Host extensions

This CL introduces HostExtension, an interface for classes that
extend the host with non-core functionality.

Extensions are added to the ChromotingHost. They are used to compile
the list of capabilities reported to the client, which can be used by
the client to determine the availability of the extension.

When a client connects, a HostExtension has the opportunity to create
an HostExtensionSession to hold client/extension state, and to handle
extension messages from that client.

BUG=

NOTRY=true

Review URL: https://codereview.chromium.org/301453003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@273901 0039d316-1c4b-4281-b951-d872f2087c98

10 files changed, 366 insertions, 5 deletions

diff --git a/remoting/host/chromoting_host.cc b/remoting/host/chromoting_host.cc
index ef2ea19..942e643 100644
--- a/remoting/host/chromoting_host.cc
+++ b/remoting/host/chromoting_host.cc
@@ -141,6 +141,10 @@ void ChromotingHost::RemoveStatusObserver(HostStatusObserver* observer) {
   status_observers_.RemoveObserver(observer);
 }
 
+void ChromotingHost::AddExtension(scoped_ptr<HostExtension> extension) {
+  extensions_.push_back(extension.release());
+}
+
 void ChromotingHost::RejectAuthenticatingClient() {
   DCHECK(authenticating_client_);
   reject_authenticating_client_ = true;
@@ -230,6 +234,19 @@ void ChromotingHost::OnSessionChannelsConnected(ClientSession* client) {
                     OnClientConnected(client->client_jid()));
 }
 
+void ChromotingHost::OnSessionClientCapabilities(ClientSession* client) {
+  DCHECK(CalledOnValidThread());
+
+  // Create extension sessions from each registered extension for this client.
+  for (HostExtensionList::iterator extension = extensions_.begin();
+       extension != extensions_.end(); ++extension) {
+    scoped_ptr<HostExtensionSession> extension_session =
+        (*extension)->CreateExtensionSession(client);
+    if (extension_session)
+      client->AddExtensionSession(extension_session.Pass());
+  }
+}
+
 void ChromotingHost::OnSessionAuthenticationFailed(ClientSession* client) {
   DCHECK(CalledOnValidThread());
 
@@ -320,6 +337,13 @@ void ChromotingHost::OnIncomingSession(
       desktop_environment_factory_,
       max_session_duration_,
       pairing_registry_);
+
+  // Registers capabilities provided by host extensions.
+  for (HostExtensionList::iterator extension = extensions_.begin();
+       extension != extensions_.end(); ++extension) {
+    client->AddHostCapabilities((*extension)->GetCapabilities());
+  }
+
   clients_.push_back(client);
 }
 
diff --git a/remoting/host/chromoting_host.h b/remoting/host/chromoting_host.h
index a6642d4..73e8442 100644
--- a/remoting/host/chromoting_host.h
+++ b/remoting/host/chromoting_host.h
@@ -8,14 +8,16 @@
 #include <list>
 #include <string>
 
-#include "base/memory/scoped_ptr.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/memory/scoped_vector.h"
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
 #include "base/threading/non_thread_safe.h"
 #include "base/threading/thread.h"
 #include "net/base/backoff_entry.h"
 #include "remoting/host/client_session.h"
+#include "remoting/host/host_extension.h"
 #include "remoting/host/host_status_monitor.h"
 #include "remoting/host/host_status_observer.h"
 #include "remoting/protocol/authenticator.h"
@@ -91,6 +93,9 @@ class ChromotingHost : public base::NonThreadSafe,
   virtual void AddStatusObserver(HostStatusObserver* observer) OVERRIDE;
   virtual void RemoveStatusObserver(HostStatusObserver* observer) OVERRIDE;
 
