mgo: fix possible race

session.go

 // mgo - MongoDB driver for Go
 //·
 // Copyright (c) 2010-2012 - Gustavo Niemeyer <gustavo@niemeyer.net>
 //·
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are met:·
 //·
 // 1. Redistributions of source code must retain the above copyright notice, this
@@ skipping 2876 matching lines @@
 }
 
 // ---------------------------------------------------------------------------
 // Internal session handling helpers.
 
 func (s *Session) acquireSocket(slaveOk bool) (*mongoSocket, error) {
 
         // Read-only lock to check for previously reserved socket.
         s.m.RLock()
         if s.masterSocket != nil {
+                sock := s.masterSocket
+                sock.Acquire()
                 s.m.RUnlock()
-                s.masterSocket.Acquire()
-                return s.masterSocket, nil
+                return sock, nil
         }
         if s.slaveSocket != nil && s.slaveOk && slaveOk {
+                sock := s.slaveSocket
+                sock.Acquire()
                 s.m.RUnlock()
-                s.slaveSocket.Acquire()
-                return s.slaveSocket, nil
+                return sock, nil
         }
         s.m.RUnlock()
 
         // No go.  We may have to request a new socket and change the session,
         // so try again but with an exclusive lock now.
         s.m.Lock()
         defer s.m.Unlock()
 
         if s.masterSocket != nil {
                 s.masterSocket.Acquire()
@@ skipping 151 matching lines @@
         }
         panic("unreachable")
 }
 
 func hasErrMsg(d []byte) bool {
         return len(d) > 16 &&
                 d[5] == 'e' && d[6] == 'r' && d[7] == 'r' &&
                 d[8] == 'm' && d[9] == 's' && d[10] == 'g' &&
                 d[11] == '\x00' && d[4] == '\x02'
 }