[JSch-users] Resolved: Jsch hangs on reading from "exec" channel.

[Alexander K. <al...@tm...>]

Hello All,

Recently I wrote that sometimes my program hangs on reading from jsch
InputStream that is provided by ChannelExec object. I've managed to
reproduce the same problem when using jsch in the other application
(http://tmate.org/svn/) and found out that the reason of the problem was in
the calls order:

The code that may hang:

            mySession = jsch.getSession(credentials.getName(), host, port);
            mySession.connect();
        
            myChannel = (ChannelExec) mySession.openChannel("exec");
            myChannel.setCommand("svnserve -t");
            
            myChannel.connect();
            myInputStream = new RollbackInputStream(new
BufferedInputStream(myChannel.getInputStream()));
            myOutputStream = myChannel.getOutputStream();


As you can see output and input streams are fetched _after_
myChannel.connect() call. In case when channel's read thread starts before
myChannel.getInputStream() is called it will terminate and no data will be
read and redirected to the channels input stream. See below ChannelExec
"run" method code, that checks whether InputStream is already created:

      // this loop will not be started if io.in is NULL and it is NULL till
getInputStream() is called.
      while(thread!=null && io!=null && io.in!=null){
          // code that reads data and redirects it to the user's input
stream.
      }
      // thread is terminated here

Obviously, the workaround is to fetch input and output stream before calling
Channel.connect() method, but what do you think about a) documenting this
problem and better b) not relying on the order of calls, i.e. creating input
stream in "connect" method or channel constructor, not in getInputStream()
method. Also, _creating_ InputStream in "get" method breaks common sense
contract of the "get" method.

Thanks,
Alexander Kitaev.