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Date: Tue, 14 Jun 2022 16:46:58 +0100
From: Adam Dinwoodie <adam AT dinwoodie DOT org>
To: cygwin AT cygwin DOT com
Subject: Re: Wish list: Cygwin `mv` to move OneDrive files without
 downloading them
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On Tue, Jun 14, 2022 at 04:32:21PM +0100, Sam Edge wrote:
> On 14/06/2022 12:25, Adam Dinwoodie wrote:
> > Microsoft OneDrive has a "Files On-Demand" function, where it will
> > synchronise file metadata to a local system, but won't actually download
> > the file content until an application attempts to read the content.
> > When moving a file within Cygwin using `mv`, the file always gets
> > downloaded, which seems like it shouldn't be necessary.  Is there any
> > way to have `mv` (and presumably the underlying rename call) work
> > without downloading the file content in this circumstance? It'd
> > definitely make some of my life easier, but I'm not sure if it's a
> > trivial issue, one that would require years of work, or somwhere in
> > between...
> >
> > As best I can tell, `mv` doesn't need to know the content of the file,
> > at least as long as it's not moving the file outside of OneDrive; it
> > feels very similar to me to moving a file within a partition on *nix:
> > generally it's just a case of updating the directory records, with no
> > need to look at the file content.
> >
> > This does work as desired from within PowerShell: PowerShell's
> > `Move-Item` won't download a file that's not currently stored on the
> > local system.  So it's presumably possible to achieve this, although I
> > don't know if it'd rely on non-public Microsoft APIs, and/or lots more
> > complexity in the Cygwin code.
> >
> > Simple demonstration below, showing the different behaviours of
> > PowerShell versus Cygwin.  I've not included a cygcheck.out, as I'm
> > pretty sure this is much closer to a feature request than a problem
> > report, but let me know if I've got that wrong.
> 
> Greetings Adam.
> 
> I wasn't even aware you could do that with OneDrive - there's still no
> Explorer
> way of doing it in Windows 10 21H2 as far as I can see. Thanks for the
> info. :-)

FWIW, this also works exactly the same using click-and-drag or Ctrl+C /
Ctrl+V in File Explorer for me, on all my Windows systems (currently
running a mix of Win11 Enterprise and Win11 Pro, both the 22H2
releases), and I *think* I remember it working on Win10 too, but I'm
less confident there.

> I imagine Cygwin doesn't know about the extended attributes being used
> within
> the local copy of your OneDrive directory so it is doing the move using the
> underlying POSIX 'rename within mount' API which hopefully uses the Win32
> 'rename' but not in a way that carries the new 'O' attribute.

That seems very plausible; I know Cygwin's mv/rename seems to keep at
least some of the Windows file attributes, but I definitely don't
profess any expertise here.

> Presumably PowerShell's Move-Item does use the right flags to the Win32 API
> call.
> 
> One of the problems the Cygwin maintainers have is that Microsoft introduces
> 'enhancements' to NTFS & the Win32 API arbitrarily in Windows updates
> without
> announcing them so it's a game of whack'a'mole.

Yeah, I am painfully familiar with that sort of problem from Microsoft,
not just with Windows!

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