[JSch-users] Sftp Hangs for large files - the fix

[Lindani P. <lp...@nc...>]

I found that sending (put)  any files larger than 1K were hanging the 
jsch client.
The problem in my case was that my server had stringent  window size 
requirements  (max  1K).
The client was then hanging in the write  method of  Session.java:


public /*synchronized*/ void write(Packet packet, Channel c, int length) 
throws Exception{
 
    while(true){
      if(c.rwsize>length){
        c.rwsize-=length;
        break;
      }
      try{
       Thread.sleep(10);
     
      }catch(Exception e){};
    }
    write(packet);
  }
 

In my case c.rwsize>length  will never be true for files larger than 1K 
(put method attempts to read in 1024 bytes at a time)  , so I got stuck 
in the while loop.

My fix for this was to check for the available remote window size 
(rwsize)  in  the put method of  ChannelSftp and make sure that I only 
attempt
to send  a length smaller then the available window space (minus any 
packet padding  such as handle.length) . This also improved my 
performance as I can send much more than the hardcoded 1024 bytes at a 
time, if the window space is available.
Also I changed the above code to :

public /*synchronized*/ void write(Packet packet, Channel c, int length) 
throws Exception{
    int retry = 0;
    while(true){
      if(c.rwsize>length){
        c.rwsize-=length;
        break;
      }
     
       Thread.sleep(10);
       retry++;
        if  (retry > 10000) throw Exception("Unable to negotiate 
adequate window size"); //wait for only 100s
    }
    write(packet);
  }

So the caller can at least get an exception if the required window size 
is not obtained in after a certain wait.

LD.