6ee46a5 created the issue

Here's the scenario. guest has read access to a directory, but not write permission. The user (matthew) has write permission. Here is what I see on upload say a 100 byte file.

HEAD /myfile.txt (guest)
200 OK

PUT /myfile.txt (guest, filesize=100)
401 Unauthorized

PUT /myfile.txt (matthew, filesize=0)
200 OK

The HEAD is coming from DAVSession.login(). This succeeds and PUT tries to use the same credentials. When this fails, another PUT happens, but with 0 bytes (maybe the file pointer wasn't restored?).

My fix was to make my server return 401 on the HEAD command, but this requires user-agent snooping as HEAD can't return 401 to the browser. To fix authentication related weirdness in other client agent's I've made my server always require login for OPTIONS and PROPFIND. That has the advantage of only affecting WebDav clients, not normal HTTP usage.

I think there are two issues

a) the second PUT sends a zero-length file
b) recommend maybe calling OPTIONS or PROPFIND in DavSession.login() since those are webdav specific methods rather than HEAD which the browser also calls.