用作Golang中Yaml数据目标的默认结构

Question

I try to improve the user experience of a cli I maintain. A main goal is to provide reasonable defaults. It uses yaml extensively for configuration.

A basic demo implementation of the configuration can be found here: https://github.com/unprofession-al/configuration/tree/bf5a89b3eee7338899b28c047f3795546ce3d2e6

General

The main configuration looks like this:

type Config map[string]ConfigSection

type ConfigSection struct {
    Input  InputConfig  `yaml:"input"`
    Output OutputConfig `yaml:"output"`
}

Config holds a bunch of ConfigSections. This allow the user to define variations of the configuration (lets say prod, dev and testing for example) and use YAML Achors to do so.

The parts of the ConfigSection (Input and Output) would be defined in the package that consumes the configuration. Each of this part provides a Defaults() and a custom UnmarshalYAML() func. Also ConfigSection itself provides an UnmarshalYAML() func. This idea is stolen from https://github.com/go-yaml/yaml/issues/165#issuecomment-255223956.

Question

In data.go in the repo some test input as well as the expected output is defined. Running the tests (go test -v) shows:

• Having nothing defined in the ConfigSection (empty example) no defaults are applied.
• If a part (of a ConfigSection) with no data field is defined, this part will have no defaults. The "undefined" part has defaults (see input, output).
• If both parts are defined (as is section both) but have no data fields, any defaults are set.

I see no pattern at all and ran out of ideas why this works like this and how to get the expected results (eg. get the test to pass).

Answer 1

Ok, so the pattern I did not see was quite obviouse: The "deepest leaf" of a configuration overwrites everything below, either with the given data or with the go defaults for empty values:

That means that a struct like this...

[key_string]:
  input:
    listener: [string]
    static: [string]
  output:
    listener: [string]
    details:
      filter: [string]
      retention: [string]

... defaulted with the data...

defaults:
  input:
    listener: 127.0.0.1:8910
    static: default
  output:
    listener: 127.0.0.1:8989
    details:
      filter: '*foo*'
      retention: 3h

... feeded with a yaml of this form..

empty:

both:
  input:
  output:

input: &input
  input:

input-modified-with-anchor:
  <<: *input
  input:
    static: NOTDEFAULT

input-modified-without-anchor:
  input:
    static: NOTDEFAULT

output: &output
  output:

output-modified-with-anchor:
  <<: *output
  output:
    details:
      filter: NOTDEFAULT

output-modified-without-anchor:
  output:
    details:
      filter: NOTDEFAULT

... turn out as...

both:
  input:
    listener: ""
    static: ""
  output:
    listener: ""
    details:
      filter: ""
      retention: ""
empty:
  input:
    listener: ""
    static: ""
  output:
    listener: ""
    details:
      filter: ""
      retention: ""
input:
  input:
    listener: ""
    static: ""
  output:
    listener: 127.0.0.1:8989
    details:
      filter: '*foo*'
      retention: 3h
input-modified-with-anchor:
  input:
    listener: 127.0.0.1:8910
    static: NOTDEFAULT
  output:
    listener: 127.0.0.1:8989
    details:
      filter: '*foo*'
      retention: 3h
input-modified-without-anchor:
  input:
    listener: 127.0.0.1:8910
    static: NOTDEFAULT
  output:
    listener: 127.0.0.1:8989
    details:
      filter: '*foo*'
      retention: 3h

For my use case this is a too complicated behavior, I therefore try a different approach: I if demanded I inject a default config into the yaml and put a reference to its anchor in each section. I feel like this is more transparent and reproducible to the end user. Heres an ugly draft of the func:

func injectYAML(data []byte) ([]byte, error) {
    // render a default section an add an anchor
    key := "injected_defaults"
    defaultData := Config{key: Defaults()}
    var defaultSection []byte
    defaultSection, _ = yaml.Marshal(defaultData)
    defaultSection = bytes.Replace(defaultSection, []byte(key+":"), []byte(key+": &"+key), 1)

    // get list of sections in input data
    c := Config{}
    err := yaml.Unmarshal(data, &c)
    if err != nil {
        return data, fmt.Errorf("Error while reading sections from yaml: %s", err.Error())
    }

    // remove "---" at beginning when present
    data = bytes.TrimLeft(data, "---")

    // add reference to default section to each section
    lines := bytes.Split(data, []byte("
"))
    var updatedLines [][]byte
    for _, line := range lines {
        updatedLines = append(updatedLines, line)
        for section := range c {
            if bytes.HasPrefix(line, []byte(section+":")) {
                updatedLines = append(updatedLines, []byte("  <<: *"+key))
            }
        }
    }
    updatedData := bytes.Join(updatedLines, []byte("
"))

    // compose injected yaml
    out := []byte("---
")
    out = append(out, defaultSection...)
    out = append(out, updatedData...)
    return out, nil
}

Full example at: https://github.com/unprofession-al/configuration/tree/7c2eb7da58b51f52b50f2a0fbac193c799c9eb08