The application KDE Connect allows remotely browsing an Android device from a desktop computer through SFTP. Since Android 4.4, developers don't have write permission to SD cards directly through the filesystem anymore. So I am trying to port the SFTP module using the Storage Access Framework (DocumentFile, etc.)
I am taking the permission with an Intent.ACTION_OPEN_DOCUMENT_TREE and FLAG_GRANT_WRITE_URI_PERMISSION and passing the context to my classes.
I am able to create new empty files, rename files and delete files on the SD card inside my class so I believe I am getting the necessary permissions. However, transferring a file results in an empty file (0 bytes) being created. I can see the transfer taking a certain time and a progress bar on the desktop side, so it doesn't just abort.
Here is the relevant part of the SftpSubsystem class from the Apache SSHD library (see doc here) with my own comments to explain what's going on:
public class SftpSubsystem implements Command, Runnable, SessionAware, FileSystemAware {
// This method receives a buffer from an InputStream and processes it
// according to its type. In this situation, it would also contain
// a block of the file being transferred (4096 bytes)
protected void process(Buffer buffer) {
int type = buffer.getByte();
switch (type) {
case WRITE:
FileHandle fh = getHandleFromString(buffer.getString());
long offset = buffer.getLong();
byte[] data = buffer.getBytes();
fh.write(data, offset);
break;
// other cases
}
}
// This class is a handle to a file (duh) with
// an OutputStream to write and InputStream to read
protected static class FileHandle {
SshFile file;
OutputStream output;
long outputPos;
InputStream input;
long inputPos;
// Method called inside process()
public void write(byte[] data, long offset) throws IOException {
if (output != null && offset != outputPos) {
IoUtils.closeQuietly(output);
output = null;
}
if (output == null) {
// This is called once at the start of the transfer.
// This is what I think I need to rewrite to make
// it work with DocumentFile objects.
output = file.createOutputStream(offset);
}
output.write(data);
outputPos += data.length;
}
}
}
The original implementation of createOutputStream() that I want to rewrite because RandomAccessFile doesn't work with DocumentFile:
public class NativeSshFile implements SshFile {
private File file;
public OutputStream createOutputStream(final long offset)
throws IOException {
// permission check
if (!isWritable()) {
throw new IOException("No write permission : " + file.getName());
}
// move to the appropriate offset and create output stream
final RandomAccessFile raf = new RandomAccessFile(file, "rw");
try {
raf.setLength(offset);
raf.seek(offset);
// The IBM jre needs to have both the stream and the random access file
// objects closed to actually close the file
return new FileOutputStream(raf.getFD()) {
public void close() throws IOException {
super.close();
raf.close();
}
};
} catch (IOException e) {
raf.close();
throw e;
}
}
}
One of the ways I tried to implement it:
class SimpleSftpServer {
static class AndroidSshFile extends NativeSshFile {
// This is the DocumentFile that is stored after
// create() created the empty file
private DocumentFile docFile;
public OutputStream createOutputStream(final long offset) throws IOException {
// permission check
if (!isWritable()) {
throw new IOException("No write permission : " + docFile.getName());
}
ParcelFileDescriptor pfd = context.getContentResolver().openFileDescriptor(docFile.getUri(), "rw");
FileDescriptor fd = pfd.getFileDescriptor();
try {
android.system.Os.lseek(fd, offset, OsConstants.SEEK_SET);
} catch (ErrnoException e) {
Log.e("SimpleSftpServer", "" + e);
return null;
}
return new FileOutputstream(fd, offset);
}
}
}
I also tried a simple (the offset is ignored but it's just a test):
public OutputStream createOutputStream(final long offset) throws IOException {
// permission check
if (!isWritable()) {
throw new IOException("No write permission : " + docFile.getName());
}
return context.getContentResolver().openOutputStream(docFile.getUri());
}
I also tried with a FileChannel and to flush and sync the FileOutputStream.
Any idea why I end up with an empty file?
EDIT: here is a small example of a test I did to just write a new file from an existing file. It works, but this is not what I actually want to do (see code above) but I thought I'd provide an example to show that I understand the basics of how to write to an OutputStream.
private void createDocumentFileFromFile() {
File fileToRead = new File("/storage/0123-4567/lady.m4a");
File fileToWrite = new File("/storage/0123-4567/lady2.m4a");
File dir = fileToWrite.getParentFile();
DocumentFile docDir = DocumentFile.fromTreeUri(context, SimpleSftpServer.externalStorageUri);
try {
DocumentFile createdFile = docDir.createFile(null, fileToWrite.getName());
Uri uriToRead = Uri.fromFile(fileToRead);
InputStream in = context.getContentResolver().openInputStream(uriToRead);
OutputStream out = context.getContentResolver().openOutputStream(createdFile.getUri());
try {
int nbOfBytes = 0;
final int BLOCKSIZE = 4096;
byte[] bytesRead = new byte[BLOCKSIZE];
while (true) {
nbOfBytes = in.read(bytesRead);
if (nbOfBytes == -1) {
break;
}
out.write(bytesRead, 0, nbOfBytes);
}
} finally {
in.close();
out.close();
}
} catch (IOException e) {
}
}
