feat: Add !merge YAML tag support

[glb]

This extends the YAML schema to support a !merge tag as described in serverless/serverless#8037:

Example: merge objects

provider:
  environment: !merge
    - FOO: value1
    - BAR: value2

results in:

provider:
  environment:
    FOO: value1
    BAR: value2

which may seem less-than-thrilling but if you consider the case of

provider:
  environment: !merge
    - ${file(stage-env.json)}
    - ${file(common-env.json)}

or:

resources:
  Resources: !merge
    - ${file(resources1.yaml)}
    - ${file(resources2.yaml)}

it becomes more interesting.

🚨 This will throw an error if there are key collisions in the list of objects given to !merge; it will not simply overwrite the keys like what happens with Object.assign({}, ...).

Example: "merge" arrays

If !merge is given a list of arrays, it will do a shallow flattening of the provided arrays and return a list that is the equivalent of concatenating all of the lists together. There is no detection or removal of duplicate elements.

resources:
  Resources:
    example:
      Type: AWS::S3::Bucket
      DependsOn: !merge
        - ["a", "b"]
        - ["c", "d"]

becomes

resources:
  Resources:
    example:
      Type: AWS::S3::Bucket
      DependsOn:
        - "a"
        - "b"
        - "c"
        - "d"

which again is more interesting if you consider the opportunities for merging more complex content.

[codecov]

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The diff coverage is 100.00%.

@@            Coverage Diff            @@
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Impacted Files	Coverage Δ
cloudformation-schema.js	100.00% <100.00%> (ø)

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[medikoo]

@glb great thanks for this! Still can you follow the specification and propose it in https://github.com/serverless/serverless/ repository?

If you have some concerns towards specification or something is not clear, please ask at serverless/serverless#8037

[glb]

@glb great thanks for this! Still can you follow the specification and propose it in https://github.com/serverless/serverless/ repository?

If you have some concerns towards specification or something is not clear, please ask at serverless/serverless#8037

@medikoo could you please help me understand where I've diverged from the specification in serverless/serverless#8037 in this PR? This is the change for part 2.

I'm actually a little unsure whether this will work as desired; I am trying to do an integration test of @serverless/serverless and this @serverless/utils branch with a serverless.yml file that has

provider:
  environment: !merge
    - ${file(stage-env.json)}
    - ${file(common-env.json)}

but running into what I think is an unrelated problem.

I'm not at all sure whether the !merge tag processing will work as desired, as I think it happens during YAML parsing and before variable resolution...

[medikoo]

@medikoo could you please help me understand where I've diverged from the specification in serverless/serverless#8037 in this PR? This is the change for part 2.

Specification mentions:

Let's put definition of new YAML types to lib/configuration/yaml-merge.js

And that's in context of serverless/serverless repository.

We do not expect any PR to be made in this repository