Is python's shutil.copyfile() atomic?

Question

I'm writing a python script that copies a file using shutil.copyfile() on Linux. During the copying, other processes may be trying to read the file. Is the following sufficient to ensure that an external process doesn't get a corrupted view of the file?

os.unlink(dest)
shutil.copyfile(src, dest)

That is, is shutil.copyfile() atomic such that other processes cannot read the destination file until after the copy operation is complete?

Answer 1

No, shutil.copyfile is not atomic. This is part of the definition of shutil.copyfile:

def copyfile(src, dst, *, follow_symlinks=True):    
    ...
    with open(src, 'rb') as fsrc:
        with open(dst, 'wb') as fdst:
            copyfileobj(fsrc, fdst)

where copyfileobj is defined like this:

def copyfileobj(fsrc, fdst, length=16*1024):
    while 1:
        buf = fsrc.read(length)
        if not buf:
            break
        fdst.write(buf)

The thread calling copyfile could be stopped inside this while-loop at which point some other process could try to open the file to be read. It would get a corrupted view of the file.