On Wed, Jul 05, 2023 at 08:04:41PM -0400, Jeff Layton wrote:

I don't believe it's an issue. I've seen nothing in the POSIX spec that
mandates that timestamp updates to different inodes involved in an
operation be set to the _same_ value. It just says they must be updated.

It's also hard to believe that any software would depend on this either,
given that it's very inconsistent across filesystems today. AFAICT, this
was mostly done in the past just as a matter of convenience.

I've seen this assumption in several programs:

mutt buffy.c
https://sources.debian.org/src/mutt/2.2.9-1/buffy.c/?hl=625#L625

  if (mailbox->newly_created &&
      (sb->st_ctime != sb->st_mtime || sb->st_ctime != sb->st_atime))
    mailbox->newly_created = 0;


neomutt mbox/mbox.c
https://sources.debian.org/src/neomutt/20220429+dfsg1-4.1/mbox/mbox.c/?hl=1820#L1820

  if (m->newly_created && ((st.st_ctime != st.st_mtime) || (st.st_ctime != st.st_atime)))
    m->newly_created = false;


screen logfile.c
https://sources.debian.org/src/screen/4.9.0-4/logfile.c/?hl=130#L130

  if ((!s->st_dev && !s->st_ino) ||             /* stat failed, that's new! */
      !s->st_nlink ||                           /* red alert: file unlinked */
      (s->st_size < o.st_size) ||               /*           file truncated */
      (s->st_mtime != o.st_mtime) ||            /*            file modified */
      ((s->st_ctime != o.st_ctime) &&           /*     file changed (moved) */
       !(s->st_mtime == s->st_ctime &&          /*  and it was not a change */
         o.st_ctime < s->st_ctime)))            /* due to delayed nfs write */
  {

nemo libnemo-private/nemo-vfs-file.c
https://sources.debian.org/src/nemo/5.6.5-1/libnemo-private/nemo-vfs-file.c/?hl=344#L344

		/* mtime is when the contents changed; ctime is when the
		 * contents or the permissions (inc. owner/group) changed.
		 * So we can only know when the permissions changed if mtime
		 * and ctime are different.
		 */
		if (file->details->mtime == file->details->ctime) {
			return FALSE;
		}


While looking for more examples, I found a perl test that seems to suggest
that at least Solaris, AFS, AmigaOS, DragonFly BSD do as you suggest:
https://sources.debian.org/src/perl/5.36.0-7/t/op/stat.t/?hl=158#L140


Thanks