[Mono-dev] Making Open more atomic/consistent

[LCID Fire]

Looking at the System.IO.File.Open/System.IO.MonoIO.Copy method I 
noticed some oddities in the implementation.
First off it does a File.Exists check on the destination file which is 
IMO too early since the destination file is created far later on - there 
is the possibility that the file could be deleted till the actual create 
is attempted. In this case the method would have failed for no reason.
Furthermore should the file be created after the File.Exists check it 
attempts a file creation via calling open. Open then fails - but looking 
at System.IO.MonoIO.GetException the error message for 
MonoIOError.ERROR_FILE_EXISTS differs from the one that is returned from 
the earlier check ("Could not create file \"{0}\". File already exists." 
vs "{0} already exists").
So for consistency and to get the actual check more atomic I'd propose 
to just delete the first check in System.IO.MonoIO.Copy.