Avoiding race conditions when copying files

When copying files in Python, shutil.copy (and shutil.copy2) are able to silently overwrite the destination file if it already exists. At times this is the desired behavior, but I find that more often, I want to prevent overwriting the destination. A "naive" check would be this:
def supposedlySaferCopy(srcfile, destfile):
    if not exists(destfile):
        raise IOError('destination already exists')
    
    shutil.copy(srcfile, destfile)
But it has a race condition: there is a small window of time between checking for existence and running the copy. Sometimes this is check is a safeguard, for example to make sure file operations in a complex script are not overwriting data when not expected to. In general this pattern can also be a security issue, e.g. a type of symlink race.

In Windows one can make a call directly to the Windows api; both CopyFile and MoveFile take a parameter for preventing overwrite. This can be done in pure Python because ctypes is built into Python's standard library (in 2.5 and later). In Posix systems, I wrote a copyFilePosixWithoutOverwrite function. The O_CREAT flag ensures the file is new, and the O_EXCL will hold the file handle exclusively. Here are my open and copy implementations:
def copy(srcfile, destfile, overwrite):
    if not exists(srcfile):
        raise IOError('source path does not exist')
        
    if srcfile == destfile:
        pass
    elif sys.platform == 'win32':
        from ctypes import windll, c_wchar_p, c_int
        failIfExists = c_int(0) if overwrite else c_int(1)
        res = windll.kernel32.CopyFileW(c_wchar_p(srcfile), c_wchar_p(destfile), failIfExists)
        if not res:
            raise IOError('CopyFileW failed')
    else:
        if overwrite:
            shutil.copy(srcfile, destfile)
        else:
            copyFilePosixWithoutOverwrite(srcfile, destfile)

    assertTrue(exists(destfile))
        
def move(srcfile, destfile, overwrite):
    if not exists(srcfile):
        raise IOError('source path does not exist')
        
    if srcfile == destfile:
        pass
    elif sys.platform == 'win32':
        from ctypes import windll, c_wchar_p, c_int
        replaceExisting = c_int(1) if overwrite else c_int(0)
        res = windll.kernel32.MoveFileExW(c_wchar_p(srcfile), c_wchar_p(destfile), replaceExisting)
        if not res:
            raise IOError('MoveFileExW failed')
    else:
        copy(srcfile, destfile, overwrite)
        os.unlink(srcfile)
    
    assertTrue(exists(destfile))
    
def copyFilePosixWithoutOverwrite(srcfile, destfile):
    # fails if destination already exist. O_EXCL prevents other files from writing to location.
    # raises OSError on failure.
    flags = os.O_CREAT | os.O_EXCL | os.O_WRONLY
    file_handle = os.open(destfile, flags)
    with os.fdopen(file_handle, 'wb') as fdest:
        with open(srcfile, 'rb') as fsrc:
            while True:
                buffer = fsrc.read(64 * 1024)
                if not buffer:
                    break
                fdest.write(buffer)
Fairly comprehensive tests and more file utilities can be found in tests.py and files.py on my GitHub page here.