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

backend.go

@@ -24,6 +24,10 @@ type Backend struct {
 	startupMessage  StartupMessage
 	sync            Sync
 	terminate       Terminate
+
+	bodyLen    int
+	msgType    byte
+	partialMsg bool
 }
 
 func NewBackend(r io.Reader, w io.Writer) (*Backend, error) {
@@ -57,16 +61,19 @@ func (b *Backend) ReceiveStartupMessage() (*StartupMessage, error) {
 }
 
 func (b *Backend) Receive() (FrontendMessage, 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 FrontendMessage
-	switch msgType {
+	switch b.msgType {
 	case 'B':
 		msg = &b.bind
 	case 'C':
@@ -88,14 +95,16 @@ func (b *Backend) Receive() (FrontendMessage, error) {
 	case 'X':
 		msg = &b.terminate
 	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
 }