+  // Registers a host extension.
+  void AddExtension(scoped_ptr<HostExtension> extension);
+
   // This method may be called only from
   // HostStatusObserver::OnClientAuthenticated() to reject the new
   // client.
@@ -118,6 +123,7 @@ class ChromotingHost : public base::NonThreadSafe,
   virtual void OnSessionAuthenticating(ClientSession* client) OVERRIDE;
   virtual bool OnSessionAuthenticated(ClientSession* client) OVERRIDE;
   virtual void OnSessionChannelsConnected(ClientSession* client) OVERRIDE;
+  virtual void OnSessionClientCapabilities(ClientSession* client) OVERRIDE;
   virtual void OnSessionAuthenticationFailed(ClientSession* client) OVERRIDE;
   virtual void OnSessionClosed(ClientSession* session) OVERRIDE;
   virtual void OnSessionSequenceNumber(ClientSession* session,
@@ -154,6 +160,7 @@ class ChromotingHost : public base::NonThreadSafe,
   friend class ChromotingHostTest;
 
   typedef std::list<ClientSession*> ClientList;
+  typedef ScopedVector<HostExtension> HostExtensionList;
 
   // Immediately disconnects all active clients. Host-internal components may
   // shutdown asynchronously, but the caller is guaranteed not to receive
@@ -204,6 +211,9 @@ class ChromotingHost : public base::NonThreadSafe,
   // The pairing registry for PIN-less authentication.
   scoped_refptr<protocol::PairingRegistry> pairing_registry_;
 
+  // List of host extensions.
+  HostExtensionList extensions_;
+
   base::WeakPtrFactory<ChromotingHost> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ChromotingHost);
diff --git a/remoting/host/client_session.cc b/remoting/host/client_session.cc
index 2db968e5..55e4dec 100644
--- a/remoting/host/client_session.cc
+++ b/remoting/host/client_session.cc
@@ -18,6 +18,7 @@
 #include "remoting/host/audio_capturer.h"
 #include "remoting/host/audio_scheduler.h"
 #include "remoting/host/desktop_environment.h"
+#include "remoting/host/host_extension_session.h"
 #include "remoting/host/input_injector.h"
 #include "remoting/host/screen_controls.h"
 #include "remoting/host/screen_resolution.h"
@@ -98,6 +99,25 @@ ClientSession::~ClientSession() {
   connection_.reset();
 }
 
+void ClientSession::AddExtensionSession(
+    scoped_ptr<HostExtensionSession> extension_session) {
+  DCHECK(CalledOnValidThread());
+
+  extension_sessions_.push_back(extension_session.release());
+}
+
+void ClientSession::AddHostCapabilities(const std::string& capabilities) {
+  DCHECK(CalledOnValidThread());
+
+  if (capabilities.empty())
+    return;
+
+  if (!host_capabilities_.empty())
+    host_capabilities_.append(" ");
+
+  host_capabilities_.append(capabilities);
+}
+
 void ClientSession::NotifyClientResolution(
     const protocol::ClientResolution& resolution) {
   DCHECK(CalledOnValidThread());
@@ -178,6 +198,7 @@ void ClientSession::SetCapabilities(
     *client_capabilities_ = capabilities.capabilities();
 
   VLOG(1) << "Client capabilities: " << *client_capabilities_;
+  event_handler_->OnSessionClientCapabilities(this);
 
   // Calculate the set of capabilities enabled by both client and host and
   // pass it to the desktop environment if it is available.
@@ -214,6 +235,13 @@ void ClientSession::DeliverClientMessage(
         HOST_LOG << "gnubby auth is not enabled";
       }
       return;
+    } else {
+      for(HostExtensionSessionList::iterator it = extension_sessions_.begin();
+          it != extension_sessions_.end(); ++it) {
+        // Extension returns |true| to indicate that the message was handled.
+        if ((*it)->OnExtensionMessage(this, message))
+          return;
+      }
     }
   }
   HOST_LOG << "Unexpected message received: "
@@ -263,7 +291,7 @@ void ClientSession::OnConnectionAuthenticated(
     return;
   }
 
-  host_capabilities_ = desktop_environment_->GetCapabilities();
+  AddHostCapabilities(desktop_environment_->GetCapabilities());
 
