Created attachment 158752 [details]
1. Put a document on a Windows file share
2. Open that document on a Linux machine
3. Open the same document on a Windows machine (choose read-only in the dialog)
4. Try to save the document on Linux
Saving should work, since Linux has exclusive write access to that document
Note: This has something to do with system file locking. When disabling the option "UseDocumentSystemFileLocking" on the Windows machine, saving on Linux works fine.
Attached the test protocol.
So a guess would be that when opening it read-only on Windows, LibreOffice doesn't use correct mode (read-only, shared) - see CreateFile's dwDesiredAccess and dwShareMode . Note that from Windows share PoV, the file is not open from Linux side, it was atomically opened when read, then was closed, than will be attempted to open for writing later (IIUC the mechanics of Linux file operations). LO only detects the opened state using lock file.