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https://issues.apache.org/jira/browse/SSHD-1123?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17285347#comment-17285347
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Achim Hügen commented on SSHD-1123:
-----------------------------------
Thanks for the assessment. So there still are a lot of unknowns.
What do you think about a feature toggle to control the chunking behaviour?
> ChannelAsyncOutputStream breaks downloads of sftp client by not chunking when
> the remote window is smaller than the packet size
> -------------------------------------------------------------------------------------------------------------------------------
>
> Key: SSHD-1123
> URL: https://issues.apache.org/jira/browse/SSHD-1123
> Project: MINA SSHD
> Issue Type: Bug
> Affects Versions: 2.6.0
> Reporter: Achim Hügen
> Priority: Major
> Attachments: download.js, package.json
>
>
> After the upgrade from 2.4.0 to 2.6.0 we experienced issues with a SFTP
> Client used in our test automation (Node.JS ssh2). The download of a file
> hangs after about 1MB of 5MB has been downloaded.
> Although this ssh2 version (0.4.15) is admittedly quite old and meanwhile a
> fix is available, we are reluctant to rollout 2.6.0 to production where a
> multitude of sftp clients is used by customers and might experience the same
> issue.
> Short story: the specific ssh2 client only adjusts its local windows size, if
> the window size is down to 0. But mina in version 2.6 usually will stop
> sending data before. So both parties are waiting for each other.
> Example: the window size of the client is 1048576 and packet size is 32768.
> After successfully requesting 1015808 of file data, when the clients reads
> the next 32768 bytes, the remote window size as calculated by the MINA is
> 31706 bytes. Thus the data to send is larger then the remaining space in the
> window.
> The old logic in 2.4 just filled the window to the brim by chunking the data.
> Some newly introduced logic (see
> https://github.com/apache/mina-sshd/commit/536d066349f41a6f6a7f95501506cd1ba66dad7a)
> now decides to stop sending more data, because the remote window size is
> smaller then the packet size.
> The server waits for the client to send a SSH_MSG_CHANNEL_WINDOW_ADJUST and
> the client doesn't do this, because the window still has some space.
> Unfortunately the commit doesn't contain any reference to why this was
> changed. As I understand the SSH specification, the packet size is a maximum
> packet size. So, it should be ok to send a smaller packet.
> This is the code causing the issue in ChannelAsyncOutputStream.
> {code:java}
> if (total > remoteWindowSize) {
> // if we have a big message and there is enough space, send the next chunk
> if (remoteWindowSize >= packetSize) {
> // send the first chunk as we have enough space in the window
> length = packetSize;
> } else {
> // do not chunk when the window is smaller than the packet size
> length = 0;
> // do a defensive copy in case the user reuses the buffer
> IoWriteFutureImpl f = new IoWriteFutureImpl(future.getId(), new
> ByteArrayBuffer(buffer.getCompactData()));
> f.addListener(w -> future.setValue(w.getException() != null ?
> w.getException() : w.isWritten()));
> pendingWrite.set(f);
> if (log.isTraceEnabled()) {
> log.trace("doWriteIfPossible({})[resume={}] waiting for window
> space {}",
> this, resume, remoteWindowSize);
> }
> }
> {code}
> I have attached code for reproducing the issue. To run you need node.js and
> npm installed:
> {code}
> npm install
> nodejs download.js
> {code}
>
>
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