Add "structured data" resource

[sed-i]

Describe the feature:
Server validation often involves structured data, such as json/yaml/toml. Being able to assert against objects inside some structured data could obviate the need for bespoke goss resources.

Describe the solution you'd like
A general purpose resource for poking into structured data and making exact/contains assertions. For example:

structured-data:
  format: json  # another option: yaml
  source:
    command: "juju status --format=json"
    # mutually exclusive option: url
  mode: contains  # another option: exact-match
  assertions:
    ".applications | keys":  # this is a standard jq-style query
      - prom
      - alertmanager
    ".applications.prom.relations | keys":
      - alertmanager
      - prometheus-peers

Describe alternatives you've considered
While we can certainly use the command resource for those checks, on a larger scale it may result in checks that are not maintainable.

[aelsabbahy]

Can you provide an example json and checks you're making?

Goss has a gjson matcher, which may meet your needs.

You can use it against the stdout of a command.

[sed-i]

I was not aware of gjson, looks incredible @aelsabbahy.
A trimmed down json sample is below.

json sample

{
  "model": {
    "name": "welcome-k8s"
  },
  "applications": {
    "am": {
      "charm": "alertmanager-k8s",
      "base": {
        "name": "ubuntu",
        "channel": "20.04"
      },
      "relations": {
        "alerting": [
          {
            "related-application": "prom",
            "interface": "alertmanager_dispatch",
            "scope": "global"
          }
        ],
        "replicas": [
          {
            "related-application": "am",
            "interface": "alertmanager_replica",
            "scope": "global"
          }
        ]
      },
      "version": "0.27.0"
    },
    "prom": {
      "charm": "prometheus-k8s",
      "base": {
        "name": "ubuntu",
        "channel": "20.04"
      },
      "relations": {
        "alertmanager": [
          {
            "related-application": "am",
            "interface": "alertmanager_dispatch",
            "scope": "global"
          }
        ],
        "prometheus-peers": [
          {
            "related-application": "prom",
            "interface": "prometheus_peers",
            "scope": "global"
          }
        ]
      },
      "version": "2.52.0"
      }
    }
  }
}

I tried to run the following:

command:
  app-names:
    exec: juju status --format=json
    exit-status: 0
    stdout:
      gjson:
        applications:
          and:
            - {have-key: "prom"}
            - {have-key: "am"}
        application.prom.relations:
          and:
            - {have-key: "alertmanager"}

But got a nil pointer deref.

goss -g check2.yaml validate
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x84e53b]

goroutine 7 [running]:
github.com/goss-org/goss/matchers.(*AndMatcher).FailureResult(0x4119db?, {0x0?, 0x0?})
	/home/travis/gopath/src/github.com/goss-org/goss/matchers/and.go:32 +0x5b
github.com/goss-org/goss/matchers.(*WithSafeTransformMatcher).FailureResult(0xb567f8?, {0xa676c0, 0xc000382110})
	/home/travis/gopath/src/github.com/goss-org/goss/matchers/with_safe_transform.go:45 +0x1bc
github.com/goss-org/goss/matchers.(*AndMatcher).FailureResult(0x0?, {0xa676c0?, 0xc000382110?})
	/home/travis/gopath/src/github.com/goss-org/goss/matchers/and.go:32 +0x62
github.com/goss-org/goss/resource.ValidateGomegaValue({_, _}, {_, _}, {_, _}, {_, _}, _)
	/home/travis/gopath/src/github.com/goss-org/goss/resource/validate.go:199 +0x788
github.com/goss-org/goss/resource.ValidateValue({_, _}, {_, _}, {_, _}, {_, _}, _)
	/home/travis/gopath/src/github.com/goss-org/goss/resource/validate.go:126 +0x174
github.com/goss-org/goss/resource.(*Command).Validate(0xc0000bc380, 0xc0000be0b0)
	/home/travis/gopath/src/github.com/goss-org/goss/resource/command.go:65 +0x3bd
github.com/goss-org/goss.validate.func2()
	/home/travis/gopath/src/github.com/goss-org/goss/validate.go:184 +0x86
created by github.com/goss-org/goss.validate in goroutine 1
	/home/travis/gopath/src/github.com/goss-org/goss/validate.go:181 +0x187

Likely I got the syntax wrong?

[sed-i]

Oops, a typo on my part

-application.prom.relations:
+applications.prom.relations:

This works fine:

command:
  app-names:
    exec: juju status --format=json
    exit-status: 0
    stdout:
      gjson:
        applications:
          and:
            - {have-key: "prom"}
            - {have-key: "am"}
        applications.prom.relations:
          and:
            - {have-key: "alertmanager"}

[sed-i]

Filed #982 with a minimal repro.
Closing this issue, as the requested feature already exists.