Failed to rename remote file due to permission failure

[sakama]

I changed upload logic at #37 before.
In this change, plugin will create temporary file at remote SFTP server and rename it after all task completed since plugin user can't know if task was finished or not when some of tasks failed.

However, this change sometimes got permission failure while renaming remote file.

SFTP rename remote file failed. Retrying 1/10 after 0 seconds. Message: Could not determine if file sftp://***/path/to/somewhere is writeable. 

I don't know why file rename failed even if plugin is able to upload temporary file.
Anyway, we need to improve upload logic.

[sakama]

I found the cause.

Apache Commons VFS2 is checking parent directory's permission before renaming remote files on SFTP server.

    @Override
    public void moveTo(final FileObject destFile) throws FileSystemException {
        if (canRenameTo(destFile)) {
            if (!getParent().isWriteable()) {
                throw new FileSystemException("vfs.provider/rename-parent-read-only.error", getName(),
                        getParent().getName());
            }
        } else {
            if (!isWriteable()) {
                throw new FileSystemException("vfs.provider/rename-read-only.error", getName());
            }
        }
        ...

https://github.com/apache/commons-vfs/blob/commons-vfs2-2.2-rc1/commons-vfs2/src/main/java/org/apache/commons/vfs2/provider/AbstractFileObject.java#L1572-L1581

This works if SFTP account has permission to exec command over SSH.
However, doesn't work if SSH command isn't permitted.

Same problems were already reported at JIRA.
https://issues.apache.org/jira/browse/VFS-590