classification
Title: ctime: I don't think that word means what you think it means.
Type: Stage:
Components: Library (Lib) Versions: Python 3.4
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: Zooko.Wilcox-O'Hearn, benjamin.peterson, ezio.melotti, pitrou, zooko
Priority: normal Keywords:

Created on 2009-04-07 21:57 by zooko, last changed 2013-08-23 23:55 by Zooko.Wilcox-O'Hearn.

Messages (11)
msg85750 - (view) Author: Zooko O'Whielacronx (zooko) Date: 2009-04-07 21:57
The stat module currently uses the "st_ctime" slot to hold two kinds
values which are semantically different but which are frequently
confused with one another.  It chooses which kind of value to put in
there based on platform -- Windows gets the file creation time and all
other platforms get the "ctime".  The only sane way to use this API is
then to switch on platform:

if platform.system() == "Windows":
    metadata["creation time"] = s.st_ctime
else:
    metadata["unix ctime"] = s.st_ctime

(That is an actual code snippet from the Allmydata-Tahoe project.)

Many or even most programmers incorrectly think that unix ctime is file
creation time, so instead of using the sane idiom above, they write the
following:

metadata["ctime"] = s.st_ctime

thus passing on the confusion to the users of their metadata, who may
not know on which platform this metadata was created.  This is the
situation we have found ourselves in for the Allmydata-Tahoe project --
we now have a bunch of "ctime" values stored in our filesystem and no
way to tell which kind they were.

More and more filesystems such as ZFS and Macintosh apparently offer
creation time nowadays.

I propose the following changes:

1.  Add a "st_crtime" field which gets populated on filesystems
(Windows, ZFS, Mac) which can do so.

That is hopefully not too controversial and we could proceed to do so
even if the next proposal gets bogged down:

2.  Add a "st_unixctime" field which gets populated *only* by the unix
ctime and never by any other value (even on Windows, where the unix
ctime *is* available even though nobody cares about it), and deprecate
the hopelessly ambiguous "st_ctime" field.

You may be interested in http://allmydata.org/trac/tahoe/ticket/628
("mtime" and "ctime": I don't think that word means what you think it
means.) where the Allmydata-Tahoe project is carefully unpicking the
mess we made for ourselves by confusing ctime with file-creation time.
msg85987 - (view) Author: Antoine Pitrou (pitrou) * (Python committer) Date: 2009-04-15 10:18
Sounds like a good idea, perhaps you could launch a discussion on
python-dev?
msg89336 - (view) Author: Zooko O'Whielacronx (zooko) Date: 2009-06-13 17:44
Okay, I posted to python-dev:

http://mail.python.org/pipermail/python-dev/2009-June/090021.html
msg195477 - (view) Author: Ezio Melotti (ezio.melotti) * (Python committer) Date: 2013-08-17 14:39
What was the outcome?
msg196050 - (view) Author: Zooko Wilcox-O'Hearn (Zooko.Wilcox-O'Hearn) Date: 2013-08-23 22:17
Well, read the thread! http://mail.python.org/pipermail/python-dev/2009-June/090021.html

Basically just a couple of +1's, and a good suggestion to name it something clearer than "crtime". 


Please fix it!
msg196052 - (view) Author: Benjamin Peterson (benjamin.peterson) * (Python committer) Date: 2013-08-23 22:40
I don't think we should hijack st_ctime. All the other members of the stat object correspond exactly to what you get back from the Unix stat() call. Let's not break that. How about just leaving st_ctime as it is and adding st_creationtime, which will have the nicer semantics?
msg196056 - (view) Author: Zooko Wilcox-O'Hearn (Zooko.Wilcox-O'Hearn) Date: 2013-08-23 23:35
Benjamin Peterson: what do you mean "hijack ctime"? I don't think I — or anyone — has proposed anything that fits that description. Please be more specific.  My proposal in http://bugs.python.org/issue5720#msg85750 does not break anything.
msg196058 - (view) Author: Benjamin Peterson (benjamin.peterson) * (Python committer) Date: 2013-08-23 23:44
Indeed, "hijacking" is a bit strong. I mean filling it with a value that is not in the underlying st_ctime field from stat() call.
msg196059 - (view) Author: Zooko Wilcox-O'Hearn (Zooko.Wilcox-O'Hearn) Date: 2013-08-23 23:48
Benjamin: I'm sorry, I still don't understand. Do you think my proposal would involve setting something named "ctime" to contain a value that didn't come from the underlying stat "ctime"?
msg196060 - (view) Author: Benjamin Peterson (benjamin.peterson) * (Python committer) Date: 2013-08-23 23:52
You're quite right. I stupidly misread "crtime" as "ctime".
msg196061 - (view) Author: Zooko Wilcox-O'Hearn (Zooko.Wilcox-O'Hearn) Date: 2013-08-23 23:55
Aha! Mystery solved. I wouldn't say that you were stupid — I would say that "crtime" is way too close to "ctime", and I strongly agree with the suggestion (http://mail.python.org/pipermail/python-dev/2009-June/090026.html) on the mailing list by Greg Ewing that we name the new thing something more obvious.
History
Date User Action Args
2013-08-23 23:55:49Zooko.Wilcox-O'Hearnsetmessages: + msg196061
2013-08-23 23:52:03benjamin.petersonsetmessages: + msg196060
2013-08-23 23:48:31Zooko.Wilcox-O'Hearnsetmessages: + msg196059
2013-08-23 23:44:56benjamin.petersonsetmessages: + msg196058
2013-08-23 23:35:24Zooko.Wilcox-O'Hearnsetmessages: + msg196056
2013-08-23 22:40:25benjamin.petersonsetnosy: + benjamin.peterson
messages: + msg196052
2013-08-23 22:17:50Zooko.Wilcox-O'Hearnsetnosy: + Zooko.Wilcox-O'Hearn
messages: + msg196050
2013-08-17 14:39:11ezio.melottisetnosy: + ezio.melotti
messages: + msg195477
2012-11-03 15:41:41Ramchandra Aptesetversions: + Python 3.4
2009-06-13 17:44:27zookosetmessages: + msg89336
2009-04-15 10:18:45pitrousetnosy: + pitrou
messages: + msg85987
2009-04-07 21:57:48zookocreate