bug#81508: 31.0.50; package.el archive contents download robustness issues

Sean Whitton <[email protected]> writes:

> If there is an ENOSPC in the middle of writing the archive-contents
> file, the truncated file is left behind.  This is because
> package--download-one-archive uses plain write-region instead of writing
> into a temporary file and then atomically renaming it into place.

I feel like this is not the only place where something like this could
go wrong, so it would be useful to perhaps have some functionality
directly in url.el to download a file and rename it atomically, since
IIRC there are a number of edge-cases to consider here.

> Worse, if a truncated file has been left behind,
> package--read-archive-file errors instead of trying to delete and
> re-download the file.

Assuming we fix the above, should we also make this function more robust
by e.g. trying to re-download the archive if it is malformed?