Add SafeHtmlAttr, SafeCSS template function

This allows a template user to keep a safe HTML attribute or CSS string
as is in a template.

This is implementation of @anthonyfok great insight

Fix #784, #347

tpl/template_test.go

@@ -1,6 +1,7 @@
 package tpl
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"html/template"
@@ -826,3 +827,107 @@ func TestMarkdownify(t *testing.T) {
 		t.Errorf("Markdownify: got '%s', expected '%s'", result, expect)
 	}
 }
+
+func TestSafeHtml(t *testing.T) {
+	for i, this := range []struct {
+		str                 string
+		tmplStr             string
+		expectWithoutEscape string
+		expectWithEscape    string
+	}{
+		{`<div></div>`, `{{ . }}`, `&lt;div&gt;&lt;/div&gt;`, `<div></div>`},
+	} {
+		tmpl, err := template.New("test").Parse(this.tmplStr)
+		if err != nil {
+			t.Errorf("[%d] unable to create new html template %q: %s", this.tmplStr, err)
+			continue
+		}
+
+		buf := new(bytes.Buffer)
+		err = tmpl.Execute(buf, this.str)
+		if err != nil {
+			t.Errorf("[%d] execute template with a raw string value returns unexpected error: %s", i, err)
+		}
+		if buf.String() != this.expectWithoutEscape {
+			t.Errorf("[%d] execute template with a raw string value, got %v but expected %v", i, buf.String(), this.expectWithoutEscape)
+		}
+
+		buf.Reset()
+		err = tmpl.Execute(buf, SafeHtml(this.str))
+		if err != nil {
+			t.Errorf("[%d] execute template with an escaped string value by SafeHtml returns unexpected error: %s", i, err)
+		}
+		if buf.String() != this.expectWithEscape {
+			t.Errorf("[%d] execute template with an escaped string value by SafeHtml, got %v but expected %v", i, buf.String(), this.expectWithEscape)
+		}
+	}
+}
+
+func TestSafeHtmlAttr(t *testing.T) {
+	for i, this := range []struct {
+		str                 string
+		tmplStr             string
+		expectWithoutEscape string
+		expectWithEscape    string
+	}{
+		{`href="irc://irc.freenode.net/#golang"`, `<a {{ . }}>irc</a>`, `<a ZgotmplZ>irc</a>`, `<a href="irc://irc.freenode.net/#golang">irc</a>`},
+	} {
+		tmpl, err := template.New("test").Parse(this.tmplStr)
+		if err != nil {
+			t.Errorf("[%d] unable to create new html template %q: %s", this.tmplStr, err)
+			continue
+		}
+
+		buf := new(bytes.Buffer)
+		err = tmpl.Execute(buf, this.str)
+		if err != nil {
+			t.Errorf("[%d] execute template with a raw string value returns unexpected error: %s", i, err)
+		}
+		if buf.String() != this.expectWithoutEscape {
+			t.Errorf("[%d] execute template with a raw string value, got %v but expected %v", i, buf.String(), this.expectWithoutEscape)
+		}
+
+		buf.Reset()
+		err = tmpl.Execute(buf, SafeHtmlAttr(this.str))
+		if err != nil {
+			t.Errorf("[%d] execute template with an escaped string value by SafeHtmlAttr returns unexpected error: %s", i, err)
+		}
+		if buf.String() != this.expectWithEscape {
+			t.Errorf("[%d] execute template with an escaped string value by SafeHtmlAttr, got %v but expected %v", i, buf.String(), this.expectWithEscape)
+		}
+	}
+}
+
+func TestSafeCSS(t *testing.T) {
+	for i, this := range []struct {
+		str                 string
+		tmplStr             string
+		expectWithoutEscape string
+		expectWithEscape    string
+	}{
+		{`width: 60px;`, `<div style="{{ . }}"></div>`, `<div style="ZgotmplZ"></div>`, `<div style="width: 60px;"></div>`},
+	} {
+		tmpl, err := template.New("test").Parse(this.tmplStr)
+		if err != nil {
+			t.Errorf("[%d] unable to create new html template %q: %s", this.tmplStr, err)
+		}
+
+		buf := new(bytes.Buffer)
+		err = tmpl.Execute(buf, this.str)
+		if err != nil {
+			t.Errorf("[%d] execute template with a raw string value returns unexpected error: %s", i, err)
+		}
+		if buf.String() != this.expectWithoutEscape {
+			t.Errorf("[%d] execute template with a raw string value, got %v but expected %v", i, buf.String(), this.expectWithoutEscape)
+		}
+
+		buf.Reset()
+		err = tmpl.Execute(buf, SafeCSS(this.str))
+		if err != nil {
+			t.Errorf("[%d] execute template with an escaped string value by SafeCSS returns unexpected error: %s", i, err)
+		}
+		if buf.String() != this.expectWithEscape {
+			t.Errorf("[%d] execute template with an escaped string value by SafeCSS, got %v but expected %v", i, buf.String(), this.expectWithEscape)
+		}
+	}
+}