Re: avoid multiple hard links to same WAL file after a crash

On Fri, Apr 08, 2022 at 09:00:36PM -0400, Robert Haas wrote:
> On Fri, Apr 8, 2022 at 12:53 PM Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote:
>> I think there might be another problem. The man page for rename() seems to
>> indicate that overwriting an existing file also introduces a window where
>> the old and new path are hard links to the same file. This isn't a problem
>> for the WAL files because we should never be overwriting an existing one,
>> but I wonder if it's a problem for other code paths. My guess is that many
>> code paths that overwrite an existing file are first writing changes to a
>> temporary file before atomically replacing the original. Those paths are
>> likely okay, too, as you can usually just discard any existing temporary
>> files.
>
> I wonder if this is really true. I thought rename() was supposed to be atomic.

Not always. For example, some old versions of MacOS have a non-atomic
implementation of rename(), like prairiedog with 10.4. Even 10.5 does
not handle atomicity as far as I call. In short, it looks like a bad
idea to me to rely on this idea at all. Some FSes have their own way
of handling things, as well, but I am not much into this world.

Saying that, it would be nice to see durable_rename_excl() gone as it
has created quite a bit of pain for us in the past years.
--
Michael