spring-integration-sftp change in Spring Boot 3

[Ducius6]

When we upgraded out application to Spring boot 3 we started getting issues like this:

Caused by: org.apache.sshd.common.io.WritePendingException: A write operation is already pending; cannot write 21 bytes
at org.apache.sshd.common.channel.ChannelAsyncOutputStream.writeBuffer(ChannelAsyncOutputStream.java:102)
at org.apache.sshd.sftp.client.impl.DefaultSftpClient.send(DefaultSftpClient.java:299)
at org.apache.sshd.sftp.client.impl.AbstractSftpClient.checkHandle(AbstractSftpClient.java:230)
at org.apache.sshd.sftp.client.impl.AbstractSftpClient.openDir(AbstractSftpClient.java:829)
at org.apache.sshd.sftp.client.impl.SftpDirEntryIterator.(SftpDirEntryIterator.java:60)
at org.apache.sshd.sftp.client.impl.SftpIterableDirEntry.iterator(SftpIterableDirEntry.java:64)
at org.apache.sshd.sftp.client.impl.SftpIterableDirEntry.iterator(SftpIterableDirEntry.java:34)
at java.base/java.lang.Iterable.spliterator(Iterable.java:101)
at org.springframework.integration.sftp.session.SftpSession.doList(SftpSession.java:103)
at org.springframework.integration.sftp.session.SftpSession.list(SftpSession.java:69)
at org.springframework.integration.sftp.session.SftpSession.list(SftpSession.java:52)
at org.springframework.integration.file.remote.session.CachingSessionFactory$CachedSession.list(CachingSessionFactory.java:227)
at org.springframework.integration.file.remote.RemoteFileTemplate.lambda$list$5(RemoteFileTemplate.java:422)
at org.springframework.integration.file.remote.RemoteFileTemplate.execute(RemoteFileTemplate.java:452)
... 18 common frames omitted

We found out that issues is when you call RemoteFileTemplate.list method concurrently. Since ChannelAsyncOutputStream.writeBuffer cannot be caller concurrently.

Before upgrading to SB3 we had no issues like this.

So my question is, is this a bug in new implementation in SB3 using apache mina SSHD or is this expected behaviour and we should not call this method from different threads?

[artembilan]

Although this has nothing to do with Spring Boot, I still confirm that we have a problem in Spring Integration around this Apache MINA SFTP client usage.

According to the ChannelAsyncOutputStream JavaDocs we indeed cannot perform concurrent operations on this SFTP client:

    /**
     * {@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 {

Converting this to issue to fix respectively.

[artembilan]

Doesn't look like there is a connection back to the issue I've created from this one, so pasting it explicitly: #8708