

$ env FOO=FOO BAR=BAR docker compose --progress none up --no-log-prefix 
Attaching to service-1
echo "FOO:${FOO:-default}, BAR:${BAR:-default}"
FOO:FOO, BAR:BAR
service-1 exited with code 0

Compose Version

Docker Compose version v2.24.6

Docker Environment

No response

Anything else?

No response

[1arp]

This is expected behaviour as per https://github.com/compose-spec/compose-go/blob/b32c8a1e23e7b84aea4d134066a3a74023e92675/cli/options.go#L450.\
I can go ahead and update if this is a bug @ndeloof, but I don't think it is.

The latest version on the main branch of the spec follows the same behaviour https://github.com/compose-spec/compose-go/blob/07e9470486344cc0a2071851f684af595d0807d0/cli/options.go#L460-L482.

[luciangabor]

Maybe here's the issue:

compose/cmd/compose/config.go

Line 143 in 5c4f337

content = escapeDollarSign(content)

And that breaks other cases too - like when a variable is used to point to an included/extended file ...

[ndeloof]

This is not interpolation. When you set BAR without a value, you ask compose to just propagate your environment to container, which he did. interpolation is about replacing variables in your compose.yaml file