mime/quotedprintable: accept trailing soft line-break at the end of message

Fixes #15486.

Change-Id: Id879dc9acef9232003df9a0f6f54312191374a60
Reviewed-on: https://go-review.googlesource.com/27530
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/mime/quotedprintable/reader.go

@@ -74,6 +74,9 @@ func (r *Reader) Read(p []byte) (n int, err error) {
 	// 1. in addition to "=\r\n", "=\n" is also treated as soft line break.
 	// 2. it will pass through a '\r' or '\n' not preceded by '=', consistent
 	//    with other broken QP encoders & decoders.
+	// 3. it accepts soft line-break (=) at end of message (issue 15486); i.e.
+	//    the final byte read from the underlying reader is allowed to be '=',
+	//    and it will be silently ignored.
 	for len(p) > 0 {
 		if len(r.line) == 0 {
 			if r.rerr != nil {
@@ -89,7 +92,8 @@ func (r *Reader) Read(p []byte) (n int, err error) {
 			if bytes.HasSuffix(r.line, softSuffix) {
 				rightStripped := wholeLine[len(r.line):]
 				r.line = r.line[:len(r.line)-1]
-				if !bytes.HasPrefix(rightStripped, lf) && !bytes.HasPrefix(rightStripped, crlf) {
+				if !bytes.HasPrefix(rightStripped, lf) && !bytes.HasPrefix(rightStripped, crlf) &&
+					!(len(rightStripped) == 0 && len(r.line) > 0 && r.rerr == io.EOF) {
 					r.rerr = fmt.Errorf("quotedprintable: invalid bytes after =: %q", rightStripped)
 				}
 			} else if hasLF {

src/mime/quotedprintable/reader_test.go

@@ -58,6 +58,9 @@ func TestReader(t *testing.T) {
 		{in: "foo=\nbar", want: "foobar"},
 		{in: "foo=\rbar", want: "foo", err: "quotedprintable: invalid hex byte 0x0d"},
 		{in: "foo=\r\r\r \nbar", want: "foo", err: `quotedprintable: invalid bytes after =: "\r\r\r \n"`},
+		// Issue 15486, accept trailing soft line-break at end of input.
+		{in: "foo=", want: "foo"},
+		{in: "=", want: "", err: `quotedprintable: invalid bytes after =: ""`},
 
 		// Example from RFC 2045:
 		{in: "Now's the time =\n" + "for all folk to come=\n" + " to the aid of their country.",