bake: check condition and error_message are set during validation

[crazy-max]

fixes #3100

We forgot to check if condition and error_message are set and also check if result value has the expected type.

cc @rcjsuen

[rcjsuen]

@crazy-max Could you try to add this test? What should be the expected case if condition = false? It seems like if the user puts that in they will get This check failed, but has an invalid error message?

func TestHello(t *testing.T) {
	fp := File{
		Name: "docker-bake.hcl",
		Data: []byte(`
variable "PORT" {
  default = 3000
  validation {
    condition = false
  }
}
`),
	}

	_, _, err := ReadTargets(context.TODO(), []File{fp}, []string{"app"}, nil, nil, &EntitlementConf{})
	require.Error(t, err)
	fmt.Println(">>>>>>")
	fmt.Println(err)
	fmt.Println(">>>>>>")
	require.Contains(t, err.Error(), "Condition expression must return either true or false, not null")
}

[crazy-max]

It seems like if the user puts that in they will get This check failed, but has an invalid error message?

Yes because the error message is required similar to how terraform does when evaluating conditions: https://github.com/hashicorp/terraform/blob/ea767aa7add3e14b7d292efead3ef23d25cd49e1/internal/terraform/eval_conditions.go#L190-L193

[rcjsuen]

It seems like if the user puts that in they will get This check failed, but has an invalid error message?

Yes because the error message is required similar to how terraform does when evaluating conditions: https://github.com/hashicorp/terraform/blob/ea767aa7add3e14b7d292efead3ef23d25cd49e1/internal/terraform/eval_conditions.go#L190-L193

Gotcha, another Terraform thing, eh? Thanks for clarifying. 👍