Fix reading interrupted messages

When an message is received and a timeout occurs after reading the
header but before reading the entire body the connection state could
be corrupted due to the header being consumed. The next read would
consider the body of the previous message as the header for the next.

fixes #348

frontend.go

@@ -34,6 +34,10 @@ type Frontend struct {
 	parseComplete        ParseComplete
 	readyForQuery        ReadyForQuery
 	rowDescription       RowDescription
+
+	bodyLen    int
+	msgType    byte
+	partialMsg bool
 }
 
 func NewFrontend(r io.Reader, w io.Writer) (*Frontend, error) {
@@ -47,16 +51,19 @@ func (b *Frontend) Send(msg FrontendMessage) error {
 }
 
 func (b *Frontend) Receive() (BackendMessage, error) {
-	header, err := b.cr.Next(5)
-	if err != nil {
-		return nil, err
+	if !b.partialMsg {
+		header, err := b.cr.Next(5)
+		if err != nil {
+			return nil, err
+		}
+
+		b.msgType = header[0]
+		b.bodyLen = int(binary.BigEndian.Uint32(header[1:])) - 4
+		b.partialMsg = true
 	}
 
-	msgType := header[0]
-	bodyLen := int(binary.BigEndian.Uint32(header[1:])) - 4
-
 	var msg BackendMessage
-	switch msgType {
+	switch b.msgType {
 	case '1':
 		msg = &b.parseComplete
 	case '2':
@@ -100,14 +107,16 @@ func (b *Frontend) Receive() (BackendMessage, error) {
 	case 'Z':
 		msg = &b.readyForQuery
 	default:
-		return nil, errors.Errorf("unknown message type: %c", msgType)
+		return nil, errors.Errorf("unknown message type: %c", b.msgType)
 	}
 
-	msgBody, err := b.cr.Next(bodyLen)
+	msgBody, err := b.cr.Next(b.bodyLen)
 	if err != nil {
 		return nil, err
 	}
 
+	b.partialMsg = false
+
 	err = msg.Decode(msgBody)
 	return msg, err
 }