Get proper disk space for mounted volume

DiskSpaceForMounted.md

This siutation is encountered in Azure Function where we want to have telemetries for available space for temp as well as home. It is a general issue of getting available space for mounted folder.

See the result, notice the difference between using different ways to get free space for the same folder? Why?

Assuming temp folder location: c:\users\abc\temp, disk c has 1G free, and temp is mounted from another volume that has 5G.

This gets the free disk space for the driver, not for the mounted folder - 1G:

ulong availableSpace = GetDiskFreeSpace(tempFolder);

And this gets the actual available space for the mount point (5G):

[DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
[return: MarshalAs(UnmanagedType.Bool)]
private static extern bool GetDiskFreeSpaceEx(string lpDirectoryName,
    out ulong lpFreeBytesAvailable,
    out ulong lpTotalNumberOfBytes,
    out ulong lpTotalNumberOfFreeBytes);

private static ulong GetDiskFreeSpace(string path)
{
    // Making sure there's tailing slash.
    // Refer to https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-getdiskfreespaceexa
    path = path.TrimEnd('\\') + '\\';

    if (string.IsNullOrEmpty(path))
    {
        throw new ArgumentNullException(nameof(path));
    }

    if (!GetDiskFreeSpaceEx(path, out ulong freeSpace, out _, out _))
    {
        throw new Win32Exception(Marshal.GetLastWin32Error());
    }

    return freeSpace;
}

Notice it was a PInvoke on Win32 API, means it won't work on Linux.