Add echo provider for testing purpose

Ref roboll/helmfile#920 (comment)

README.md

@@ -221,6 +221,17 @@ Examples:
 - `ref+sops://path/to/file#/foo/bar` reads `path/to/file` as a `yaml` file and returns the value at `foo.bar`.
 - `ref+sops://path/to/file?format=json#/foo/bar` reads `path/to/file` as a `json` file and returns the value at `foo.bar`.
 
+### Echo
+
+Echo provider echoes the string for testing purpose. Please read [the original proposal](https://github.com/roboll/helmfile/pull/920#issuecomment-548213738) to get why we might need this.
+
+- `ref+echo://KEY1/KEY2/VALUE[#/path/to/the/value]`
+
+Examples:
+
+- `ref+echo://foo/bar` generates `foo/bar`
+- `ref+echo://foo/bar/baz#/foo/bar` generates `baz`. This works by the host and the path part `foo/bar/baz` generating an object `{"foo":{"bar":"baz"}}` and the fragment part `#/foo/bar` results in digging the object to obtain the value at `$.foo.bar`.
+
 ## Non-Goals
 
 ### String-Interpolation / Template Functions

pkg/providers/echo/echo.go

@@ -0,0 +1,47 @@
+package echo
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/variantdev/vals/pkg/api"
+)
+
+type provider struct {
+}
+
+func New(cfg api.StaticConfig) *provider {
+	p := &provider{}
+	return p
+}
+
+// Get gets an AWS SSM Parameter Store value
+func (p *provider) GetString(key string) (string, error) {
+	return key, nil
+}
+
+func (p *provider) GetStringMap(key string) (map[string]interface{}, error) {
+	keys := strings.Split(key, "/")
+
+	res := map[string]interface{}
+	cur := res
+
+	if len(keys) < 2 {
+		return nil, fmt.Errorf("key must have two or more components separated by \"/\": got %q", key)
+	}
+
+	return res, nil
+
+	for i := 0; i < len(keys)-1; i++ {
+		k := keys[i]
+		if i == len(keys)-2 {
+			cur[k] = keys[i+1]
+		} else {
+			newm := map[string]interface{}{}
+			cur[k] = newm
+			cur = newm
+		}
+	}
+
+	return res, nil
+}

vals.go

@@ -6,6 +6,7 @@ import (
 	"github.com/variantdev/vals/pkg/api"
 	"github.com/variantdev/vals/pkg/expansion"
 	"github.com/variantdev/vals/pkg/providers/awssec"
+	"github.com/variantdev/vals/pkg/providers/echo"
 	"github.com/variantdev/vals/pkg/providers/sops"
 	"github.com/variantdev/vals/pkg/providers/ssm"
 	"github.com/variantdev/vals/pkg/providers/vault"
@@ -45,6 +46,7 @@ const (
 	ProviderSSM            = "awsssm"
 	ProviderSecretsManager = "awssecrets"
 	ProviderSOPS           = "sops"
+	ProviderEcho           = "echo"
 )
 
 type Evaluator interface {
@@ -118,6 +120,9 @@ func (r *Runtime) Eval(template map[string]interface{}) (map[string]interface{},
 		case ProviderSOPS:
 			p := sops.New(conf)
 			return p, nil
+		case ProviderEcho:
+			p := echo.New(conf)
+			return p, nil
 		}
 		return nil, fmt.Errorf("no provider registered for scheme %q", scheme)
 	}