   // Ignore protocol::Capabilities messages from the client if it does not
   // support any capabilities.
@@ -271,6 +299,8 @@ void ClientSession::OnConnectionAuthenticated(
     VLOG(1) << "The client does not support any capabilities.";
 
     client_capabilities_ = make_scoped_ptr(new std::string());
+    event_handler_->OnSessionClientCapabilities(this);
+
     desktop_environment_->SetCapabilities(*client_capabilities_);
   }
 
diff --git a/remoting/host/client_session.h b/remoting/host/client_session.h
index f892329..a86839e 100644
--- a/remoting/host/client_session.h
+++ b/remoting/host/client_session.h
@@ -8,6 +8,7 @@
 #include <string>
 
 #include "base/memory/ref_counted.h"
+#include "base/memory/scoped_vector.h"
 #include "base/memory/weak_ptr.h"
 #include "base/sequenced_task_runner_helpers.h"
 #include "base/threading/non_thread_safe.h"
@@ -15,6 +16,8 @@
 #include "base/timer/timer.h"
 #include "remoting/host/client_session_control.h"
 #include "remoting/host/gnubby_auth_handler.h"
+#include "remoting/host/host_extension.h"
+#include "remoting/host/host_extension_session.h"
 #include "remoting/host/mouse_clamping_filter.h"
 #include "remoting/host/remote_input_filter.h"
 #include "remoting/protocol/clipboard_echo_filter.h"
@@ -64,6 +67,9 @@ class ClientSession
     // Called after we've finished connecting all channels.
     virtual void OnSessionChannelsConnected(ClientSession* client) = 0;
 
+    // Called after client has reported capabilities.
+    virtual void OnSessionClientCapabilities(ClientSession* client) = 0;
+
     // Called after authentication has failed. Must not tear down this
     // object. OnSessionClosed() is notified after this handler
     // returns.
@@ -103,6 +109,13 @@ class ClientSession
       scoped_refptr<protocol::PairingRegistry> pairing_registry);
   virtual ~ClientSession();
 
+  // Adds an extension to client to handle extension messages.
+  void AddExtensionSession(scoped_ptr<HostExtensionSession> extension_session);
+
+  // Adds extended capabilities to advertise to the client, e.g. those
+  // implemented by |DesktopEnvironment| or |HostExtension|s.
+  void AddHostCapabilities(const std::string& capability);
+
   // protocol::HostStub interface.
   virtual void NotifyClientResolution(
       const protocol::ClientResolution& resolution) OVERRIDE;
@@ -148,7 +161,13 @@ class ClientSession
 
   bool is_authenticated() { return auth_input_filter_.enabled();  }
 
+  const std::string* client_capabilities() const {
+    return client_capabilities_.get();
+  }
+
  private:
+  typedef ScopedVector<HostExtensionSession> HostExtensionSessionList;
+
   // Creates a proxy for sending clipboard events to the client.
   scoped_ptr<protocol::ClipboardStub> CreateClipboardProxy();
 
@@ -244,6 +263,9 @@ class ClientSession
   // Used to proxy gnubby auth traffic.
   scoped_ptr<GnubbyAuthHandler> gnubby_auth_handler_;
 
+  // Host extension sessions, used to handle extension messages.
+  HostExtensionSessionList extension_sessions_;
+
   DISALLOW_COPY_AND_ASSIGN(ClientSession);
 };
 
diff --git a/remoting/host/client_session_unittest.cc b/remoting/host/client_session_unittest.cc
index f28050a..d648ba1 100644
--- a/remoting/host/client_session_unittest.cc
+++ b/remoting/host/client_session_unittest.cc
@@ -2,17 +2,24 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <algorithm>
+#include <string>
+#include <vector>
+
 #include "base/message_loop/message_loop.h"
+#include "base/strings/string_util.h"
 #include "base/test/test_simple_task_runner.h"
 #include "remoting/base/auto_thread_task_runner.h"
 #include "remoting/base/constants.h"
 #include "remoting/host/audio_capturer.h"
 #include "remoting/host/client_session.h"
 #include "remoting/host/desktop_environment.h"
+#include "remoting/host/host_extension.h"
 #include "remoting/host/host_mock_objects.h"
 #include "remoting/host/screen_capturer_fake.h"
 #include "remoting/protocol/protocol_mock_objects.h"
 #include "testing/gmock/include/gmock/gmock-matchers.h"
+#include "testing/gmock_mutant.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/webrtc/modules/desktop_capture/desktop_geometry.h"
 #include "third_party/webrtc/modules/desktop_capture/desktop_region.h"
@@ -31,9 +38,11 @@ using protocol::SessionConfig;
 using testing::_;
 using testing::AnyNumber;
 using testing::AtMost;
+using testing::CreateFunctor;
 using testing::DeleteArg;
 using testing::DoAll;
 using testing::Expectation;
+using testing::Invoke;
 using testing::Return;
 using testing::ReturnRef;
 using testing::Sequence;
@@ -42,6 +51,8 @@ using testing::StrictMock;
 
