Introduce pgproto3 package

pgproto3 will wrap the message encoding and decoding for the PostgreSQL
frontend/backend protocol version 3.

authentication.go

@@ -0,0 +1,54 @@
+package pgproto3
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+)
+
+const (
+	AuthTypeOk                = 0
+	AuthTypeCleartextPassword = 3
+	AuthTypeMD5Password       = 5
+)
+
+type Authentication struct {
+	Type uint32
+
+	// MD5Password fields
+	Salt [4]byte
+}
+
+func (*Authentication) Backend() {}
+
+func (dst *Authentication) UnmarshalBinary(src []byte) error {
+	*dst = Authentication{Type: binary.BigEndian.Uint32(src[:4])}
+
+	switch dst.Type {
+	case AuthTypeOk:
+	case AuthTypeCleartextPassword:
+	case AuthTypeMD5Password:
+		copy(dst.Salt[:], src[4:8])
+	default:
+		return fmt.Errorf("unknown authentication type: %d", dst.Type)
+	}
+
+	return nil
+}
+
+func (src *Authentication) MarshalBinary() ([]byte, error) {
+	var bigEndian BigEndianBuf
+	buf := &bytes.Buffer{}
+	buf.WriteByte('R')
+	buf.Write(bigEndian.Uint32(0))
+	buf.Write(bigEndian.Uint32(src.Type))
+
+	switch src.Type {
+	case AuthTypeMD5Password:
+		buf.Write(src.Salt[:])
+	}
+
+	binary.BigEndian.PutUint32(buf.Bytes()[1:5], uint32(buf.Len()-1))
+
+	return buf.Bytes(), nil
+}