Clean up tech debt in YAML unmarshaling

[drevell]

TL;DR

Our current YAML parsing works, but there might be a less-boilerplatey way to do YAML unmarshaling while:

• enforcing required fields
• enforcing that no extra unknown fields were present
• doing custom validation on field values

The upstream YAML parser has an interesting bug go-yaml/yaml#460 where it will enforce that no extra fields are present, but that feature doesn't work if you call node.Decode(). That may limit our options for doing this cleanly.

Example of problematic code: in spec.go, there is some repeated almost-copy-pasted boilerplate code that looks like this:

// UnmarshalYAML implements yaml.Unmarshaler.
func (s *Spec) UnmarshalYAML(n *yaml.Node) error {
	knownYAMLFields := []string{"apiVersion", "kind", "desc", "inputs", "steps"}
	if err := extraFields(n, knownYAMLFields); err != nil {
		return err
	}

	type shadowType Spec
	shadow := &shadowType{} // see "Q2" in file comment above
	if err := n.Decode(shadow); err != nil {
		return err
	}

	*s = Spec(*shadow)
	s.Pos = yamlPos(n)
	return nil
}

Copy-pasted code like this is almost always harmful.

Detailed design

No response

Alternatives considered

No response

Additional information

No response