allow override to REMOVE elements by using x-null

[ndeloof]

Add support for special extension x-null to be used in override files. When set, during merge, source value is reset to empty/null.

note: we can't just use value: null as I expected because this is used in a few places for a distinct purpose: environment variables to inherit local environment, or build.ssh key to be propagated by ssh-agent

This is an attempt to fix compose-spec/compose-spec#284

[milas]

note: we can't just use value: null as I expected because this is used in a few places for a distinct purpose: environment variables to inherit local environment, or build.ssh key to be propagated by ssh-agent

That is a good realization. The current behavior on null for those is pretty unfortunate (hindsight is 20/20 but I wish we'd used a special string value).

---

I'll be honest, I'm not crazy about the x-null: syntax.

Brainstorming ⬇️

Base compose.yaml

services:
  web:
    build:
      context: .
      ssh:
        - default
        - mykey=~/.sshmykey.pem
    ports:
      - 8080:8000
    environment:
      FOO: 'bar'

Goals:

• Unset FOO from the env
• Remove default SSH agent

Examples below are hypothetical overrides.

(A)

services:
  web:
    build:
      ssh:
       "unset:default":
    environment:
      "unset:FOO":

Q: Should it be x-unset:FOO for the key name?
Q: Do all k-v lists in Compose support map syntax?

(B)

services:
  web:
    build:
      ssh:
        _unset_: [ default ]
    environment:
      _unset_: [ FOO ]

Q: Maybe _remove_ is a better name? That applies cleaner to lists + maps.

(C)

services:
  web:
    build:
      ssh:
        default: '_compose_:unset'
    environment:
      FOO: '_compose_:unset'

Q: Do all k-v lists in Compose support map syntax?

Similar to x-null virtual object, but look at any values that are strings to see if they start with a _compose_: meta directive 🤷🏻

[milas]

For reference, I think I'm leaning towards (B) and dunno if I like either (A) or (C)

[ndeloof]

Without a major change to the json-schema, we have to stick with x-* syntax. But obviously there's no requirement to use x-nul, could be x-reset-value or x-unset

the two examples you described (remove element from list or environment map) is not covered by this PR. We could indeed support a custom value, or more easily use a custom yaml tag:

services:
  web:
    build:
      ssh:
        !unset  default: 
    environment:
        !unset FOO: 

[milas]

Oh! That's way better. I was looking at the YAML spec but did not realize you could use custom tags 😅

[milas]

the two examples you described (remove element from list or environment map) is not covered by this PR

Oh, I misunderstood then 😅 I was thinking (using my same example as before) that you'd be able to do:

services:
  web:
    build:
      context: .
      ssh:
        default:
          x-null:
    environment:
      FOO:
        x-null:

[ndeloof]

Doing so would fail during JSON-Schema validation :'(

services.web.environment.FOO must be a string, number, boolean or null

[ndeloof]

Created #380 to explore the yaml tag approach