Skip template parsing for commented out lines (#351)

* Skip template parsing for commented out lines

* fix: execute template only once

* refactor: modify test input

---------

Co-authored-by: Harsha Dixit <[email protected]>

Author: mheap

pkg/file/template.go

@@ -96,7 +96,22 @@ func renderTemplate(content string, mockEnvVars bool) (string, error) {
 	}
 	t := template.New("state").Funcs(templateFuncs).Delims("${{", "}}")
 
-	t, err := t.Parse(content)
+	// Parse content line by line, and ignore lines that start with #
+	var allContent bytes.Buffer
+	lines := strings.Split(content, "\n")
+	for i := 0; i < len(lines); i++ {
+		line := lines[i]
+		if !strings.HasPrefix(strings.TrimSpace(line), "#") {
+			allContent.WriteString(line + "\n")
+		}
+	}
+
+	result := allContent.String()
+	if !strings.HasSuffix(content, "\n") {
+		result = strings.TrimSuffix(result, "\n")
+	}
+
+	t, err := t.Parse(result)
 	if err != nil {
 		return "", err
 	}

pkg/file/template_test.go

@@ -72,3 +72,40 @@ func Test_renderTemplateCustomPrefix(t *testing.T) {
 	os.Unsetenv("PREFIX_MY_VARIABLE")
 	SetEnvVarPrefix(oldPrefix)
 }
+
+func Test_renderTemplateIgnoresComments(t *testing.T) {
+	content := `Hello, ${{ env "DECK_MY_VARIABLE" }}!
+  # Also, ${{ env "DECK_NOT_SET_DOESNT_ERROR" }}!`
+
+	expectedOutput := `Hello, my_value!`
+	mockEnvVars := false
+
+	os.Setenv("DECK_MY_VARIABLE", "my_value")
+
+	output, err := renderTemplate(content, mockEnvVars)
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	if output != expectedOutput {
+		t.Errorf("Expected output %q, but got %q", expectedOutput, output)
+	}
+	os.Unsetenv("DECK_MY_VARIABLE")
+}
+
+func Test_renderTemplateErrorWhenNotSet(t *testing.T) {
+	content := `
+Hello, ${{ env "DECK_MY_VARIABLE" }}!
+Also, ${{ env "DECK_NOT_SET_ERRORS" }}!`
+
+	mockEnvVars := false
+
+	os.Setenv("DECK_MY_VARIABLE", "my_value")
+
+	_, err := renderTemplate(content, mockEnvVars)
+	if err == nil {
+		t.Errorf("expected error but did not receive one")
+	}
+
+	os.Unsetenv("DECK_MY_VARIABLE")
+}