 namespace {
 
+const char kDefaultTestCapability[] = "default";
+
 ACTION_P2(InjectClipboardEvent, connection, event) {
   connection->clipboard_stub()->InjectClipboardEvent(event);
 }
@@ -67,8 +78,75 @@ ACTION_P2(DeliverClientMessage, client_session, message) {
   client_session->DeliverClientMessage(message);
 }
 
+ACTION_P2(AddHostCapabilities, client_session, capability) {
+  client_session->AddHostCapabilities(capability);
 }
 
+// Matches a |protocol::Capabilities| argument against a list of capabilities
+// formatted as a space-separated string.
+MATCHER_P(EqCapabilities, expected_capabilities, "") {
+  if (!arg.has_capabilities())
+    return false;
+
+  std::vector<std::string> words_args;
+  std::vector<std::string> words_expected;
+  Tokenize(arg.capabilities(), " ", &words_args);
+  Tokenize(expected_capabilities, " ", &words_expected);
+  std::sort(words_args.begin(), words_args.end());
+  std::sort(words_expected.begin(), words_expected.end());
+  return words_args == words_expected;
+}
+
+// |HostExtension| implementation that can handle an extension message type and
+// provide capabilities.
+class FakeExtension : public HostExtension {
+ public:
+  FakeExtension(const std::string& message_type,
+                const std::string& capabilities);
+  virtual ~FakeExtension();
+
+  virtual std::string GetCapabilities() OVERRIDE;
+  virtual scoped_ptr<HostExtensionSession> CreateExtensionSession(
+      ClientSession* client_session) OVERRIDE;
+
+  bool message_handled() {
+    return message_handled_;
+  }
+
+ private:
+  class FakeExtensionSession : public HostExtensionSession {
+   public:
+    FakeExtensionSession(FakeExtension* extension);
+    virtual ~FakeExtensionSession();
+
+    virtual bool OnExtensionMessage(
+        ClientSession* client_session,
+        const protocol::ExtensionMessage& message) OVERRIDE;
+
+   private:
+    FakeExtension* extension_;
+  };
+
+  std::string message_type_;
+  std::string capabilities_;
+  bool message_handled_;
+};
+
+typedef std::vector<HostExtension*> HostExtensionList;
+
+void CreateExtensionSessions(const HostExtensionList& extensions,
+                             ClientSession* client_session) {
+  for (HostExtensionList::const_iterator extension = extensions.begin();
+       extension != extensions.end(); ++extension) {
+    scoped_ptr<HostExtensionSession> extension_session =
+        (*extension)->CreateExtensionSession(client_session);
+    if (extension_session)
+      client_session->AddExtensionSession(extension_session.Pass());
+  }
+}
+
+}  // namespace
+
 class ClientSessionTest : public testing::Test {
  public:
   ClientSessionTest() : client_jid_("user@domain/rest-of-jid") {}
@@ -100,6 +178,10 @@ class ClientSessionTest : public testing::Test {
   // the input pipe line and starts video capturing.
   void ConnectClientSession();
 
+  // Creates expectations to send an extension message and to disconnect
+  // afterwards.
+  void SetSendMessageAndDisconnectExpectation(const std::string& message_type);
+
   // Invoked when the last reference to the AutoThreadTaskRunner has been
   // released and quits the message loop to finish the test.
   void QuitMainMessageLoop();
@@ -131,6 +213,41 @@ class ClientSessionTest : public testing::Test {
   scoped_ptr<MockDesktopEnvironmentFactory> desktop_environment_factory_;
 };
 
