Modify existing yaml file and add new data and comments

Question

I recently saw that the go yaml lib has new version (V3)

with the nodes capabilities (which in my opinion is a killer feature :) ) which can helps a lots with modifying

Answer 1

First, let Me Start off by saying using yaml.Node does not produce valid yaml when marshalled from a valid yaml, given by the following example. Probably should file an issue.

package main

import (
    "fmt"
    "log"

    "gopkg.in/yaml.v3"
)

var (
    sourceYaml = `version: 1
type: verbose
kind : bfr

# my list of applications
applications:

#  First app
  - name: app1
    kind: nodejs
    path: app1
    exec:
      platforms: k8s
      builder: test
`
)

func main() {
    t := yaml.Node{}

    err := yaml.Unmarshal([]byte(sourceYaml), &t)
    if err != nil {
        log.Fatalf("error: %v", err)
    }

    b, err := yaml.Marshal(&t)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(string(b))
}

Produces the following invalid yaml in go version go1.12.3 windows/amd64

version: 1
type: verbose
kind: bfr


# my list of applications
applications:
-   #  First app
name: app1
    kind: nodejs
    path: app1
    exec:
        platforms: k8s
        builder: test

---

Secondly, using a struct such as

type VTS struct {
    Version string       `yaml:"version" json:"version"`
    Types   string       `yaml:"type" json:"type"`
    Kind    string       `yaml:"kind,omitempty" json:"kind,omitempty"`
    Apps    yaml.Node `yaml:"applications,omitempty" json:"applications,omitempty"`
}

From ubuntu's blog and the source documentation it made it seem that it would correctly identify fields within the struct that are nodes and build that tree separately, but that is not the case. When unmarshalled, it will give a correct node tree, but when remarshalled it will produce the following yaml with all of the fields that yaml.Node exposes. Sadly we cannot go this route, must find another way.

version: "1"
type: verbose
kind: bfr
applications:
    kind: 2
    style: 0
    tag: '!!seq'
    value: ""
    anchor: ""
    alias: null
    content:
    -   #  First app
name: app1
        kind: nodejs
        path: app1
        exec:
            platforms: k8s
            builder: test
    headcomment: ""
    linecomment: ""
    footcomment: ""
    line: 9
    column: 3

---

Overlooking the first issue and the marshal bug for yaml.Nodes in a struct (on gopkg.in/yaml.v3 v3.0.0-20190409140830-cdc409dda467) we can now go about manipulating the Nodes that the package exposes. Unfortunately, there is no abstraction that will add Nodes with ease, so uses might vary and identifying nodes can be a pain. Reflection might help here a bit, so I leave that as an exercise for you.

You will find comment spew.Dumps that dump the entire node Tree in a nice format, this helped with debugging when adding Nodes to the source tree.

You can certainly remove nodes as well, you will just need to identify which particular nodes that need to be removed. You just have to ensure that you remove the parent nodes if it were a map or sequence.

package main

import (
    "encoding/json"
    "fmt"
    "log"

    "gopkg.in/yaml.v3"
)

var (
    sourceYaml = `version: 1
type: verbose
kind : bfr

# my list of applications
applications:

#  First app
  - name: app1
    kind: nodejs
    path: app1
    exec:
      platforms: k8s
      builder: test
`
    modifyJsonSource = `
[

    {
        "comment": "Second app",
        "name": "app2",
        "kind": "golang",
        "path": "app2",
        "exec": {
            "platforms": "dockerh",
            "builder": "test"
        }
    }
]
`
)

// VTS Need to Make Fields Public otherwise unmarshalling will not fill in the unexported fields.
type VTS struct {
    Version string       `yaml:"version" json:"version"`
    Types   string       `yaml:"type" json:"type"`
    Kind    string       `yaml:"kind,omitempty" json:"kind,omitempty"`
    Apps    Applications `yaml:"applications,omitempty" json:"applications,omitempty"`
}

type Applications []struct {
    Name string `yaml:"name,omitempty" json:"name,omitempty"`
    Kind string `yaml:"kind,omitempty" json:"kind,omitempty"`
    Path string `yaml:"path,omitempty" json:"path,omitempty"`
    Exec struct {
        Platforms string `yaml:"platforms,omitempty" json:"platforms,omitempty"`
        Builder   string `yaml:"builder,omitempty" json:"builder,omitempty"`
    } `yaml:"exec,omitempty" json:"exec,omitempty"`
    Comment string `yaml:"comment,omitempty" json:"comment,omitempty"`
}

func main() {
    t := yaml.Node{}

    err := yaml.Unmarshal([]byte(sourceYaml), &t)
    if err != nil {
        log.Fatalf("error: %v", err)
    }

