Fix incorrect line folding of RFC2047-encoded strings

Use only whitespace characters as a separator to fold lines according to
the section 2.2.3 of RFC5322. It may increase number of cases where hard limit
fallback is used, but it should prevent damaging encoded subjects.

textproto/header.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"io"
 	"net/textproto"
-	"regexp"
 	"strings"
 )
 
@@ -494,9 +493,6 @@ func ReadHeader(r *bufio.Reader) (Header, error) {
 	}
 }
 
-// Regexp that detects Quoted Printable (QP) characters
-var qpReg = regexp.MustCompile("(=[0-9A-Z]{2,2})+")
-
 func foldLine(v string, maxlen int) (line, next string, ok bool) {
 	ok = true
 
@@ -512,24 +508,8 @@ func foldLine(v string, maxlen int) (line, next string, ok bool) {
 			folding = "\r\n"
 		}
 	} else {
-		// Find the last QP character before limit
-		foldAtQP := qpReg.FindAllStringIndex(v[:foldBefore], -1)
 		// Find the closest whitespace before maxlen
-		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 length limit
-				foldAt = foldAtQPLastIndex
-			}
-		}
+		foldAt = strings.LastIndexAny(v[:foldBefore], " \t\n")
 
 		if foldAt == 0 {
 			// The whitespace we found was the previous folding WSP
@@ -551,7 +531,7 @@ func foldLine(v string, maxlen int) (line, next string, ok bool) {
 			// extra space in the string, so this should be avoided if
 			// possible.
 			folding = "\r\n "
-			ok = len(foldAtQP) > 0
+			ok = false
 		}
 	}
 

textproto/header_test.go

@@ -404,18 +404,24 @@ var formatHeaderFieldTests = []struct {
 	{
 		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",
+		formatted: "Subject: =?utf-8?q?=E2=80=9CDeveloper_reads_customer_requested_change.=E2=80=9D=0A?= =?utf-8?q?=0ACaravaggio=0A=0AOil_on...?=\r\n",
 	},
+	// Spaces should not appear in RFC2047-encoded string but it should be fine in case they do
 	{
 		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",
+		formatted: "Subject: =?utf-8?q?=E2=80=9CShort subject=E2=80=9D=0A?=\r\n =?utf-8?q?=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:         "Subject",
+		v:         "InCaseOfVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryLongStringWeStillShouldComplyToTheHardLimitOf998Symbols",
+		formatted: "Subject: InCaseOfVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryVeryLongStringWeStillSho\r\n uldComplyToTheHardLimitOf998Symbols\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",