Add support for identifying authentication messages

The pgprotocol overloads 'p' messages with PasswordMessage,
SASLInitialResponse, SASLResponse, and GSSResponse. This patch allows
contextual identification of the message by setting the authType in the
frontend and then setting this value in the backend when a
AuthenticationResponseMessage is received.

backend.go

@@ -12,27 +12,27 @@ type Backend struct {
 	w  io.Writer
 
 	// Frontend message flyweights
-	bind            Bind
-	cancelRequest   CancelRequest
-	_close          Close
-	copyFail        CopyFail
-	copyData        CopyData
-	copyDone        CopyDone
-	describe        Describe
-	execute         Execute
-	flush           Flush
-	gssEncRequest   GSSEncRequest
-	parse           Parse
-	passwordMessage PasswordMessage
-	query           Query
-	sslRequest      SSLRequest
-	startupMessage  StartupMessage
-	sync            Sync
-	terminate       Terminate
+	bind           Bind
+	cancelRequest  CancelRequest
+	_close         Close
+	copyFail       CopyFail
+	copyData       CopyData
+	copyDone       CopyDone
+	describe       Describe
+	execute        Execute
+	flush          Flush
+	gssEncRequest  GSSEncRequest
+	parse          Parse
+	query          Query
+	sslRequest     SSLRequest
+	startupMessage StartupMessage
+	sync           Sync
+	terminate      Terminate
 
 	bodyLen    int
 	msgType    byte
 	partialMsg bool
+	authType   uint32
 }
 
 // NewBackend creates a new Backend.
@@ -127,7 +127,19 @@ func (b *Backend) Receive() (FrontendMessage, error) {
 	case 'P':
 		msg = &b.parse
 	case 'p':
-		msg = &b.passwordMessage
+		switch b.authType {
+		case AuthTypeSASL:
+			msg = &SASLInitialResponse{}
+		case AuthTypeSASLContinue:
+			msg = &SASLResponse{}
+		case AuthTypeSASLFinal:
+			msg = &SASLResponse{}
+		case AuthTypeCleartextPassword, AuthTypeMD5Password:
+			fallthrough
+		default:
+			// to maintain backwards compatability
+			msg = &PasswordMessage{}
+		}
 	case 'Q':
 		msg = &b.query
 	case 'S':
@@ -148,3 +160,36 @@ func (b *Backend) Receive() (FrontendMessage, error) {
 	err = msg.Decode(msgBody)
 	return msg, err
 }
+
+// SetAuthType sets the authentication type in the backend.
+// Since multiple message types can start with 'p', SetAuthType allows
+// contextual identification of FrontendMessages. For example, in the
+// PG message flow documentation for PasswordMessage:
+//
+// 		Byte1('p')
+//
+//      Identifies the message as a password response. Note that this is also used for
+//		GSSAPI, SSPI and SASL response messages. The exact message type can be deduced from
+//		the context.
+//
+// Since the Frontend does not know about the state of a backend, it is important
+// to call SetAuthType() after an authentication request is received by the Frontend.
+func (b *Backend) SetAuthType(authType uint32) error {
+	switch authType {
+	case AuthTypeOk,
+		AuthTypeCleartextPassword,
+		AuthTypeMD5Password,
+		AuthTypeSCMCreds,
+		AuthTypeGSS,
+		AuthTypeGSSCont,
+		AuthTypeSSPI,
+		AuthTypeSASL,
+		AuthTypeSASLContinue,
+		AuthTypeSASLFinal:
+		b.authType = authType
+	default:
+		return fmt.Errorf("authType not recognized: %d", authType)
+	}
+
+	return nil
+}