Set canceled flag in task's context object so that we cancel out of active tasks when they try to resume. Wrap a try-catch block around tcp and udp resolve requests to convert any non-fc exceptions to fc exceptions. Minor change to clarify design intent that request_time_task should always be running in _ntp_thread.

include/fc/thread/future.hpp

@@ -58,7 +58,7 @@ namespace fc {
 
       const char* get_desc()const;
                    
-      void cancel();
+      virtual void cancel();
       bool canceled()const { return _canceled; }
       bool ready()const;
       bool error()const;

include/fc/thread/task.hpp

@@ -10,7 +10,9 @@ namespace fc {
 
   class task_base : virtual public promise_base {
     public:
-      void        run(); 
+              void run(); 
+      virtual void cancel() override;
+
     protected:
       ~task_base();
       /// Task priority looks like unsupported feature.

src/asio.cpp

@@ -118,22 +118,32 @@ namespace fc {
     }
 
     namespace tcp {
-        std::vector<boost::asio::ip::tcp::endpoint> resolve( const std::string& hostname, const std::string& port) {
-            resolver res( fc::asio::default_io_service() );
-            promise<std::vector<boost::asio::ip::tcp::endpoint> >::ptr p( new promise<std::vector<boost::asio::ip::tcp::endpoint> >("tcp::resolve completion") );
-            res.async_resolve( boost::asio::ip::tcp::resolver::query(hostname,port), 
-                             boost::bind( detail::resolve_handler<boost::asio::ip::tcp::endpoint,resolver_iterator>, p, _1, _2 ) );
-            return p->wait();;
+      std::vector<boost::asio::ip::tcp::endpoint> resolve( const std::string& hostname, const std::string& port) 
+      {
+        try 
+        {
+          resolver res( fc::asio::default_io_service() );
+          promise<std::vector<boost::asio::ip::tcp::endpoint> >::ptr p( new promise<std::vector<boost::asio::ip::tcp::endpoint> >("tcp::resolve completion") );
+          res.async_resolve( boost::asio::ip::tcp::resolver::query(hostname,port), 
+                            boost::bind( detail::resolve_handler<boost::asio::ip::tcp::endpoint,resolver_iterator>, p, _1, _2 ) );
+          return p->wait();;
         }
+        FC_RETHROW_EXCEPTIONS(warn, "")
+      }
     }
     namespace udp {
-                std::vector<udp::endpoint> resolve( resolver& r, const std::string& hostname, const std::string& port) {
-                resolver res( fc::asio::default_io_service() );
-                promise<std::vector<endpoint> >::ptr p( new promise<std::vector<endpoint> >("udp::resolve completion") );
-                res.async_resolve( resolver::query(hostname,port), 
-                                    boost::bind( detail::resolve_handler<endpoint,resolver_iterator>, p, _1, _2 ) );
-                return p->wait();
+      std::vector<udp::endpoint> resolve( resolver& r, const std::string& hostname, const std::string& port) 
+      {
+        try 
+        {
+          resolver res( fc::asio::default_io_service() );
+          promise<std::vector<endpoint> >::ptr p( new promise<std::vector<endpoint> >("udp::resolve completion") );
+          res.async_resolve( resolver::query(hostname,port), 
+                              boost::bind( detail::resolve_handler<endpoint,resolver_iterator>, p, _1, _2 ) );
+          return p->wait();
         }
+        FC_RETHROW_EXCEPTIONS(warn, "")
+      }
     }
   
 } } // namespace fc::asio

src/network/ntp.cpp

@@ -68,12 +68,14 @@ namespace fc
                }
            } // request_now
 
+           //started for first time in ntp() constructor, canceled in ~ntp() destructor
            void request_time_task()
            {
+              assert(_ntp_thread.is_current());
               request_now();
-              _request_time_task_done = _ntp_thread.schedule( [=](){ request_time_task(); }, 
-                                                              fc::time_point::now() + fc::seconds(_request_interval_sec), 
-                                                              "request_time_task" );
+              _request_time_task_done = schedule( [=](){ request_time_task(); }, 
+                                                  fc::time_point::now() + fc::seconds(_request_interval_sec), 
+                                                  "request_time_task" );
            } // request_loop
 
            void read_loop()

src/thread/task.cpp

@@ -3,6 +3,7 @@
 #include <fc/thread/unique_lock.hpp>
 #include <fc/thread/spin_lock.hpp>
 #include <fc/fwd_impl.hpp>
+#include "context.hpp"
 
 #include <fc/log/logger.hpp>
 #include <boost/exception/all.hpp>
@@ -34,6 +35,7 @@ namespace fc {
     }
 #endif
   }
+
   void task_base::run_impl() {
     try {
       if( !canceled() )
@@ -50,6 +52,16 @@ namespace fc {
        set_exception( std::make_shared<unhandled_exception>( FC_LOG_MESSAGE( warn, "unhandled exception: ${diagnostic}", ("diagnostic",boost::current_exception_diagnostic_information()) ) ) );
     }
   }
+
+  void task_base::cancel()
+  {
+    promise_base::cancel();
+    if (_active_context)
+    {
+      _active_context->canceled = true;
+    }
+  }
+
   task_base::~task_base() {
     _destroy_functor( _functor );
   }