Avoid a bug in Poco SecureSocket stack.

The session must be aborted manually before any destructors from
the other objects are called.

It is hard to pin down but at some point between v1.36 & v1.46 the bug
was fixed by ensuring that destructors don't throw exceptions.
Refs #8924

Code/Mantid/Framework/DataHandling/src/DownloadInstrument.cpp

@@ -59,6 +59,28 @@ namespace Mantid
     using Mantid::Kernel::Direction;
     using Mantid::API::WorkspaceProperty;
 
+    namespace
+    {
+
+      /**
+       * Circumvents a bug in the Poco HTTPS session stack whereby an exception
+       * is thrown from a destructor while another is aleady active.
+       * This simply calls abort() on the session when this object is destroyed
+       * to ensure the disconnection happens early enough
+       */
+      struct AutoAbortedHTTPSSession : public Poco::Net::HTTPSClientSession
+      {
+        AutoAbortedHTTPSSession(const std::string& host, Poco::UInt16 port)
+          : Poco::Net::HTTPSClientSession(host, port) {}
+
+        ~AutoAbortedHTTPSSession()
+        {
+          this->abort();
+        }
+      };
+
+    }
+
     // Register the algorithm into the AlgorithmFactory
     DECLARE_ALGORITHM(DownloadInstrument)
 
@@ -322,7 +344,7 @@ namespace Mantid
     {
       Poco::Path entryPath(filename);
       std::string entryExt = entryPath.getExtension();
-      std::string entryName = entryPath.getBaseName();              
+      std::string entryName = entryPath.getBaseName();
       std::stringstream ss;
       ss << entryName << "_" << entryExt;
       return ss.str();
@@ -378,12 +400,8 @@ namespace Mantid
         const Poco::Net::Context::Ptr context = new Poco::Net::Context(Poco::Net::Context::CLIENT_USE, "", "", "", Poco::Net::Context::VERIFY_NONE);
         // Create a singleton for holding the default context. E.g. any future requests to publish are made to this certificate and context.
         Poco::Net::SSLManager::instance().initializeClient(NULL, certificateHandler,context);
-
-        //Session takes ownership of socket
-        Poco::Net::SecureStreamSocket* socket = new Poco::Net::SecureStreamSocket(context);
-        Poco::Net::HTTPSClientSession session(*socket);
-        session.setHost(uri.getHost());
-        session.setPort(uri.getPort());
+        // Create the session
+        AutoAbortedHTTPSSession session(uri.getHost(), static_cast<Poco::UInt16>(uri.getPort()));
 
         //set the proxy
         if (!m_isProxySet)
@@ -501,4 +519,4 @@ namespace Mantid
 
 
   } // namespace DataHandling
-} // namespace Mantid
+} // namespace Mantid