Initial port of 3709d2e02bf67ccc272e1f2311e5db125a922ba0 from 'ssh'

Re #17

paramiko/transport.py

@@ -1505,12 +1505,17 @@ class Transport (threading.Thread):
         # indefinitely, creating a GC cycle and not letting Transport ever be
         # GC'd. it's a bug in Thread.)
 
+        # Hold reference to 'sys' so we can test sys.modules to detect
+        # interpreter shutdown.
+        self.sys = sys
+
         # active=True occurs before the thread is launched, to avoid a race
         _active_threads.append(self)
         if self.server_mode:
             self._log(DEBUG, 'starting thread (server mode): %s' % hex(long(id(self)) & 0xffffffffL))
         else:
             self._log(DEBUG, 'starting thread (client mode): %s' % hex(long(id(self)) & 0xffffffffL))
+        try:
             try:
                 self.packetizer.write_all(self.local_version + '\r\n')
                 self._check_banner()
@@ -1600,6 +1605,13 @@ class Transport (threading.Thread):
                 finally:
                     self.lock.release()
             self.sock.close()
+        except:
+            # Don't raise spurious 'NoneType has no attribute X' errors when we
+            # wake up during interpreter shutdown. Or rather -- raise
+            # everything *if* sys.modules (used as a convenient sentinel)
+            # appears to still exist.
+            if self.sys.modules is not None:
+                raise
 
 
     ###  protocol stages