Add helm-secrets-encrypted values template file (#1701)

Secret files ending with .gotmpl are now also rendered as a gotemplate.

```
releases:
- name: myapp
  secrets:
  - secrets.yaml.gotmpl
```

Note that currently, .gotmpl files must be valid YAML files as well.

The expected use-case of this feature is to compose a YAML array from values and encrypted secrets.

Without this feature, you would have tried to do something like the below, which didn't work.

**Example (doesn't work!)**

`values.yaml.gotmpl`:

```
environment:
  -   name: MY_EXTERNAL_IP
      value: |
          {{ exec "./get-external-ip.sh" (list "") }}
```

`secrets.yaml`:
```
_sops:
  #...
environment:
  - name: MY_SECRET_VALUE
    value: (encrypted by sops)
```

`helmfile.yaml`:

```
releases:
- name: foo
  values:
  - values.yaml
  secrets:
  - secrets.yaml
```

This doesn't work because `values.yaml` and the decrypted `secrets.yaml` are passed to `helm` to be merged, and helm overrides the array instead of merging or concatenating the arrays.

**Example (works!)**

Instead of `values.yaml` and `secrets.yaml`, you provide a single `secrets.yaml.gotmpl` that is a valid YAML and encrypted by sops:

```
_sops:
  #...
environment:
  -   name: MY_EXTERNAL_IP
      value: |
          {{ exec "./get-external-ip.sh" (list "") }}
  - name: MY_SECRET_VALUE
    value: (encrypted by sops)
```

`helmfile.yaml`:

```
releases:
- name: foo
  secrets:
  - secrets.yaml.gotmpl
```

Helmfile decrypts the gotmpl by handing it over to helm-secrets and then renders the result as a gotmpl file. The end result is that you have a two-element array `environments` that can be just passed to helm.

Resolves #1700

Co-authored-by: Yusuke Kuoka <ykuoka@gmail.com>

pkg/helmexec/exec.go

@@ -278,7 +278,14 @@ func (helm *execer) DecryptSecret(context HelmContext, name string, flags ...str
 		if len(decSuffix) == 0 {
 			decSuffix = ".yaml.dec"
 		}
-		decFilename := strings.Replace(absPath, ".yaml", decSuffix, 1)
+
+		// helm secrets replaces the extension with its suffix ONLY when the extension is ".yaml"
+		var decFilename string
+		if strings.HasSuffix(absPath, ".yaml") {
+			decFilename = strings.Replace(absPath, ".yaml", decSuffix, 1)
+		} else {
+			decFilename = absPath + decSuffix
+		}
 
 		secretBytes, err := ioutil.ReadFile(decFilename)
 		if err != nil {
@@ -308,7 +315,9 @@ func (helm *execer) DecryptSecret(context HelmContext, name string, flags ...str
 
 	if tempFile == nil {
 		tempFile = func(content []byte) (string, error) {
-			tmpFile, err := ioutil.TempFile("", "secret")
+			dir := filepath.Dir(name)
+			extension := filepath.Ext(name)
+			tmpFile, err := ioutil.TempFile(dir, "secret*"+extension)
 			if err != nil {
 				return "", err
 			}

pkg/helmexec/exec_test.go

@@ -289,6 +289,29 @@ Found secret in cache %s/secretName
 	}
 }
 
+func Test_DecryptSecretWithGotmpl(t *testing.T) {
+	var buffer bytes.Buffer
+	logger := NewLogger(&buffer, "debug")
+	helm := MockExecer(logger, "dev")
+
+	tmpFilePath := "path/to/temp/file"
+	helm.writeTempFile = func(content []byte) (string, error) {
+		return tmpFilePath, nil
+	}
+
+	secretName := "secretName.yaml.gotmpl"
+	_, decryptErr := helm.DecryptSecret(HelmContext{}, secretName)
+	cwd, err := filepath.Abs(".")
+	if err != nil {
+		t.Errorf("Error: %v", err)
+	}
+
+	expected := fmt.Sprintf(`%s/%s.yaml.dec`, cwd, secretName)
+	if d := cmp.Diff(expected, decryptErr.(*os.PathError).Path); d != "" {
+		t.Errorf("helmexec.DecryptSecret(): want (-), got (+):\n%s", d)
+	}
+}
+
 func Test_DiffRelease(t *testing.T) {
 	var buffer bytes.Buffer
 	logger := NewLogger(&buffer, "debug")