Add support for executing sub-templates

Signed-off-by: Knut Ahlers <knut@ahlers.me>

README.md

@@ -47,6 +47,14 @@
   $ echo '{{ now "2006-01-02 15:04:05" }}' | korvike
   2017-04-17 16:27:34
   ```
+- `{{ tplexec (file "my.tpl") }}`  
+  Execute the given template with the same function set and variables as the parent template.
+  ```console
+  $ export FOO=bar
+  $ echo '{{ env "FOO" }}' >my.tpl
+  $ echo '{{ tplexec (file "my.tpl") }}' | korvike
+  bar
+  ```
 - `{{ vault <path> <key> [default value] }}`  
   Read a key from Vault using `VAULT_ADDR` and `VAULT_TOKEN` environment variables (or `~/.vault-token` file) for authentication.
   ```console

app.go

@@ -77,7 +77,13 @@ func main() {
 		log.Fatalf("Unable to parse template: %s", err)
 	}
 
-	if err := tpl.Execute(out, env.ListToMap(cfg.KeyPairs)); err != nil {
+	vars := map[string]interface{}{}
+	for k, v := range env.ListToMap(cfg.KeyPairs) {
+		vars[k] = v
+	}
+
+	korvike.SetSubTemplateVariables(vars)
+	if err := tpl.Execute(out, vars); err != nil {
 		log.Fatalf("Unable to execute template: %s", err)
 	}
 }

functions/func_test.go

@@ -6,6 +6,7 @@ import (
 	"io/ioutil"
 	"math/rand"
 	"os"
+	"strings"
 	"testing"
 	"text/template"
 	"time"
@@ -17,6 +18,8 @@ func renderHelper(tpl string, ctx map[string]interface{}) string {
 		panic(err)
 	}
 
+	SetSubTemplateVariables(ctx)
+
 	buf := bytes.NewBufferString("")
 	if err := t.Execute(buf, ctx); err != nil {
 		panic(err)
@@ -75,3 +78,15 @@ func Test_now(t *testing.T) {
 		t.Errorf("[now] did not produce expected time format")
 	}
 }
+
+func Test_tplexec(t *testing.T) {
+	result := randomString()
+	os.Setenv("KORVIKE_TESTING", result)
+
+	if res := renderHelper(`{{ tplexec "{{ .var }}:{{ env \"KORVIKE_TESTING\" }}" }}`, map[string]interface{}{"var": "test"}); res != strings.Join([]string{
+		"test",
+		result,
+	}, ":") {
+		t.Errorf("[template] did not produce expected result %q != test", res)
+	}
+}

functions/func_tplexec.go

@@ -0,0 +1,27 @@
+package functions
+
+import (
+	"bytes"
+	"fmt"
+	"text/template"
+)
+
+var subTemplateVariables map[string]interface{}
+
+func SetSubTemplateVariables(m map[string]interface{}) { subTemplateVariables = m }
+
+func init() {
+	registerFunction("tplexec", func(rawTpl string) (string, error) {
+		tpl, err := template.New("in").Funcs(GetFunctionMap()).Parse(rawTpl)
+		if err != nil {
+			return "", fmt.Errorf("parse template: %w", err)
+		}
+
+		out := new(bytes.Buffer)
+		if err := tpl.Execute(out, subTemplateVariables); err != nil {
+			return "", fmt.Errorf("execute template: %w", err)
+		}
+
+		return out.String(), nil
+	})
+}