chylek-qr opened a new issue, #733:
URL: https://github.com/apache/mina-sshd/issues/733
### Version
2.15.0
### Bug description
Hi! There seems to be an issue with the `SftpRemotePathChannel#transferTo`
implementation.
Demonstration code, assumes that `ClientSession session` is already
obtained, and expects a `source-file` to be present on the SFTP server. In my
case, the file is about 256 kB.
```java
SftpFileSystem fileSystem =
SftpClientFactory.instance().createSftpFileSystem(session);
Path sourcePath = fileSystem.getPath("source-file");
Path targetPath = Path.of("/tmp/test");
try (FileChannel sourceChannel = FileChannel.open(sourcePath,
StandardOpenOption.READ);
FileChannel targetChannel = FileChannel.open(targetPath,
StandardOpenOption.CREATE, StandardOpenOption.WRITE,
StandardOpenOption.TRUNCATE_EXISTING)) {
long totalBytes = sourceChannel.size();
long copiedBytes = 0L;
while (copiedBytes < totalBytes) {
copiedBytes +=
sourceChannel.transferTo(targetChannel.position(), /* count = */ 1024,
targetChannel);
}
}
```
I'm copying the data using `FileChannel#transferTo` in a loop, so I can
track the progress. The count is only 1024 bytes is for demonstration, because
it has to be less than half the size of the file downloaded from SFTP to
trigger the bug.
Note that `transferFrom` does not have this issue. I always want to call the
transfer method on the SftpRemotePathChannel for best performance, so in the
real code, I do the following to decide which one to use, depending on whether
the destination is local (`sourceFileChannel` is `SftpRemotePathChannel` and
transfers **to** a local file) or not (`writer` is `SftpRemotePathChannel` and
transfers **from** a local file). Only `transferFrom` does not work.
```java
long copyTo(FileChannel writer, long count) throws IOException {
if (isDestinationLocal) {
return sourceFileChannel.transferTo(writer.position(), count,
writer);
}
else {
return writer.transferFrom(sourceFileChannel,
sourceFileChannel.position(), count);
}
}
````
### Actual behavior
The code successfully copies 2048 bytes (2 iterations of the loop), then
throws an exception:
```
java.lang.NullPointerException: Cannot invoke
"org.apache.sshd.common.util.buffer.Buffer.available()" because "this.buffer"
is null
at
org.apache.sshd.sftp.client.impl.SftpInputStreamAsync.sendRequests(SftpInputStreamAsync.java:245)
at
org.apache.sshd.sftp.client.impl.SftpInputStreamAsync.doRead(SftpInputStreamAsync.java:195)
at
org.apache.sshd.sftp.client.impl.SftpInputStreamAsync.transferTo(SftpInputStreamAsync.java:152)
at
org.apache.sshd.sftp.client.impl.SftpRemotePathChannel.transferTo(SftpRemotePathChannel.java:369)
```
The crash happens in this condition, which is only triggered when `position`
> `count`, so it only happens on the 3rd iteration of the loop when `position =
2048` and `count = 1024`.
<img width="715" alt="Image"
src="https://github.com/user-attachments/assets/cb28efd2-8da8-4f81-85e1-9b3e99703130";
/>
The issue seems to be that `fileSize` is set to `count`, i.e. the number of
bytes I want to download, and not the actual size of the file on the SFTP
server.
### Expected behavior
Download the file in 1024-byte chunks.
### Relevant log output
```Shell
```
### Other information
_No response_
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