fbb: Ignore leading whitespace when parsing msg

Resolves issue #86 (hopefully)

fbb/message.go

@@ -290,12 +290,34 @@ func (m *Message) Cc() (cc []Address) {
 	return
 }
 
+// copied from from stdlib's bytes/bytes.go
+var asciiSpace = [256]uint8{'\t': 1, '\n': 1, '\v': 1, '\f': 1, '\r': 1, ' ': 1}
+
+// trimLeft advances the reader until the first byte not
+func trimLeftSpace(r *bufio.Reader) {
+	for {
+		b, err := r.Peek(1)
+		if err != nil || asciiSpace[b[0]] == 0 {
+			break
+		}
+		r.Discard(len(b))
+	}
+}
+
 // Implements ReaderFrom for Message.
 //
 // Reads the given io.Reader and fills in values fetched from the stream.
 func (m *Message) ReadFrom(r io.Reader) error {
 	reader := bufio.NewReader(r)
 
+	// Trim leading whitespace before reading the header:
+	// Got a mysterious bug that traced back to the possibility of a
+	// received message with leading CRLFs. Trimming space characters
+	// before reading the header should be safe, as the worst case scenario
+	// is that we fail to parse the header as opposed to definitely
+	// failing.
+	trimLeftSpace(reader)
+
 	if h, err := textproto.NewReader(reader).ReadMIMEHeader(); err != nil {
 		return err
 	} else {

fbb/message_test.go

@@ -13,6 +13,38 @@ import (
 	"unicode"
 )
 
+func TestReadMessageWithWhitespaceBeforeHeader(t *testing.T) {
+	m1 := NewMessage(Private, "one-long-mid")
+	m1.AddTo("N0CALL")
+	m1.SetFrom("LA5NTA")
+	m1.SetBody("Hello world")
+
+	// Write the message with leading whitespace garbage
+	var buf bytes.Buffer
+	buf.WriteString("\r\n\r\n\t ")
+	if err := m1.Write(&buf); err != nil {
+		t.Fatal(err)
+	}
+
+	// Read the message and verify that we decoded the header successfully.
+	m2 := &Message{}
+	if err := m2.ReadFrom(bytes.NewReader(buf.Bytes())); err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(m1.Header, m2.Header) {
+		t.Error("parsed message header differs", m1, m2)
+	}
+}
+
+func TestEmptyMessageReadError(t *testing.T) {
+	if err := (&Message{}).ReadFrom(strings.NewReader("")); err == nil {
+		t.Errorf("Reading empty message did not error")
+	}
+	if err := (&Message{}).ReadFrom(strings.NewReader("\r\n\r\nfoobar")); err == nil {
+		t.Errorf("Reading headerless message did not error")
+	}
+}
+
 func TestParseDate(t *testing.T) {
 	tests := []string{
 		"2016/12/30 01:00", // The correct format according to winlink.org/b2f.