Safely renaming a file without overwriting

[Wolfgang Draxinger]

Steven D'Aprano wrote:

> Open "the" file? There are potentially two files -- the source
> and destination. I only want to do the rename if the
> destination *doesn't* exist, so there is no destination file to
> open. How will it help me to lock the source file? Have I
> misunderstood?

I forgot to say, to open the source file with O_CREAT | O_EXCL.
The open will fail if the file already exists. By locking the
file no other process will be able to access it, but the process
that holds the lock can do with it anything. This includes to
rename an existing file to the name of the opened one - the
previously created placeholder file will get unlinked before,
but there is _probably_ no way that any process can intercept
this. It is an interesting thing, that files that are opened
remain fully usable and accessible if you unlink them as long
you don't close them. You have to close it, to remove all
remains of it.

Wolfgang Draxinger
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