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.

frontend.go

@@ -45,6 +45,7 @@ type Frontend struct {
 	bodyLen    int
 	msgType    byte
 	partialMsg bool
+	authType   uint32
 }
 
 // NewFrontend creates a new Frontend.
@@ -146,10 +147,16 @@ func (f *Frontend) Receive() (BackendMessage, error) {
 }
 
 // Authentication message type constants.
+// See src/include/libpq/pqcomm.h for all
+// constants.
 const (
 	AuthTypeOk                = 0
 	AuthTypeCleartextPassword = 3
 	AuthTypeMD5Password       = 5
+	AuthTypeSCMCreds          = 6
+	AuthTypeGSS               = 7
+	AuthTypeGSSCont           = 8
+	AuthTypeSSPI              = 9
 	AuthTypeSASL              = 10
 	AuthTypeSASLContinue      = 11
 	AuthTypeSASLFinal         = 12
@@ -159,15 +166,23 @@ func (f *Frontend) findAuthenticationMessageType(src []byte) (BackendMessage, er
 	if len(src) < 4 {
 		return nil, errors.New("authentication message too short")
 	}
-	authType := binary.BigEndian.Uint32(src[:4])
+	f.authType = binary.BigEndian.Uint32(src[:4])
 
-	switch authType {
+	switch f.authType {
 	case AuthTypeOk:
 		return &f.authenticationOk, nil
 	case AuthTypeCleartextPassword:
 		return &f.authenticationCleartextPassword, nil
 	case AuthTypeMD5Password:
 		return &f.authenticationMD5Password, nil
+	case AuthTypeSCMCreds:
+		return nil, errors.New("AuthTypeSCMCreds is unimplemented")
+	case AuthTypeGSS:
+		return nil, errors.New("AuthTypeGSS is unimplemented")
+	case AuthTypeGSSCont:
+		return nil, errors.New("AuthTypeGSSCont is unimplemented")
+	case AuthTypeSSPI:
+		return nil, errors.New("AuthTypeSSPI is unimplemented")
 	case AuthTypeSASL:
 		return &f.authenticationSASL, nil
 	case AuthTypeSASLContinue:
@@ -175,6 +190,12 @@ func (f *Frontend) findAuthenticationMessageType(src []byte) (BackendMessage, er
 	case AuthTypeSASLFinal:
 		return &f.authenticationSASLFinal, nil
 	default:
-		return nil, fmt.Errorf("unknown authentication type: %d", authType)
+		return nil, fmt.Errorf("unknown authentication type: %d", f.authType)
 	}
 }
+
+// GetAuthType returns the authType used in the current state of the frontend.
+// See SetAuthType for more information.
+func (f *Frontend) GetAuthType() uint32 {
+	return f.authType
+}