+FakeExtension::FakeExtension(const std::string& message_type,
+                             const std::string& capabilities)
+    : message_type_(message_type),
+      capabilities_(capabilities),
+      message_handled_(false) {
+}
+
+FakeExtension::~FakeExtension() {}
+
+std::string FakeExtension::GetCapabilities() {
+  return capabilities_;
+}
+
+scoped_ptr<HostExtensionSession> FakeExtension::CreateExtensionSession(
+    ClientSession* client_session) {
+  return scoped_ptr<HostExtensionSession>(new FakeExtensionSession(this));
+}
+
+FakeExtension::FakeExtensionSession::FakeExtensionSession(
+    FakeExtension* extension)
+    : extension_(extension) {
+}
+
+FakeExtension::FakeExtensionSession::~FakeExtensionSession() {}
+
+bool FakeExtension::FakeExtensionSession::OnExtensionMessage(
+    ClientSession* client_session,
+    const protocol::ExtensionMessage& message) {
+  if (message.type() == extension_->message_type_) {
+    extension_->message_handled_ = true;
+    return true;
+  }
+  return false;
+}
+
 void ClientSessionTest::SetUp() {
   // Arrange to run |message_loop_| until no components depend on it.
   scoped_refptr<AutoThreadTaskRunner> ui_task_runner = new AutoThreadTaskRunner(
@@ -180,6 +297,11 @@ void ClientSessionTest::SetUp() {
       desktop_environment_factory_.get(),
       base::TimeDelta(),
       NULL));
+
+  // By default, client will report the same capabilities as the host.
+  EXPECT_CALL(client_stub_, SetCapabilities(_))
+      .Times(AtMost(1))
+      .WillOnce(Invoke(client_session_.get(), &ClientSession::SetCapabilities));
 }
 
 void ClientSessionTest::TearDown() {
@@ -211,7 +333,8 @@ DesktopEnvironment* ClientSessionTest::CreateDesktopEnvironment() {
   EXPECT_CALL(*desktop_environment, CreateVideoCapturerPtr())
       .WillOnce(Invoke(this, &ClientSessionTest::CreateVideoCapturer));
   EXPECT_CALL(*desktop_environment, GetCapabilities())
-      .Times(AtMost(1));
+      .Times(AtMost(1))
+       .WillOnce(Return(kDefaultTestCapability));
   EXPECT_CALL(*desktop_environment, SetCapabilities(_))
       .Times(AtMost(1));
 
@@ -232,6 +355,23 @@ void ClientSessionTest::ConnectClientSession() {
   client_session_->OnConnectionChannelsConnected(client_session_->connection());
 }
 
+void ClientSessionTest::SetSendMessageAndDisconnectExpectation(
+    const std::string& message_type) {
+  protocol::ExtensionMessage message;
+  message.set_type(message_type);
+  message.set_data("data");
+
+  Expectation authenticated =
+      EXPECT_CALL(session_event_handler_, OnSessionAuthenticated(_))
+      .WillOnce(Return(true));
+  EXPECT_CALL(session_event_handler_, OnSessionChannelsConnected(_))
+      .After(authenticated)
+      .WillOnce(DoAll(
+          DeliverClientMessage(client_session_.get(), message),
+          InvokeWithoutArgs(this, &ClientSessionTest::DisconnectClientSession),
+          InvokeWithoutArgs(this, &ClientSessionTest::StopClientSession)));
+}
+
 void ClientSessionTest::QuitMainMessageLoop() {
   message_loop_.PostTask(FROM_HERE, base::MessageLoop::QuitClosure());
 }
@@ -599,4 +739,63 @@ TEST_F(ClientSessionTest, EnableGnubbyAuth) {
   message_loop_.Run();
 }
 
