Date: Wed, 9 Feb 2022 16:42:30 -0600
From: Justin Pryzby <pryzby@telsasoft.com>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-hackers@postgresql.org, Robert Haas <robertmhaas@gmail.com>,
	Thomas Munro <thomas.munro@gmail.com>
Subject: Re: wrong fds used for refilenodes after pg_upgrade relfilenode
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On Wed, Feb 09, 2022 at 02:00:04PM -0800, Andres Freund wrote:
> On linux we can do so by a) checking if readlink(/proc/self/fd/$fd) points to
> a filename ending in " (deleted)", b) doing fstat(fd) and checking if st_nlink
> == 0.

You could also stat() the file in proc/self/fd/N and compare st_ino.  It
"looks" like a symlink (in which case that wouldn't work) but it's actually a
Very Special File.  You can even recover deleted, still-opened files that way..

PS. I didn't know pg_upgrade knew about Reply-To ;)