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

On Mon, Apr 11, 2022 at 5:12 AM Kyotaro Horiguchi
<horikyota(dot)ntt(at)gmail(dot)com> wrote:
> So, the only thing we need to care is segment switch. Without it, the
> segment that InstallXLogFileSegment found by the stat loop is known to
> be safe to overwrite even if exists.
>
> When segment switch finds an existing file, it's no problem since the
> segment switch doesn't create a new segment. Otherwise segment switch
> always calls InstallXLogFileSegment. The section from searching for
> an empty segmetn slot until calling durable_rename_excl() is protected
> by ControlFileLock. Thus if a process is in the section, no other
> process can switch to a newly-created segment.
>
> If this diagnosis is correct, the comment is proved to be paranoid.

It's sometimes difficult to understand what problems really old code
comments are worrying about. For example, could they have been
worrying about bugs in the code? Could they have been worrying about
manual interference with the pg_wal directory? It's hard to know.

--
Robert Haas
EDB: http://www.enterprisedb.com