Extending an array config only overrides specific elements of the array

[danepowell]

I'm opening this in reference to a downstream issue with BLT (which uses consolidation/config): acquia/blt#3697

BLT works by providing a default set of configuration, and then calling ConfigProcessor::extend() to override any values with user-provided variants (via blt.yml and other sources).

As you can see from that issue, the problem this user is facing is that when they want to override an array, they have to override every element of that array individually. I think it would make more sense that if you override any part of an array, it resets the entire array.

For instance, if the array is this:

sync:
  commands:
    - op1
    - op2

We extend it as:

sync:
  commands:
    - opfoo

We expect the final overridden config to be:

sync:
  commands:
    - opfoo

But we actually get:

sync:
 commands:
   - opfoo
   - op2

Is this expected behavior? Is there some other workaround that I'm missing?

[greg-1-anderson]

Unordered lists are a weak spot in configuration override. Configuration is combined using @grasmash 's ArrayUtil::mergeRecursiveDistinct. I think that whether you want to replace or merge array elements might depend on the context.

If we did change the behavior here, it would have to either be via an option (a method call a client could make), or be part of a new major release, as it would be a backwards-incompatible change.