fix(GODT-770): handle extraneous end-of-mail

pkg/message/parser/parser.go

@@ -32,7 +32,7 @@ type Parser struct {
 func New(r io.Reader) (*Parser, error) {
 	p := new(Parser)
 
-	entity, err := message.Read(r)
+	entity, err := message.Read(newEndOfMailTrimmer(r))
 	if err != nil && !message.IsUnknownCharset(err) {
 		return nil, err
 	}

pkg/message/parser/trimmer.go

@@ -0,0 +1,56 @@
+// Copyright (c) 2020 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package parser
+
+import (
+	"bytes"
+	"io"
+)
+
+const endOfMail = "\r\n.\r\n"
+
+// endOfMailTrimmer wraps a reader to trim the End-Of-Mail indicator at the end
+// of the input, if present.
+//
+// During SMTP sending of a message, the DATA command indicates that you are
+// about to send the text (or body) of the message. The message text must end
+// with "\r\n.\r\n." I'm 99% sure that these 5 bytes should not be considered
+// part of the message body. However, some mail servers keep them as part of
+// the message, which our parser sometimes doesn't like. Therefore, we strip
+// them if we find them.
+type endOfMailTrimmer struct {
+	r   io.Reader
+	buf bytes.Buffer
+}
+
+func newEndOfMailTrimmer(r io.Reader) *endOfMailTrimmer {
+	return &endOfMailTrimmer{r: r}
+}
+
+func (r *endOfMailTrimmer) Read(p []byte) (int, error) {
+	_, err := io.CopyN(&r.buf, r.r, int64(len(p)+len(endOfMail)-r.buf.Len()))
+	if err != nil && err != io.EOF {
+		return 0, err
+	}
+
+	if err == io.EOF && bytes.HasSuffix(r.buf.Bytes(), []byte(endOfMail)) {
+		r.buf.Truncate(r.buf.Len() - len(endOfMail))
+	}
+
+	return r.buf.Read(p)
+}

pkg/message/parser/trimmer_test.go

@@ -0,0 +1,55 @@
+// Copyright (c) 2020 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package parser
+
+import (
+	"io"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestEndOfMailTrimmer(t *testing.T) {
+	var tests = []struct {
+		in  string
+		out string
+	}{
+		{"string without eom", "string without eom"},
+		{"string with eom\r\n.\r\n", "string with eom"},
+		{"string with eom\r\n.\r\nin the middle", "string with eom\r\n.\r\nin the middle"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.in, func(t *testing.T) {
+			res := dumbRead(newEndOfMailTrimmer(strings.NewReader(tt.in)))
+			assert.Equal(t, tt.out, string(res))
+		})
+	}
+}
+
+func dumbRead(r io.Reader) []byte {
+	out := []byte{}
+
+	b := make([]byte, 1)
+	for _, err := r.Read(b); err == nil; _, err = r.Read(b) {
+		out = append(out, b...)
+	}
+
+	return out
+}