+// Verifies that messages can be handled by extensions.
+TEST_F(ClientSessionTest, ExtensionMessages_MessageHandled) {
+  FakeExtension extension1("ext1", "cap1");
+  FakeExtension extension2("ext2", "cap2");
+  FakeExtension extension3("ext3", "cap3");
+  HostExtensionList extensions;
+  extensions.push_back(&extension1);
+  extensions.push_back(&extension2);
+  extensions.push_back(&extension3);
+
+  EXPECT_CALL(session_event_handler_, OnSessionClientCapabilities(_))
+      .WillOnce(Invoke(CreateFunctor(&CreateExtensionSessions, extensions)));
+
+  SetSendMessageAndDisconnectExpectation("ext2");
+  ConnectClientSession();
+  message_loop_.Run();
+
+  EXPECT_FALSE(extension1.message_handled());
+  EXPECT_TRUE(extension2.message_handled());
+  EXPECT_FALSE(extension3.message_handled());
+}
+
+// Verifies that extension messages not handled by extensions don't result in a
+// crash.
+TEST_F(ClientSessionTest, ExtensionMessages_MessageNotHandled) {
+  FakeExtension extension1("ext1", "cap1");
+  HostExtensionList extensions;
+  extensions.push_back(&extension1);
+
+  EXPECT_CALL(session_event_handler_, OnSessionClientCapabilities(_))
+      .WillOnce(Invoke(CreateFunctor(&CreateExtensionSessions, extensions)));
+
+  SetSendMessageAndDisconnectExpectation("extX");
+  ConnectClientSession();
+  message_loop_.Run();
+
+  EXPECT_FALSE(extension1.message_handled());
+}
+
+TEST_F(ClientSessionTest, ReportCapabilities) {
+  Expectation authenticated =
+      EXPECT_CALL(session_event_handler_, OnSessionAuthenticated(_))
+      .WillOnce(DoAll(
+          AddHostCapabilities(client_session_.get(), "capX capZ"),
+          AddHostCapabilities(client_session_.get(), ""),
+          AddHostCapabilities(client_session_.get(), "capY"),
+          Return(true)));
+  EXPECT_CALL(client_stub_,
+              SetCapabilities(EqCapabilities("capX capY capZ default")));
+  EXPECT_CALL(session_event_handler_, OnSessionChannelsConnected(_))
+      .After(authenticated)
+      .WillOnce(DoAll(
+          InvokeWithoutArgs(this, &ClientSessionTest::DisconnectClientSession),
+          InvokeWithoutArgs(this, &ClientSessionTest::StopClientSession)));
+
+  ConnectClientSession();
+  message_loop_.Run();
+}
+
 }  // namespace remoting
diff --git a/remoting/host/host_extension.h b/remoting/host/host_extension.h
new file mode 100644
index 0000000..6014f61
--- /dev/null
+++ b/remoting/host/host_extension.h
@@ -0,0 +1,40 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef REMOTING_HOST_HOST_EXTENSION_H_
+#define REMOTING_HOST_HOST_EXTENSION_H_
+
+#include <string>
+
+#include "base/memory/scoped_ptr.h"
+
+namespace remoting {
+
+class ClientSession;
+class HostExtensionSession;
+
+// Extends |ChromotingHost| with new functionality, and can use extension
+// messages to communicate with the client.
+class HostExtension {
+ public:
+  virtual ~HostExtension() {}
+
+  // Returns a space-separated list of capabilities provided by this extension.
+  // Capabilities may be used to inform the client of the availability of an
+  // extension. They are merged into the capabilities the host reports to the
+  // client.
+  virtual std::string GetCapabilities() = 0;
+
+  // Creates an extension session, which can handle extension messages for a
+  // client session.
+  // NULL may be returned if |client_session| cannot support this
+  // extension.
+  // |client_session| must outlive the resulting |HostExtensionSession|.
+  virtual scoped_ptr<HostExtensionSession> CreateExtensionSession(
+      ClientSession* client_session) = 0;
+};
+
+}  // namespace remoting
+
+#endif  // REMOTING_HOST_HOST_EXTENSION_H_
