rpc: Ignore and log errors during cancel

Cancelling the RPC acceptors can sometimes result in an error about
a bad file descriptor.

As this is the shutdown sequence we need to continue nevertheless,
ignore these errors, log a warning and proceed.

Fixes #4352.

src/rpcserver.cpp

@@ -700,11 +700,20 @@ void StopRPCThreads()
     // 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::system::error_code ec;
     BOOST_FOREACH(const boost::shared_ptr<ip::tcp::acceptor> &acceptor, rpc_acceptors)
-        acceptor->cancel();
+    {
+        acceptor->cancel(ec);
+        if (ec)
+            LogPrintf("%s: Warning: %s when cancelling acceptor", __func__, ec.message());
+    }
     rpc_acceptors.clear();
     BOOST_FOREACH(const PAIRTYPE(std::string, boost::shared_ptr<deadline_timer>) &timer, deadlineTimers)
-        timer.second->cancel();
+    {
+        timer.second->cancel(ec);
+        if (ec)
+            LogPrintf("%s: Warning: %s when cancelling timer", __func__, ec.message());
+    }
     deadlineTimers.clear();
 
     rpc_io_service->stop();