Fix invalid output when truncating Unicode text in templates

template/functions.go

@@ -5,7 +5,6 @@
 package template // import "miniflux.app/template"
 
 import (
-	"bytes"
 	"html/template"
 	"net/mail"
 	"strings"
@@ -80,15 +79,13 @@ func (f *funcMap) Map() template.FuncMap {
 		},
 		"dict": dict,
 		"truncate": func(str string, max int) string {
-			if len(str) > max {
-				var buffer bytes.Buffer
-
-				buffer.WriteString(str[:max-1])
-				buffer.WriteString("…")
-
-				return buffer.String()
+			runes := 0
+			for i := range str {
+				runes++
+				if runes > max {
+					return str[:i] + "…"
+				}
 			}
-
 			return str
 		},
 		"theme_color": func(theme string) string {

template/functions_test.go

@@ -0,0 +1,48 @@
+// Copyright 2018 Frédéric Guillot. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package template // import "miniflux.app/template"
+
+import (
+	"testing"
+)
+
+func TestTruncate(t *testing.T) {
+	fm := funcMap{}
+	if f, ok := fm.Map()["truncate"]; ok {
+		if truncate := f.(func(str string, max int) string); ok {
+			shortEnglishText := "Short text"
+			shortUnicodeText := "Короткий текст"
+
+			// edge case
+			if truncate(shortEnglishText, len(shortEnglishText)) != shortEnglishText {
+				t.Fatal("Invalid truncation")
+			}
+			// real case
+			if truncate(shortEnglishText, 25) != shortEnglishText {
+				t.Fatal("Invalid truncation")
+			}
+			if truncate(shortUnicodeText, len(shortUnicodeText)) != shortUnicodeText {
+				t.Fatal("Invalid truncation")
+			}
+			if truncate(shortUnicodeText, 25) != shortUnicodeText {
+				t.Fatal("Invalid truncation")
+			}
+
+			longEnglishText := "This is really pretty long English text"
+			longRussianText := "Это реально очень длинный русский текст"
+
+			if truncate(longEnglishText, 25) != "This is really pretty lon…" {
+				t.Fatal("Invalid truncation")
+			}
+			if truncate(longRussianText, 25) != "Это реально очень длинный…" {
+				t.Fatal("Invalid truncation")
+			}
+		} else {
+			t.Fatal("Type assetion for this func is failed, check func, maybe it was changed")
+		}
+	} else {
+		t.Fatal("There is no such function in this map, check key, maybe it was changed")
+	}
+}