Rename session.SessionCallbacks interface

This renaming is more involved that other ones. Interfaces are hard.

First of all, introduce a new interface with identifiers that do not
offend golint:

  - use "Callbacks" instead of "SessionCallbacks" to avoid stuttering
    when importing (it will be called "session.Callbacks" in user code)

  - use "GetPublicKeyID" method name instead of "GetPublicKeyId"

Then update all existing code to use the new interface. Secure Session
itself should call the new method, accept and store Callbacks instance.
Users of Secure Session should implement the new GetPublicKeyID method.

However, in order to maintain backwards compatibility we have to allow
the old code to continue using the old SessionCallbacks interface with
GetPublicKeyId method. We implement this using an adapter struct that
translates new method calls into the old ones. Since Go does not have
method overloading we have to accept an "interface {}" instance and
check its type dynamically. These compatibility shims can be removed
when we drop the old interface.

docs/examples/Themis-server/go/ssession_server.go

@@ -20,7 +20,7 @@ type clientTransportCallback struct {
 	serverID     []byte
 }
 
-func (clb *clientTransportCallback) GetPublicKeyForId(ss *session.SecureSession, id []byte) *keys.PublicKey {
+func (clb *clientTransportCallback) GetPublicKeyForID(ss *session.SecureSession, id []byte) *keys.PublicKey {
 	if bytes.Equal(id, clb.serverID) {
 		return &keys.PublicKey{Value: clb.serverPublic}
 	}

docs/examples/go/secure_session_client.go

@@ -11,7 +11,7 @@ import (
 type callbacks struct {
 }
 
-func (clb *callbacks) GetPublicKeyForId(ss *session.SecureSession, id []byte) *keys.PublicKey {
+func (clb *callbacks) GetPublicKeyForID(ss *session.SecureSession, id []byte) *keys.PublicKey {
 	decodedID, err := base64.StdEncoding.DecodeString(string(id[:]))
 	if nil != err {
 		return nil

docs/examples/go/secure_session_server.go

@@ -11,7 +11,7 @@ import (
 type callbacks struct {
 }
 
-func (clb *callbacks) GetPublicKeyForId(ss *session.SecureSession, id []byte) *keys.PublicKey {
+func (clb *callbacks) GetPublicKeyForID(ss *session.SecureSession, id []byte) *keys.PublicKey {
 	decodedID, err := base64.StdEncoding.DecodeString(string(id[:]))
 	if nil != err {
 		return nil

gothemis/session/session.go

@@ -34,18 +34,38 @@ const (
 	STATE_ESTABLISHED = StateEstablished
 )
 
+// Callbacks implements a delegate for SecureSession.
+type Callbacks interface {
+	GetPublicKeyForID(ss *SecureSession, id []byte) *keys.PublicKey
+	StateChanged(ss *SecureSession, state int)
+}
+
 // SessionCallbacks implements a delegate for SecureSession.
+//
+// Deprecated: Since 0.11. Use "session.Callbacks" instead.
 type SessionCallbacks interface {
 	GetPublicKeyForId(ss *SecureSession, id []byte) *keys.PublicKey
 	StateChanged(ss *SecureSession, state int)
 }
 
+type sessionCallbacksAdapter struct {
+	inner SessionCallbacks
+}
+
+func (adapter *sessionCallbacksAdapter) GetPublicKeyForID(ss *SecureSession, id []byte) *keys.PublicKey {
+	return adapter.inner.GetPublicKeyForId(ss, id)
+}
+
+func (adapter *sessionCallbacksAdapter) StateChanged(ss *SecureSession, state int) {
+	adapter.inner.StateChanged(ss, state)
+}
+
 // SecureSession is a lightweight mechanism for securing any kind of network communication
 // (both private and public networks, including the Internet). It is protocol-agnostic and
 // operates on the 5th layer of the network OSI model (the session layer).
 type SecureSession struct {
 	ctx   *C.struct_session_with_callbacks_type
-	clb   SessionCallbacks
+	clb   Callbacks
 	state int
 }
 
@@ -54,8 +74,18 @@ func finalize(ss *SecureSession) {
 }
 
 // New makes a new Secure Session with provided peer ID, private key, and callbacks.
-func New(id []byte, signKey *keys.PrivateKey, callbacks SessionCallbacks) (*SecureSession, error) {
-	ss := &SecureSession{clb: callbacks}
+func New(id []byte, signKey *keys.PrivateKey, callbacks interface{}) (*SecureSession, error) {
+	// TODO: remove these dynamic type checks together with SessionCallbacks
+	realCallbacks, newInterface := callbacks.(Callbacks)
+	if !newInterface {
+		oldCallbacks, oldInterface := callbacks.(SessionCallbacks)
+		if oldInterface {
+			realCallbacks = &sessionCallbacksAdapter{inner: oldCallbacks}
+		} else {
+			return nil, errors.New("Invalid callback interface")
+		}
+	}
+	ss := &SecureSession{clb: realCallbacks}
 
 	if nil == id || 0 == len(id) {
 		return nil, errors.New("Failed to creating secure session object with empty id")
@@ -98,7 +128,7 @@ func onPublicKeyForId(ssCtx unsafe.Pointer, idPtr unsafe.Pointer, idLen C.size_t
 	id := C.GoBytes(idPtr, C.int(idLen))
 	ss = (*SecureSession)(unsafe.Pointer(uintptr(ssCtx) - unsafe.Offsetof(ss.ctx)))
 
-	pub := ss.clb.GetPublicKeyForId(ss, id)
+	pub := ss.clb.GetPublicKeyForID(ss, id)
 	if nil == pub {
 		return int(C.GOTHEMIS_INVALID_PARAMETER)
 	}

gothemis/session/session_test.go

@@ -17,7 +17,7 @@ type testCallbacks struct {
 var clientID = []byte("client a")
 var serverID = []byte("client b")
 
-func (clb *testCallbacks) GetPublicKeyForId(ss *SecureSession, id []byte) *keys.PublicKey {
+func (clb *testCallbacks) GetPublicKeyForID(ss *SecureSession, id []byte) *keys.PublicKey {
 	if bytes.Equal(clientID, id) {
 		return clb.a.Public
 	} else if bytes.Equal(serverID, id) {