Re: [JSch-users] SCP to SFTP file streaming

[Jung, V. <vol...@so...>]

Hi Andreas,

as far as I understand, you are trying to stream data received by the SCP 'protocol' directly to a target host via SFTP.

The streams of the EXEC-channel are open as long as the EXEC channel is not closed. However, your implementation seems to assume that the InputStream is closed as soon as the data of the file is fully received via SCP. This is not the case and looks to me like a misunderstanding on your side.

I think you have to implement the SCP specific details yourself, meaning that you have to provide a new InputStream for SFTP-upload which is closed upon receiving the end-transmission-token via SCP.


Sincerely yours
Volker




Von: Sensen, Andreas <a.s...@th...>
Gesendet: Freitag, 12. April 2019 08:42
An: 'jsc...@li...' <jsc...@li...>
Betreff: [JSch-users] SCP to SFTP file streaming

Hi all,

first i'd like to thank JCraft for the JSch library. It has been really helpful so far!

But during my development i ran into a problem: trying to copy a file with SCP to an SFTP-Server
using the Streams provided by JSch. The error is easily reproducable in Version 0.1.55 by doing
the following steps:

* Retrieve an InputStream (MyPipedInputStream) from a Channel
* Try to upload this file with an ChannelSftp:

--------
    JSch jsch = new JSch();
    Session session = jsch.getSession(username, host, port);
    ChannelExec channel = (ChannelExec) session.openChannel("exec");
    ...
    InputStream sshInputStream = channel.getInputStream();

    JSch jsch = new JSch();
    Session session = jsch.getSession(username, host, port);
    ChannelSftp channel = (ChannelSftp) session.openChannel("sftp");
   channel.put(sshInputStream , file);
--------

The problem lies in ChannelSftp's _put method (lines 635 to 643):

--------
    do{
        nread=src.read(data, s, datalen);
        if(nread>0){
            s+=nread;
            datalen-=nread;
            count+=nread;
        }
    }
    while(datalen>0 && nread>0);
--------

As far as i understand it, the while-loop tries to read as many bytes as possible into the data
array. But since a (My)PipedInputStream doesn't return -1 if it's empty, but rather waits for more
data to come, this ends in an endlessly blocking read() at the end of the file transfer.

The only way i could solve this is by removing the do-while-loop around the code and adding an
SftpProgressMonitor implementation, that returns false on count(long byte) if the whole file has
been transferred. But that involves manipulating JSch source code, which i'd prefer not to do...

Is there any better solution to this particular problem?

Any help would be greatly appreciated!

Thanks
Andreas