    // Look for the Map Node with the seq array of items
    applicationNode := iterateNode(&t, "applications")

    // spew.Dump(iterateNode(&t, "applications"))

    var addFromJson Applications
    err = json.Unmarshal([]byte(modifyJsonSource), &addFromJson)
    if err != nil {
        log.Fatalf("error: %v", err)
    }

    // Delete the Original Applications the following options:
    // applicationNode.Content = []*yaml.Node{}
    // deleteAllContents(applicationNode)
    deleteApplication(applicationNode, "name", "app1")


    for _, app := range addFromJson {
        // Build New Map Node for new sequences coming in from json
        mapNode := &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}

        // Build Name, Kind, and Path Nodes
        mapNode.Content = append(mapNode.Content, buildStringNodes("name", app.Name, app.Comment)...)
        mapNode.Content = append(mapNode.Content, buildStringNodes("kind", app.Kind, "")...)
        mapNode.Content = append(mapNode.Content, buildStringNodes("path", app.Path, "")...)

        // Build the Exec Nodes and the Platform and Builder Nodes within it
        keyMapNode, keyMapValuesNode := buildMapNodes("exec")
        keyMapValuesNode.Content = append(keyMapValuesNode.Content, buildStringNodes("platform", app.Exec.Platforms, "")...)
        keyMapValuesNode.Content = append(keyMapValuesNode.Content, buildStringNodes("builder", app.Exec.Builder, "")...)

        // Add to parent map Node
        mapNode.Content = append(mapNode.Content, keyMapNode, keyMapValuesNode)

        // Add to applications Node
        applicationNode.Content = append(applicationNode.Content, mapNode)
    }
    // spew.Dump(t)
    b, err := yaml.Marshal(&t)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(string(b))
}

// iterateNode will recursive look for the node following the identifier Node,
// as go-yaml has a node for the key and the value itself
// we want to manipulate the value Node
func iterateNode(node *yaml.Node, identifier string) *yaml.Node {
    returnNode := false
    for _, n := range node.Content {
        if n.Value == identifier {
            returnNode = true
            continue
        }
        if returnNode {
            return n
        }
        if len(n.Content) > 0 {
            ac_node := iterateNode(n, identifier)
            if ac_node != nil {
                return ac_node
            }
        }
    }
    return nil
}

// deleteAllContents will remove all the contents of a node
// Mark sure to pass the correct node in otherwise bad things will happen
func deleteAllContents(node *yaml.Node) {
    node.Content = []*yaml.Node{}
}

// deleteApplication expects that a sequence Node with all the applications are present
// if the key value are not found it will not log any errors, and return silently
// this is expecting a map like structure for the applications
func deleteApplication(node *yaml.Node, key, value string) {
    state := -1
    indexRemove := -1
    for index, parentNode := range node.Content {
        for _, childNode := range parentNode.Content {
            if key == childNode.Value && state == -1 {
                state += 1
                continue // found expected move onto next
            }
            if value == childNode.Value && state == 0 {
                state += 1
                indexRemove = index
                break // found the target exit out of the loop
            } else if state == 0 {
                state = -1
            }
        }
    }
    if state == 1 {
        // Remove node from contents
        // node.Content = append(node.Content[:indexRemove], node.Content[indexRemove+1:]...)
        // Don't Do this you might have a potential memory leak source: https://github.com/golang/go/wiki/SliceTricks
        // Since the underlying nodes are pointers
        length := len(node.Content)
        copy(node.Content[indexRemove:], node.Content[indexRemove+1:])
        node.Content[length-1] = nil
        node.Content = node.Content[:length-1]
    }
}


