rpc: keep track of acceptors, and cancel them in StopRPCThreads

Fixes #4156.

The problem is that the boost::asio::io_service destructor
waits for the acceptors to finish (on windows, and boost 1.55).

Fix this by keeping track of the acceptors and cancelling them before
stopping the event loops.

src/rpcserver.cpp

@@ -39,6 +39,7 @@ static ssl::context* rpc_ssl_context = NULL;
 static boost::thread_group* rpc_worker_group = NULL;
 static boost::asio::io_service::work *rpc_dummy_work = NULL;
 static std::vector<CSubNet> rpc_allow_subnets; //!< List of subnets to allow RPC connections from
+static std::vector< boost::shared_ptr<ip::tcp::acceptor> > rpc_acceptors;
 
 void RPCTypeCheck(const Array& params,
                   const list<Value_type>& typesExpected,
@@ -593,12 +594,13 @@ void StartRPCThreads()
     asio::ip::address bindAddress = loopback ? asio::ip::address_v6::loopback() : asio::ip::address_v6::any();
     ip::tcp::endpoint endpoint(bindAddress, GetArg("-rpcport", Params().RPCPort()));
     boost::system::error_code v6_only_error;
-    boost::shared_ptr<ip::tcp::acceptor> acceptor(new ip::tcp::acceptor(*rpc_io_service));
 
     bool fListening = false;
     std::string strerr;
     try
     {
+        boost::shared_ptr<ip::tcp::acceptor> acceptor(new ip::tcp::acceptor(*rpc_io_service));
+        rpc_acceptors.push_back(acceptor);
         acceptor->open(endpoint.protocol());
         acceptor->set_option(boost::asio::ip::tcp::acceptor::reuse_address(true));
 
@@ -616,7 +618,6 @@ void StartRPCThreads()
     {
         strerr = strprintf(_("An error occurred while setting up the RPC port %u for listening on IPv6, falling back to IPv4: %s"), endpoint.port(), e.what());
     }
-
     try {
         // If dual IPv6/IPv4 failed (or we're opening loopback interfaces only), open IPv4 separately
         if (!fListening || loopback || v6_only_error)
@@ -624,7 +625,8 @@ void StartRPCThreads()
             bindAddress = loopback ? asio::ip::address_v4::loopback() : asio::ip::address_v4::any();
             endpoint.address(bindAddress);
 
-            acceptor.reset(new ip::tcp::acceptor(*rpc_io_service));
+            boost::shared_ptr<ip::tcp::acceptor> acceptor(new ip::tcp::acceptor(*rpc_io_service));
+            rpc_acceptors.push_back(acceptor);
             acceptor->open(endpoint.protocol());
             acceptor->set_option(boost::asio::ip::tcp::acceptor::reuse_address(true));
             acceptor->bind(endpoint);
@@ -668,7 +670,16 @@ void StopRPCThreads()
 {
     if (rpc_io_service == NULL) return;
 
+    // First, cancel all timers and acceptors
+    // This is not done automatically by ->stop(), and in some cases the destructor of
+    // asio::io_service can hang if this is skipped.
+    BOOST_FOREACH(const boost::shared_ptr<ip::tcp::acceptor> &acceptor, rpc_acceptors)
+        acceptor->cancel();
+    rpc_acceptors.clear();
+    BOOST_FOREACH(const PAIRTYPE(std::string, boost::shared_ptr<deadline_timer>) &timer, deadlineTimers)
+        timer.second->cancel();
     deadlineTimers.clear();
+
     rpc_io_service->stop();
     if (rpc_worker_group != NULL)
         rpc_worker_group->join_all();