Support append on existing file?

[bailaohe]

Good job! I'm doing some webdav work on this fs.

I come to some problem that, when the webdav server handle some chunked-PUT request for some large file, it may write some segment file and then re-open it to append.

I found this in namespace.cc

`

if (exist) {
    if ((flags & O_TRUNC) == 0) {
        LOG(INFO, "CreateFile %s fail: already exist!", fname.c_str());
        return kFileExists;
    } else {
        if (IsDir(file_info.type())) {
            LOG(INFO, "CreateFile %s fail: directory with same name exist", fname.c_str());
            return kFileExists;
        }
        for (int i = 0; i < file_info.blocks_size(); i++) {
            blocks_to_remove->push_back(file_info.blocks(i));
        }
    }
}` 

So this fs does not intend to support append operation on existing file?