html: lazily populate Unescape tables

Saves ~105KB of heap for callers who don't use html.UnescapeString.
(EscapeString is much more common).

Also saves 70KB of binary size, because now the linker can do dead
code elimination. (because #2559 is still open and global maps always
generate init code)

Fixes #26727
Updates #6853

Change-Id: I18fe9a273097e2c7e0cb7f88205cae1bb60fa89b
Reviewed-on: https://go-review.googlesource.com/127075
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/html/entity_test.go

@@ -9,7 +9,15 @@ import (
 	"unicode/utf8"
 )
 
+func init() {
+	UnescapeString("") // force load of entity maps
+}
+
 func TestEntityLength(t *testing.T) {
+	if len(entity) == 0 || len(entity2) == 0 {
+		t.Fatal("maps not loaded")
+	}
+
 	// We verify that the length of UTF-8 encoding of each value is <= 1 + len(key).
 	// The +1 comes from the leading "&". This property implies that the length of
 	// unescaped text is <= the length of escaped text.

src/html/escape.go

@@ -185,6 +185,7 @@ func EscapeString(s string) string {
 // UnescapeString(EscapeString(s)) == s always holds, but the converse isn't
 // always true.
 func UnescapeString(s string) string {
+	populateMapsOnce.Do(populateMaps)
 	i := strings.IndexByte(s, '&')
 
 	if i < 0 {