Introduce pgproto3 package

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

pgproto3.go

@@ -0,0 +1,88 @@
+package pgproto3
+
+import "fmt"
+
+type Message interface {
+	UnmarshalBinary(data []byte) error
+	MarshalBinary() (data []byte, err error)
+}
+
+type FrontendMessage interface {
+	Message
+	Frontend() // no-op method to distinguish frontend from backend methods
+}
+
+type BackendMessage interface {
+	Message
+	Backend() // no-op method to distinguish frontend from backend methods
+}
+
+// func ParseBackend(typeByte byte, body []byte) (BackendMessage, error) {
+// 	switch typeByte {
+// 	case '1':
+// 		return ParseParseComplete(body)
+// 	case '2':
+// 		return ParseBindComplete(body)
+// 	case 'C':
+// 		return ParseCommandComplete(body)
+// 	case 'D':
+// 		return ParseDataRow(body)
+// 	case 'E':
+// 		return ParseErrorResponse(body)
+// 	case 'K':
+// 		return ParseBackendKeyData(body)
+// 	case 'R':
+// 		return ParseAuthentication(body)
+// 	case 'S':
+// 		return ParseParameterStatus(body)
+// 	case 'T':
+// 		return ParseRowDescription(body)
+// 	case 't':
+// 		return ParseParameterDescription(body)
+// 	case 'Z':
+// 		return ParseReadyForQuery(body)
+// 	default:
+// 		return ParseUnknownMessage(typeByte, body)
+// 	}
+// }
+
+// func ParseFrontend(typeByte byte, body []byte) (FrontendMessage, error) {
+// 	switch typeByte {
+// 	case 'B':
+// 		return ParseBind(body)
+// 	case 'D':
+// 		return ParseDescribe(body)
+// 	case 'E':
+// 		return ParseExecute(body)
+// 	case 'P':
+// 		return ParseParse(body)
+// 	case 'p':
+// 		return ParsePasswordMessage(body)
+// 	case 'Q':
+// 		return ParseQuery(body)
+// 	case 'S':
+// 		return ParseSync(body)
+// 	case 'X':
+// 		return ParseTerminate(body)
+// 	default:
+// 		return ParseUnknownMessage(typeByte, body)
+// 	}
+// }
+
+type invalidMessageLenErr struct {
+	messageType string
+	expectedLen int
+	actualLen   int
+}
+
+func (e *invalidMessageLenErr) Error() string {
+	return fmt.Sprintf("%s body must have length of %d, but it is %d", e.messageType, e.expectedLen, e.actualLen)
+}
+
+type invalidMessageFormatErr struct {
+	messageType string
+}
+
+func (e *invalidMessageFormatErr) Error() string {
+	return fmt.Sprintf("%s body is invalid", e.messageType)
+}