Don't split Quoted Printable characters during folding procedure

Author: Gonzih

header.go

@@ -3,13 +3,17 @@ package message
 import (
 	"mime"
 	"net/textproto"
+	"regexp"
 	"strings"
 
 	"github.com/emersion/go-message/charset"
 )
 
 const maxHeaderLen = 76
 
+// Regexp that detects Quoted Printable (QP) characters
+var qpReg = regexp.MustCompile("(=[0-9A-Z]{2,2})+")
+
 func parseHeaderWithParams(s string) (f string, params map[string]string, err error) {
 	f, params, err = mime.ParseMediaType(s)
 	if err != nil {
@@ -59,8 +63,24 @@ func formatHeaderField(k, v string) string {
 				folding = "\r\n"
 			}
 		} else {
+			// Find the last QP character before limit
+			foldAtQP := qpReg.FindAllStringIndex(v[:foldBefore], -1)
 			// Find the closest whitespace before i
-			foldAt = strings.LastIndexAny(v[:foldBefore], " \t\n")
+			foldAtEOL := strings.LastIndexAny(v[:foldBefore], " \t\n")
+
+			// Fold at the latest whitespace by default
+			foldAt = foldAtEOL
+
+			// if there are QP characters in the string
+			if len(foldAtQP) > 0 {
+				// Get the start index of the last QP character
+				foldAtQPLastIndex := foldAtQP[len(foldAtQP)-1][0]
+				if foldAtQPLastIndex > foldAt {
+					// Fold at the latest QP character if there are no whitespaces after it and before line hard limit
+					foldAt = foldAtQPLastIndex
+				}
+			}
+
 			if foldAt == 0 {
 				// The whitespace we found was the previous folding WSP
 				foldAt = foldBefore - 1

header_test.go

@@ -59,6 +59,21 @@ var formatHeaderFieldTests = []struct {
 		v:         "This is        yet          \t  another    subject          \t                   with many         whitespace      characters",
 		formatted: "Subject: This is        yet          \t  another    subject          \t       \r\n            with many         whitespace      characters\r\n",
 	},
+	{
+		k:         "Subject",
+		v:         "=?utf-8?q?=E2=80=9CDeveloper_reads_customer_requested_change.=E2=80=9D=0A?= =?utf-8?q?=0ACaravaggio=0A=0AOil_on...?=",
+		formatted: "Subject: =?utf-8?q?=E2=80=9CDeveloper_reads_customer_requested_change.\r\n =E2=80=9D=0A?= =?utf-8?q?=0ACaravaggio=0A=0AOil_on...?=\r\n",
+	},
+	{
+		k:         "Subject",
+		v:         "=?utf-8?q?=E2=80=9CShort subject=E2=80=9D=0A?= =?utf-8?q?=0AAuthor=0A=0AOil_on...?=",
+		formatted: "Subject: =?utf-8?q?=E2=80=9CShort subject=E2=80=9D=0A?= =?utf-8?q?\r\n =0AAuthor=0A=0AOil_on...?=\r\n",
+	},
+	{
+		k:         "Subject",
+		v:         "=?utf-8?q?=E2=80=9CVery long subject very long subject very long subject very long subject=E2=80=9D=0A?= =?utf-8?q?=0ALong second part of subject long second part of subject long second part of subject long subject=0A=0AOil_on...?=",
+		formatted: "Subject: =?utf-8?q?=E2=80=9CVery long subject very long subject very long\r\n subject very long subject=E2=80=9D=0A?= =?utf-8?q?=0ALong second part of\r\n subject long second part of subject long second part of subject long\r\n subject=0A=0AOil_on...?=\r\n",
+	},
 	{
 		k:         "DKIM-Signature",
 		v:         "v=1;\r\n h=From:To:Reply-To:Subject:Message-ID:References:In-Reply-To:MIME-Version;\r\n d=example.org\r\n",