Overwrite remote share file first downloads it needlessly

[PVince81]

Steps

1. Setup two OC servers A and B
2. On A create a folder "local"
3. Create a file "local/test.txt"
4. Share "local" with a user on B
5. Connect to B with WebDAV and PUT a file as "local/test.txt" to overwrite it
6. Check the access logs on A

Expected result

B PUTs file to A through publicwebdav endpoint

Actual result

Before putting the file, B first does a GET to download the contents of the file from A.
Only then it does a PUT

Versions

ownCloud 8.1 RC2

It looks like it's happening here: https://github.com/owncloud/core/blob/master/lib/private/files/storage/dav.php#L392

From what I understand getCachedFile() was used here in case fopen was done in random access mode. But in the case of full overwrite (mode "w") there is no need to download the remote file.

@icewind1991

[PVince81]

Now I realize this isn't only for s2s, it would also affect WebDAV mounts.

It seems a similar logic is used for other external storages like SMB (but not all of them)
This means that external storage upload performance will be reduced.