[Samba] renaming over to a busy file: traces

[Jeremy Allison]

On Wed, Mar 20, 2024 at 06:18:59PM +0100, Ralph Boehme via samba wrote:
>On 3/19/24 07:22, Michael Tokarev via samba wrote:
>>It looks to me that while renaming a file, samba checks only if the SOURCE
>>file is locked, but not if the DESTINATION file is locked.
>
>d'oh! I completely missed that part...
>
>I guess the problem is that we can't lock two locking.tdb records 
>simultaneously. We already have the source file record locked, also 
>locking the the destination record is not possible.
>
>We could check the destination record without holding a lock *before* 
>acquiring the lock on the source file record, but that would be a 
>possible victim to races. It would be better then what we have now 
>though I guess. Patches welcome! :)

Renames are horribly tricky. It's only in the "destination file
exists and overwrite flag is set" in the SMB2 case we have to
fix I think.

I've been looking at the logic here:

2.1.5.15.11 FileRenameInformation

https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-fsa/87f86c9b-6c2a-4803-84b7-131a74a434fa

It seems that in this specific case Windows deletes the
target file first (see the bool variable 'RemoveTargetLink')
although I'm presuming this is externally atomic to callers.

Still thinking about how (or if) we could achieve the same..