// buildStringNodes builds Nodes for a single key: value instance
func buildStringNodes(key, value, comment string) []*yaml.Node {
    keyNode := &yaml.Node{
        Kind:        yaml.ScalarNode,
        Tag:         "!!str",
        Value:       key,
        HeadComment: comment,
    }
    valueNode := &yaml.Node{
        Kind:  yaml.ScalarNode,
        Tag:   "!!str",
        Value: value,
    }
    return []*yaml.Node{keyNode, valueNode}
}

// buildMapNodes builds Nodes for a key: map instance
func buildMapNodes(key string) (*yaml.Node, *yaml.Node) {
    n1, n2 := &yaml.Node{
        Kind:  yaml.ScalarNode,
        Tag:   "!!str",
        Value: key,
    }, &yaml.Node{Kind: yaml.MappingNode,
        Tag: "!!map",
    }
    return n1, n2
}

Produces yaml

version: 1
type: verbose
kind: bfr


# my list of applications
applications:
-   #  First app
name: app1
    kind: nodejs
    path: app1
    exec:
        platforms: k8s
        builder: test
-   # Second app
name: app2
    kind: golang
    path: app2
    exec:
        platform: dockerh
        builder: test

Answer 2

You could create a new node and directly append to the contents, without deleting the previous node. The following example illustrates this point:

package main

import (
    "fmt"
    "log"

    "gopkg.in/yaml.v3"
)

var (
    sourceYaml = `version: 1
type: verbose
kind : bfr

# my list of applications
applications:

#  First app
  - name: app1
    kind: nodejs
    path: app1
    exec:
      platforms: k8s
      builder: test
`
)

type Application struct {
    Name string `yaml:"name,omitempty" json:"name,omitempty"`
    Kind string `yaml:"kind,omitempty" json:"kind,omitempty"`
    Path string `yaml:"path,omitempty" json:"path,omitempty"`
    Exec struct {
        Platforms string `yaml:"platforms,omitempty" json:"platforms,omitempty"`
        Builder   string `yaml:"builder,omitempty" json:"builder,omitempty"`
    } `yaml:"exec,omitempty" json:"exec,omitempty"`
}

func newApplicationNode(
    name string,
    kind string,
    path string,
    platforms string,
    builder string,
    comment string) (*yaml.Node, error) {

    app := Application{
        Name: name,
        Kind: kind,
        Path: path,
        Exec: struct {
            Platforms string `yaml:"platforms,omitempty" json:"platforms,omitempty"`
            Builder   string `yaml:"builder,omitempty" json:"builder,omitempty"`
        }{platforms, builder},
    }
    marshalledApp, err := yaml.Marshal(&app)
    if err != nil {
        return nil, err
    }

    node := yaml.Node{}
    if err := yaml.Unmarshal(marshalledApp, &node); err != nil {
        return nil, err
    }
    node.Content[0].HeadComment = comment
    return &node, nil
}

func main() {
    yamlNode := yaml.Node{}

    err := yaml.Unmarshal([]byte(sourceYaml), &yamlNode)
    if err != nil {
        log.Fatalf("error: %v", err)
    }

    newApp, err := newApplicationNode("app2", "golang", "app2", "dockerh",
        "test", "Second app")
    if err != nil {
        log.Fatalf("error: %v", err)
    }

    appIdx := -1
    for i, k := range yamlNode.Content[0].Content {
        if k.Value == "applications" {
            appIdx = i + 1
            break
        }
    }

    yamlNode.Content[0].Content[appIdx].Content = append(
        yamlNode.Content[0].Content[appIdx].Content, newApp.Content[0])

    out, err := yaml.Marshal(&yamlNode)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(string(out))
}

Clearly instead of going the hacky way as I did in newApplicationNode you can unmarshal properly from your JSON. However, as stated in previous answers, it is important to notice that the key and actual value are in subsequent indexes inside Content, therefore you need to take this into account when modifying the document. (e.g., look up applications key but then consider the next index (appIdx = i + 1 in my example) for its contents.

Hope that helps!