feat: support template headers (#72)

* feat: support go template headers

* feat: use comments instead of frontmatter

template.go

@@ -1,6 +1,7 @@
 package gomplate
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"fmt"
@@ -203,6 +204,11 @@ func goTemplate(template Template, environment map[string]any) (string, error) {
 	}
 
 	if tpl == nil {
+		template, err := parseAndStripTemplateHeader(template)
+		if err != nil {
+			return "", err
+		}
+
 		tpl = gotemplate.New("")
 		if template.LeftDelim != "" {
 			tpl = tpl.Delims(template.LeftDelim, template.RightDelim)
@@ -215,7 +221,7 @@ func goTemplate(template Template, environment map[string]any) (string, error) {
 		for k, v := range template.Functions {
 			funcs[k] = v
 		}
-		var err error
+
 		tpl, err = tpl.Funcs(funcs).Parse(template.Template)
 		if err != nil {
 			return "", err
@@ -249,3 +255,59 @@ func LoadSharedLibrary(source string) error {
 	registry.Register(func() string { return string(data) })
 	return nil
 }
+
+func parseAndStripTemplateHeader(template Template) (Template, error) {
+	header, content := extractHeaderAndContent(template.Template)
+	if header == "" {
+		return template, nil
+	}
+
+	template.Template = content
+
+	fields := strings.Fields(header)
+	for _, field := range fields {
+		split := strings.SplitN(field, "=", 2)
+		if len(split) != 2 {
+			return template, fmt.Errorf("invalid header: %s", field)
+		}
+
+		switch split[0] {
+		case "right-delim":
+			template.RightDelim = split[1]
+		case "left-delim":
+			template.LeftDelim = split[1]
+		}
+	}
+
+	return template, nil
+}
+
+const templateHeaderPrefix = "# gotemplate: "
+
+func extractHeaderAndContent(template string) (string, string) {
+	scanner := bufio.NewScanner(strings.NewReader(template))
+
+	// Loop through headers.
+	// There could be multiple, we look for the gotemplate header.
+	for scanner.Scan() {
+		line := scanner.Text()
+
+		if line == "---" {
+			// Special case for yaml where the header might not start from the first line.
+			continue
+		}
+
+		// end of headers.
+		isHeader := strings.HasPrefix(line, "#")
+		if !isHeader {
+			break
+		}
+
+		if strings.HasPrefix(line, templateHeaderPrefix) {
+			header := strings.TrimPrefix(line, templateHeaderPrefix)
+			return header, strings.Replace(template, fmt.Sprintf("%s\n", line), "", 1)
+		}
+	}
+
+	return "", template
+}

tests/gomplate_test.go

@@ -1,6 +1,8 @@
 package tests
 
 import (
+	"fmt"
+	"os"
 	"testing"
 	"time"
 
@@ -80,3 +82,38 @@ func TestGomplate(t *testing.T) {
 		})
 	}
 }
+
+func TestGomplateHeaders(t *testing.T) {
+	tests := []struct {
+		env       map[string]interface{}
+		template  string
+		out       string
+		expectErr bool
+	}{
+		{map[string]interface{}{"name": "world"}, readFile(t, "testdata/gotemplate/template-multiple-headers.yaml"), readFile(t, "testdata/gotemplate/expected/template-multiple-headers.yaml"), false},
+		{map[string]interface{}{"name": "world"}, readFile(t, "testdata/gotemplate/template-header.txt"), "Hello, world", false},
+		{map[string]interface{}{"name": "world"}, readFile(t, "testdata/gotemplate/bad-template-header.txt"), "", true},
+		{map[string]interface{}{"name": "world"}, readFile(t, "testdata/gotemplate/template-header-override.txt"), "Hello, world. This ${should} not be {{touched}}", false},
+	}
+
+	for i, tc := range tests {
+		t.Run(fmt.Sprintf("%d", i+1), func(t *testing.T) {
+			out, err := gomplate.RunTemplate(tc.env, gomplate.Template{
+				Template: tc.template,
+			})
+			if tc.expectErr {
+				assert.Error(t, err)
+			} else {
+				assert.ErrorIs(t, err, nil)
+				assert.Equal(t, tc.out, out)
+			}
+		})
+	}
+}
+
+func readFile(t *testing.T, filename string) string {
+	t.Helper()
+	data, err := os.ReadFile(filename)
+	assert.ErrorIs(t, err, nil)
+	return string(data)
+}

tests/testdata/gotemplate/bad-template-header.txt

@@ -0,0 +1,2 @@
+# gotemplate: left-delim=$[ [ right-delim=]]
+Hello, $[[.name]]

tests/testdata/gotemplate/expected/template-multiple-headers.yaml

@@ -0,0 +1,3 @@
+---
+# yaml-language-server: $schema=https://raw.githubusercontent.com/fluxcd-community/flux2-schemas/main/helmrepository-source-v1beta2.json
+name: Hello world

tests/testdata/gotemplate/template-header-override.txt

@@ -0,0 +1,2 @@
+# gotemplate: left-delim=$[[ right-delim=]]
+Hello, $[[.name]]. This ${should} not be {{touched}}

tests/testdata/gotemplate/template-header.txt

@@ -0,0 +1,2 @@
+# gotemplate: left-delim=$[[ right-delim=]]
+Hello, $[[.name]]

tests/testdata/gotemplate/template-multiple-headers.yaml

@@ -0,0 +1,4 @@
+---
+# yaml-language-server: $schema=https://raw.githubusercontent.com/fluxcd-community/flux2-schemas/main/helmrepository-source-v1beta2.json
+# gotemplate: left-delim=<<< right-delim=>>>
+name: Hello <<<.name>>>