Support append on existing file?

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?

Start in namespace.cc at the CreateFile logic shown in the issue, especially the existing-file and O_TRUNC branches. Determine the intended semantics for reopening an existing file to append data from chunked PUT requests; done requires an agreed append behavior and corresponding validation, but no test location is named in the issue.

Written by the indexing model from the issue text.