Wish list: Cygwin `mv` to move OneDrive files without downloading them

[Adam Dinwoodie]

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.

    PS C:\Users\adinwoodie\OneDrive> # Create a directory and some files to experiment with
    PS C:\Users\adinwoodie\OneDrive> New-Item -Type directory -Name temp


        Directory: C:\Users\adinwoodie\OneDrive


    Mode                 LastWriteTime         Length Name
    ----                 -------------         ------ ----
    d-----        14/06/2022     12:09                temp


    PS C:\Users\adinwoodie\OneDrive> cd temp
    PS C:\Users\adinwoodie\OneDrive\temp> New-Item -Type file -Name move-with-pwsh -Value "Some arbitrary file content"


        Directory: C:\Users\adinwoodie\OneDrive\temp


    Mode                 LastWriteTime         Length Name
    ----                 -------------         ------ ----
    -a----        14/06/2022     12:10             27 move-with-pwsh


    PS C:\Users\adinwoodie\OneDrive\temp> New-Item -Type file -Name move-with-cygwin -Value "Some arbitrary file content"


        Directory: C:\Users\adinwoodie\OneDrive\temp


    Mode                 LastWriteTime         Length Name
    ----                 -------------         ------ ----
    -a----        14/06/2022     12:10             27 move-with-cygwin


    PS C:\Users\adinwoodie\OneDrive\temp> # Look at the directory listing prior to recovering the disk space
    PS C:\Users\adinwoodie\OneDrive\temp> dir


        Directory: C:\Users\adinwoodie\OneDrive\temp


    Mode                 LastWriteTime         Length Name
    ----                 -------------         ------ ----
    -a---l        14/06/2022     12:10             27 move-with-cygwin
    -a---l        14/06/2022     12:10             27 move-with-pwsh


    PS C:\Users\adinwoodie\OneDrive\temp> # Mark the files as to be stored online and the local disk space to be recovered
    PS C:\Users\adinwoodie\OneDrive\temp> attrib.exe +O +U *
    PS C:\Users\adinwoodie\OneDrive\temp> # Check the disk space is now marked as recovered; note the brackets around the file length
    PS C:\Users\adinwoodie\OneDrive\temp> dir


        Directory: C:\Users\adinwoodie\OneDrive\temp


    Mode                 LastWriteTime         Length Name
    ----                 -------------         ------ ----
    -a---l        14/06/2022     12:10           (27) move-with-cygwin
    -a---l        14/06/2022     12:10           (27) move-with-pwsh


    PS C:\Users\adinwoodie\OneDrive\temp> # Move the files
    PS C:\Users\adinwoodie\OneDrive\temp> Move-Item move-with-pwsh moved-with-pwsh
    PS C:\Users\adinwoodie\OneDrive\temp> C:\cygwin64\bin\mv.exe move-with-cygwin moved-with-cygwin
    PS C:\Users\adinwoodie\OneDrive\temp> # Note the version moved with Cygwin is now consuming local disk space again, while the version moved with PowerShell doesn't
    PS C:\Users\adinwoodie\OneDrive\temp> dir


        Directory: C:\Users\adinwoodie\OneDrive\temp


    Mode                 LastWriteTime         Length Name
    ----                 -------------         ------ ----
    -a---l        14/06/2022     12:10             27 moved-with-cygwin
    -a---l        14/06/2022     12:10           (27) moved-with-pwsh