diff --git a/remoting/host/host_extension_session.h b/remoting/host/host_extension_session.h
new file mode 100644
index 0000000..54b9c53
--- /dev/null
+++ b/remoting/host/host_extension_session.h
@@ -0,0 +1,33 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef REMOTING_HOST_HOST_EXTENSION_SESSION_H_
+#define REMOTING_HOST_HOST_EXTENSION_SESSION_H_
+
+namespace remoting {
+
+namespace protocol {
+class ExtensionMessage;
+}  // namespace protocol
+
+class ClientSession;
+
+// Created by an |HostExtension| to store |ClientSession| specific state, and to
+// handle extension messages.
+class HostExtensionSession {
+ public:
+  virtual ~HostExtensionSession() {}
+
+  // Called when the host receives an |ExtensionMessage| for the |ClientSession|
+  // associated with this |HostExtensionSession|.
+  // It returns |true| if the message was handled, and |false| otherwise.
+  virtual bool OnExtensionMessage(
+      ClientSession* client_session,
+      const protocol::ExtensionMessage& message) = 0;
+};
+
+}  // namespace remoting
+
+#endif  // REMOTING_HOST_HOST_EXTENSION_SESSION_H_
+
diff --git a/remoting/host/host_mock_objects.h b/remoting/host/host_mock_objects.h
index d916f04..383f474 100644
--- a/remoting/host/host_mock_objects.h
+++ b/remoting/host/host_mock_objects.h
@@ -71,6 +71,7 @@ class MockClientSessionEventHandler : public ClientSession::EventHandler {
   MOCK_METHOD1(OnSessionAuthenticating, void(ClientSession* client));
   MOCK_METHOD1(OnSessionAuthenticated, bool(ClientSession* client));
   MOCK_METHOD1(OnSessionChannelsConnected, void(ClientSession* client));
+  MOCK_METHOD1(OnSessionClientCapabilities, void(ClientSession* client));
   MOCK_METHOD1(OnSessionAuthenticationFailed, void(ClientSession* client));
   MOCK_METHOD1(OnSessionClosed, void(ClientSession* client));
   MOCK_METHOD2(OnSessionSequenceNumber, void(ClientSession* client,
diff --git a/remoting/protocol/session_config.cc b/remoting/protocol/session_config.cc
index 3ce7727..1728a80 100644
--- a/remoting/protocol/session_config.cc
+++ b/remoting/protocol/session_config.cc
@@ -49,7 +49,7 @@ bool SessionConfig::SupportsCapabilities() const {
 SessionConfig SessionConfig::ForTest() {
   SessionConfig result;
   result.set_control_config(ChannelConfig(ChannelConfig::TRANSPORT_MUX_STREAM,
-                                          kControlStreamVersionNoCapabilities,
+                                          kControlStreamVersion,
                                           ChannelConfig::CODEC_UNDEFINED));
   result.set_event_config(ChannelConfig(ChannelConfig::TRANSPORT_MUX_STREAM,
                                         kDefaultStreamVersion,
diff --git a/remoting/remoting_host.gypi b/remoting/remoting_host.gypi
index dc4cdc6..e5247ea 100644
--- a/remoting/remoting_host.gypi
+++ b/remoting/remoting_host.gypi
@@ -119,10 +119,10 @@
             'host/disconnect_window_win.cc',
             'host/dns_blackhole_checker.cc',
             'host/dns_blackhole_checker.h',
+            'host/gnubby_auth_handler.h',
             'host/gnubby_auth_handler_posix.cc',
             'host/gnubby_auth_handler_posix.h',
             'host/gnubby_auth_handler_win.cc',
-            'host/gnubby_auth_handler.h',
             'host/gnubby_socket.cc',
             'host/gnubby_socket.h',
             'host/heartbeat_sender.cc',
@@ -137,6 +137,8 @@
             'host/host_exit_codes.cc',
             'host/host_exit_codes.h',
             'host/host_export.h',
+            'host/host_extension.h',
+            'host/host_extension_session.h',
             'host/host_secret.cc',
             'host/host_secret.h',
             'host/host_status_monitor.h',