diff --git a/autotest/client_autotest/client_autotest.cc b/autotest/client_autotest/client_autotest.cc
index dadfe83..400f8e7 100644
--- a/autotest/client_autotest/client_autotest.cc
+++ b/autotest/client_autotest/client_autotest.cc
@@ -297,7 +297,8 @@ TEST(ClientTest, Post) {
 
 static webcc::fs::path GenerateTempFile(const std::string& data) {
   try {
-    webcc::fs::path path = webcc::fs::temp_directory_path() / webcc::RandomString(10);
+    webcc::fs::path path =
+        webcc::fs::temp_directory_path() / webcc::RandomString(10);
 
     webcc::fs::ofstream ofs;
     ofs.open(path, std::ios::binary);
diff --git a/autotest/client_timeout_autotest/client_timeout_autotest.cc b/autotest/client_timeout_autotest/client_timeout_autotest.cc
index 6f8f009..7207d84 100644
--- a/autotest/client_timeout_autotest/client_timeout_autotest.cc
+++ b/autotest/client_timeout_autotest/client_timeout_autotest.cc
@@ -101,3 +101,28 @@ TEST_F(ClientTimeoutTest, Timeout) {
   EXPECT_TRUE(!r);
   EXPECT_TRUE(timeout);
 }
+
+// Test ClientSession::Cancel()
+TEST_F(ClientTimeoutTest, SessionCancel) {
+  webcc::ClientSession session;
+  session.set_read_timeout(30);
+
+  bool canceled = false;
+
+  // Create a thread to cancel the session after 3 seconds.
+  std::thread t{ [&session, &canceled]() {
+    std::this_thread::sleep_for(std::chrono::seconds(3));
+    canceled = session.Cancel();
+  } };
+
+  // Send a request and ask the server to sleep 5 seconds before reply.
+  try {
+    auto r = session.Send(WEBCC_GET("http://localhost/sleep/5").Port(kPort)());
+  } catch (const webcc::Error&) {
+  }
+
+  t.join();
+
+  // The request should be canceled.
+  EXPECT_TRUE(canceled);
+}
diff --git a/webcc/client.cc b/webcc/client.cc
index 77eae82..4ddd25c 100644
--- a/webcc/client.cc
+++ b/webcc/client.cc
@@ -15,7 +15,6 @@ Client::Client(boost::asio::io_context& io_context,
       ssl_context_(ssl_context),
       resolver_(io_context),
       deadline_timer_(io_context) {
-
 }
 
 #else
@@ -75,29 +74,28 @@ Error Client::Request(RequestPtr request, bool stream) {
 }
 
 void Client::Close() {
-  if (!connected_) {
-    //resolver_.cancel();  // TODO
+  DoClose();
+
+  // Don't call FinishRequest() from here! It will be called in the handler
+  // OnXxx with `error::operation_aborted`.
+}
+
+void Client::DoClose() {
+  if (connected_) {
+    connected_ = false;
     if (socket_) {
-      // Cancel any async operations on the socket.
-      LOG_VERB("Close socket");
+      LOG_VERB("Shutdown & close socket");
+      socket_->Shutdown();
+      socket_->Close();
+    }
+    LOG_INFO("Socket closed");
+  } else {
+    // TODO: resolver_.cancel() ?
+    if (socket_) {
+      LOG_INFO("Close socket");
       socket_->Close();
-      // Make sure the current request, if any, could be finished.
-      FinishRequest();
     }
-    return;
-  }
-
-  connected_ = false;
-
-  if (socket_) {
-    LOG_INFO("Shutdown & close socket");
-    socket_->Shutdown();
-    socket_->Close();
-    // Make sure the current request, if any, could be finished.
-    FinishRequest();
   }
-
-  LOG_INFO("Socket closed");
 }
 
 void Client::AsyncConnect() {
@@ -153,7 +151,7 @@ void Client::OnConnect(boost::system::error_code ec, tcp::endpoint) {
 
   if (ec) {
     if (ec == boost::asio::error::operation_aborted) {
-      // Socket has been closed by OnDeadlineTimer() or Close().
+      // Socket has been closed by OnDeadlineTimer() or DoClose().
       LOG_WARN("Connect operation aborted");
     } else {
       LOG_INFO("Connect error");
@@ -219,11 +217,11 @@ void Client::OnWriteBody(boost::system::error_code ec, std::size_t legnth) {
 
 void Client::HandleWriteError(boost::system::error_code ec) {
   if (ec == boost::asio::error::operation_aborted) {
-    // Socket has been closed by OnDeadlineTimer() or Close().
+    // Socket has been closed by OnDeadlineTimer() or DoClose().
     LOG_WARN("Write operation aborted");
   } else {
     LOG_ERRO("Socket write error (%s)", ec.message().c_str());
-    Close();
+    DoClose();
   }
 
