[Python-ideas] Implement POSIX ln via shutil.link and shutil.symlink

[Greg Ewing]

> On Wed, May 29, 2019 at 10:07:38PM +0700, Tom Hale wrote:
> 
>     If
>     somebody can create a file named link_name between unlink and 
>     symlink, he can also remove and create a file named link_name after 
>     symlink.

I tbink there are some corner cases that can give different results
if the symlink is not atomic.

For example:

Process A wants to symlink f1 --> f2, replacing any existing f1.

Process B wants to create f1 if it doesn't already exist, or update
it if it does. If f1 is a symlink, the file it's linked to should
be updated.

The end result should be that f1 exists and is a symlink to f2.

If the symlink is not atomic, this can happen:

1. Process A sees that f1 already exists and deletes it.
2. Process B sees that f1 does not exist and creates a new file
    called f1.
3. Process A tries to symlink f1 to f2, which fails because there
    is now an existing file called f1.

This violates the postcondition, because f1 is not a symlink
to f2.

-- 
Greg