How to fix WritePendingException?

[vnxvd]

Hello everybody,
I am getting following error message while using Spring Integration Sftp.

WritePendingException: A write operation is already pending; cannot write 6905 bytes, stackTrace: 
org.apache.sshd.common.io.WritePendingException: A write operation is already pending; cannot write 6905 bytes 
at org.apache.sshd.common.channel.ChannelAsyncOutputStream.writeBuffer(ChannelAsyncOutputStream.java:110) 
at org.apache.sshd.sftp.client.impl.DefaultSftpClient.write(DefaultSftpClient.java:308) 
at org.apache.sshd.sftp.client.impl.SftpOutputStreamAsync.internalFlush(SftpOutputStreamAsync.java:360) 
at org.apache.sshd.sftp.client.impl.SftpOutputStreamAsync.flush(SftpOutputStreamAsync.java:293) 
at org.springframework.util.FileCopyUtils.copy(FileCopyUtils.java:112) 
at org.springframework.integration.sftp.session.SftpSession.write(SftpSession.java:140)

This is the code for creating Sftp Session Factory.

DefaultSftpSessionFactory sessionFactory = new DefaultSftpSessionFactory(true);
        sessionFactory.setHost(connection.getHost());
        if (connection.getPort() != null) {
            sessionFactory.setPort(connection.getPort());
        }
        sessionFactory.setUser(connection.getUser());
        if (StringUtils.isNotBlank(connection.getPassword())) {
            sessionFactory.setPassword(connection.getPassword());
        } else if (StringUtils.isNotBlank(connection.getPrivateKey())) {
            sessionFactory.setPrivateKey(new FileSystemResource(connection.getPrivateKey()));
            if (StringUtils.isNotBlank(connection.getPrivateKeyPassphrase())) {
                sessionFactory.setPrivateKeyPassphrase(connection.getPrivateKeyPassphrase());
            }
        }
        sessionFactory.setAllowUnknownKeys(true);
        return sessionFactory;

This is the code for creating Sftp Session and use the session.

SftpSession sftpSession = sessionFactory.getSession();
        if (!sftpSession.exists(targetFile)) {
            try (InputStream is = new FileInputStream(sourceFile)) {
                sftpSession.write(is, targetFile);
            } catch (IOException e) {// handle exception here}
        }

Library versions:
Spring Boot 3.4.0
Spring Integration Sftp 6.4.0
Apache sshd Sftp 2.14.0
As a temporary solution, I am using "synchronized" keyword on the method which using the SftpSession object to fix the error. Please let me know if you have a better idea to fix this error message.
Thanks.

[artembilan]

This is what I see from the mentioned org.apache.sshd.common.channel.ChannelAsyncOutputStream.writeBuffer() JavaDocs:

    /**
     * {@inheritDoc}
     *
     * This write operation is <em>asynchronous</em>: if there is not enough window space, it may keep the write pending
     * or write only part of the buffer and keep the rest pending. Concurrent writes are not allowed and will throw a
     * {@link WritePendingException}. Any subsequent write <em>must</em> occur only once the returned future is
     * fulfilled; for instance triggered via a listener on the returned future. Try to avoid doing a subsequent write
     * directly in a future listener, though; doing so may lead to deep chains of nested listener calls with deep stack
     * traces, and may ultimately lead to a stack overflow.
     *
     * @throws WritePendingException if a concurrent write is attempted
     */
    @Override
    public IoWriteFuture writeBuffer(Buffer buffer) throws IOException {

Concurrent writes are not allowed

That's enough for us to understand that we cannot use object concurrently.

In the DefaultSftpSessionFactory we have an internal ConcurrentSftpClient which has a send() method guarded with a Lock.

Apparently we are missing the fact that write() could be called outside of the mention send().

Converting this discussion into an issue and fixing respectively.

Thank you for the report!

NOTE: please, learn how to post code snippets here on GitHub: https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-and-highlighting-code-blocks

[artembilan]

Closed in favor of: #9796.

The workaround is to guard concurrent access to the sftpSession.write().