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Bizarre mtime behaviour

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solipsis at pitrou

Nov 1, 2009, 7:23 AM

Post #1 of 6 (551 views)
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Bizarre mtime behaviour

Hello,

I wondered if someone had a clue about the following behaviour.
While debugging an erratic test_mailbox failure on RDM's buildbot (and other
machines), it turned out that the system sometimes set the wrong mtime on a
directory:

$ date && python -c 'import os; os.link("setup.py", "t/c")' && stat t && date

Sun Nov 1 09:49:04 EST 2009
File: `t'
Size: 144 Blocks: 0 IO Block: 4096 directory
Device: 811h/2065d Inode: 223152 Links: 2
Access: (0755/drwxr-xr-x) Uid: ( 1001/ pitrou) Gid: ( 1005/ pitrou)
Access: 2009-11-01 09:10:11.000000000 -0500
Modify: 2009-11-01 09:49:03.000000000 -0500
Change: 2009-11-01 09:49:03.000000000 -0500
Sun Nov 1 09:49:04 EST 2009

As you see above, the mtime for directory 't' is set to a full second before the
actual modification has happened.

Sprinkling traces of time.time() and os.path.getmtime() on Lib/mailbox.py shows
this is exactly what trips up test_mailbox. I've got posted a patch to fix it
(see issue #6896), but I would like to know if such OS behaviour is normal.

Regards

Antoine.

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rhamph at gmail

Nov 1, 2009, 8:25 AM

Post #2 of 6 (536 views)
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Re: Bizarre mtime behaviour [In reply to]

On Sun, Nov 1, 2009 at 08:23, Antoine Pitrou <solipsis [at] pitrou> wrote:
> Hello,
>
> I wondered if someone had a clue about the following behaviour.
> While debugging an erratic test_mailbox failure on RDM's buildbot (and other
> machines), it turned out that the system sometimes set the wrong mtime on a
> directory:
>
> $ date && python -c 'import os; os.link("setup.py", "t/c")' && stat t && date
>
> Sun Nov 1 09:49:04 EST 2009
> File: `t'
> Size: 144 Blocks: 0 IO Block: 4096 directory
> Device: 811h/2065d Inode: 223152 Links: 2
> Access: (0755/drwxr-xr-x) Uid: ( 1001/ pitrou) Gid: ( 1005/ pitrou)
> Access: 2009-11-01 09:10:11.000000000 -0500
> Modify: 2009-11-01 09:49:03.000000000 -0500
> Change: 2009-11-01 09:49:03.000000000 -0500
> Sun Nov 1 09:49:04 EST 2009
>
> As you see above, the mtime for directory 't' is set to a full second before the
> actual modification has happened.
>
> Sprinkling traces of time.time() and os.path.getmtime() on Lib/mailbox.py shows
> this is exactly what trips up test_mailbox. I've got posted a patch to fix it
> (see issue #6896), but I would like to know if such OS behaviour is normal.

Looks like an OS bug to me. Linux I'm guessing?

--
Adam Olsen, aka Rhamphoryncus
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solipsis at pitrou

Nov 1, 2009, 8:29 AM

Post #3 of 6 (521 views)
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Re: Bizarre mtime behaviour [In reply to]

Adam Olsen <rhamph <at> gmail.com> writes:
>
> Looks like an OS bug to me. Linux I'm guessing?

Yes, but only on certain boxes. I could never reproduce on my home box.
RDM (David)'s buildbot is a Gentoo vserver with a reiserfs filesystem.

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eric at trueblade

Nov 1, 2009, 9:03 AM

Post #4 of 6 (531 views)
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Re: Bizarre mtime behaviour [In reply to]

Antoine Pitrou wrote:
> Adam Olsen <rhamph <at> gmail.com> writes:
>> Looks like an OS bug to me. Linux I'm guessing?
>
> Yes, but only on certain boxes. I could never reproduce on my home box.
> RDM (David)'s buildbot is a Gentoo vserver with a reiserfs filesystem.

You'll occasionally see something similar on Windows boxes running FAT,
because the timestamps stored in the filesystem have a 2 second
granularity there. Not sure if this might be similar, or just a reiserfs
issue.

Eric.
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robertc at robertcollins

Nov 1, 2009, 1:02 PM

Post #5 of 6 (517 views)
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Re: Bizarre mtime behaviour [In reply to]

On Sun, 2009-11-01 at 12:03 -0500, Eric Smith wrote:
> Antoine Pitrou wrote:
> > Adam Olsen <rhamph <at> gmail.com> writes:
> >> Looks like an OS bug to me. Linux I'm guessing?
> >
> > Yes, but only on certain boxes. I could never reproduce on my home box.
> > RDM (David)'s buildbot is a Gentoo vserver with a reiserfs filesystem.
>
> You'll occasionally see something similar on Windows boxes running FAT,
> because the timestamps stored in the filesystem have a 2 second
> granularity there. Not sure if this might be similar, or just a reiserfs
> issue.

Some years ago there was a bug where the dentry cache held timestamps
more granular than those that the filesystem could represent. We saw
this cause build failures which were troubling to reproduce :) I haven't
seen Linux arbitrarily invent time travel so far:).

The FAT rounding issue is a possibility, but I didn't think reiserfs was
short that much precision.

I'd check that the work area you had really was reiser, not a mounted AT
partition, and if its not look up the ReiserFS precision for mtime.

-Rob

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solipsis at pitrou

Nov 1, 2009, 1:08 PM

Post #6 of 6 (516 views)
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Re: Bizarre mtime behaviour [In reply to]

Le lundi 02 novembre 2009 à 08:02 +1100, Robert Collins a écrit :
>
> The FAT rounding issue is a possibility, but I didn't think reiserfs was
> short that much precision.
>
> I'd check that the work area you had really was reiser, not a mounted AT
> partition, and if its not look up the ReiserFS precision for mtime.

Yes it was reiserfs. As for the mtime granularity, since some displayed
mtimes were even and some were odd, I assume it's one second or less.

Another possibility would be that the filesystem would use a slightly
different clock than time.time(), but why?

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