   error_.Set(Error::kSocketWriteError, "Socket write error");
@@ -239,11 +237,11 @@ void Client::OnRead(boost::system::error_code ec, std::size_t length) {
 
   if (ec) {
     if (ec == boost::asio::error::operation_aborted) {
-      // Socket has been closed by OnDeadlineTimer() or Close().
+      // Socket has been closed by OnDeadlineTimer() or DoClose().
       LOG_WARN("Read operation aborted");
     } else {
       LOG_ERRO("Socket read error (%s)", ec.message().c_str());
-      Close();
+      DoClose();
     }
 
     error_.Set(Error::kSocketReadError, "Socket read error");
@@ -258,7 +256,7 @@ void Client::OnRead(boost::system::error_code ec, std::size_t length) {
   // Parse the piece of data just read.
   if (!response_parser_.Parse(buffer_.data(), length)) {
     LOG_ERRO("Failed to parse the response");
-    Close();
+    DoClose();
     error_.Set(Error::kParseError, "Response parse error");
     FinishRequest();
     return;
@@ -279,7 +277,7 @@ void Client::OnRead(boost::system::error_code ec, std::size_t length) {
     if (response_->IsConnectionKeepAlive()) {
       LOG_INFO("Keep the socket connection alive");
     } else {
-      Close();
+      DoClose();
     }
 
     // Stop trying to read once all content has been received, because some
@@ -324,7 +322,7 @@ void Client::OnDeadlineTimer(boost::system::error_code ec) {
   // Cancel the async operations on the socket.
   // OnXxx() will be called with `error::operation_aborted`.
   if (connected_) {
-    Close();
+    DoClose();
   } else {
     socket_->Close();
   }
@@ -342,22 +340,24 @@ void Client::StopDeadlineTimer() {
   try {
     // Cancel the async wait operation on this timer.
     deadline_timer_.cancel();
-  } catch (const boost::system::system_error&) {
+  } catch (const boost::system::system_error& e) {
+    LOG_ERRO("Deadline timer cancel error: %s", e.what());
   }
 
   deadline_timer_stopped_ = true;
 }
 
 void Client::FinishRequest() {
-  {
-    std::lock_guard<std::mutex> lock{ request_mutex_ };
-    if (!request_finished_) {
-      request_finished_ = true;
-    } else {
-      return;
-    }
+  request_mutex_.lock();
+
+  if (!request_finished_) {
+    request_finished_ = true;
+
+    request_mutex_.unlock();
+    request_cv_.notify_one();
+  } else {
+    request_mutex_.unlock();
   }
-  request_cv_.notify_one();
 }
 
 }  // namespace webcc
diff --git a/webcc/client.h b/webcc/client.h
index 14997c5..f94cb9b 100644
--- a/webcc/client.h
+++ b/webcc/client.h
@@ -88,6 +88,8 @@ public:
   }
 
 private:
+  void DoClose();
+
   void AsyncConnect();
 
   void AsyncResolve(const std::string& default_port);
diff --git a/webcc/client_session.cc b/webcc/client_session.cc
index 6967818..83309ad 100644
--- a/webcc/client_session.cc
+++ b/webcc/client_session.cc
@@ -211,10 +211,12 @@ ResponsePtr ClientSession::Send(RequestPtr request, bool stream,
   return DoSend(request, stream, callback);
 }
 
-void ClientSession::Cancel() {
+bool ClientSession::Cancel() {
   if (client_) {
     client_->Close();
+    return true;
   }
+  return false;
 }
 
 void ClientSession::InitHeaders() {
diff --git a/webcc/client_session.h b/webcc/client_session.h
index cc5af47..a545a21 100644
--- a/webcc/client_session.h
+++ b/webcc/client_session.h
@@ -96,7 +96,9 @@ public:
                    ProgressCallback callback = {});
 
   // Cancel any in-progress connecting, writing or reading.
-  void Cancel();
+  // Return if any client object has been closed.
+  // It could be used to exit the program as soon as possible.
+  bool Cancel();
 
 private:
   void InitHeaders();
diff --git a/webcc/globals.h b/webcc/globals.h
index 5421c65..7175898 100644
--- a/webcc/globals.h
+++ b/webcc/globals.h
@@ -109,6 +109,7 @@ const char* const kAuthorization = "Authorization";
 const char* const kContentType = "Content-Type";
 const char* const kContentLength = "Content-Length";
 const char* const kContentEncoding = "Content-Encoding";
+const char* const kContentMD5 = "Content-MD5";
 const char* const kContentDisposition = "Content-Disposition";
 const char* const kConnection = "Connection";
 const char* const kTransferEncoding = "Transfer-Encoding";
@@ -168,7 +169,7 @@ public:
   };
 
 public:
-  Error(Code code = kOK, const std::string& message = "")
+  explicit Error(Code code = kOK, const std::string& message = "")
       : code_(code), message_(message), timeout_(false) {
   }