mime: fix maximum length of encoded-words

RFC 2047 recommends a maximum length of 75 characters for
encoded-words. Due to a bug, encoded-words were limited to 77
characters instead of 75.

Change-Id: I2ff9d013ab922df6fd542464ace70b1c46dc7ae7
Reviewed-on: https://go-review.googlesource.com/20918Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/mime/encodedword.go

@@ -71,7 +71,7 @@ const (
 	maxEncodedWordLen = 75
 	// maxContentLen is how much content can be encoded, ignoring the header and
 	// 2-byte footer.
-	maxContentLen = maxEncodedWordLen - len("=?UTF-8?") - len("?=")
+	maxContentLen = maxEncodedWordLen - len("=?UTF-8?q?") - len("?=")
 )
 
 var maxBase64Len = base64.StdEncoding.DecodedLen(maxContentLen)

src/mime/encodedword_test.go

@@ -31,10 +31,8 @@ func TestEncodeWord(t *testing.T) {
 		{QEncoding, utf8, strings.Repeat("é", 11), "=?utf-8?q?" + strings.Repeat("=C3=A9", 10) + "?= =?utf-8?q?=C3=A9?="},
 		{QEncoding, iso88591, strings.Repeat("\xe9", 22), "=?iso-8859-1?q?" + strings.Repeat("=E9", 22) + "?="},
 		{QEncoding, utf8, strings.Repeat("\x80", 22), "=?utf-8?q?" + strings.Repeat("=80", 21) + "?= =?utf-8?q?=80?="},
-		{BEncoding, utf8, strings.Repeat("é", 24), "=?utf-8?b?" + strings.Repeat("w6nDqcOp", 8) + "?="},
-		{BEncoding, utf8, strings.Repeat("é", 27), "=?utf-8?b?" + strings.Repeat("w6nDqcOp", 8) + "?= =?utf-8?b?w6nDqcOp?="},
 		{BEncoding, iso88591, strings.Repeat("\xe9", 45), "=?iso-8859-1?b?" + strings.Repeat("6enp", 15) + "?="},
-		{BEncoding, utf8, strings.Repeat("\x80", 51), "=?utf-8?b?" + strings.Repeat("gICA", 16) + "?= =?utf-8?b?gICA?="},
+		{BEncoding, utf8, strings.Repeat("\x80", 48), "=?utf-8?b?" + strings.Repeat("gICA", 15) + "?= =?utf-8?b?gICA?="},
 	}
 
 	for _, test := range tests {
@@ -44,6 +42,37 @@ func TestEncodeWord(t *testing.T) {
 	}
 }
 
+func TestEncodedWordLength(t *testing.T) {
+	tests := []struct {
+		enc WordEncoder
+		src string
+	}{
+		{QEncoding, strings.Repeat("à", 30)},
+		{QEncoding, strings.Repeat("é", 60)},
+		{BEncoding, strings.Repeat("ï", 25)},
+		{BEncoding, strings.Repeat("ô", 37)},
+		{BEncoding, strings.Repeat("\x80", 50)},
+		{QEncoding, "{$firstname} Bienvendio a Apostolica, aquà inicia el camino de tu"},
+	}
+
+	for _, test := range tests {
+		s := test.enc.Encode("utf-8", test.src)
+		wordLen := 0
+		for i := 0; i < len(s); i++ {
+			if s[i] == ' ' {
+				wordLen = 0
+				continue
+			}
+
+			wordLen++
+			if wordLen > maxEncodedWordLen {
+				t.Errorf("Encode(%q) has more than %d characters: %q",
+					test.src, maxEncodedWordLen, s)
+			}
+		}
+	}
+}
+
 func TestDecodeWord(t *testing.T) {
 	tests := []struct {
 		src, exp string