Invalid dates in git log

Invalid dates in git log

[Eirik Bjørsnøs]

Hi there,

I'm experiencing a date issue with certain commits in Git in Linus's
2.6 kernel repository.

One commit has a date from 2019, while another commit has a date from
1970. Although Git has been around for a while and seeing a Linux
commit from 2019 would be cool, I think there's something wrong here.

My questions are:

1) Is this a problem in the Git software?
2) Or is it a data corruption issue in the repository?
3) Can it be fixed and should I contact anyone to get it fixed?

I'm doing some log analysis and these two dates mess up my charts.

Steps to reproduce:

$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git

$ git  log  a27ac38efd6dc6dccebfc9bcc475ab4aa5fc4a56 -1
commit a27ac38efd6dc6dccebfc9bcc475ab4aa5fc4a56
Author: Len Brown <len.brown@intel.com>
Date:   Fri Apr 5 00:07:45 2019 -0500

    [ACPI] fix merge error that broke CONFIG_ACPI_DEBUG=y build

    Signed-off-by: Len Brown <len.brown@intel.com>

$ git  log  224426f168aa4af3dcb628e6edaa824d32d60e6f  -1
commit 224426f168aa4af3dcb628e6edaa824d32d60e6f
Author: Ursula Braun <braunu@de.ibm.com>
Date:   Thu Jan 1 01:00:01 1970 +0100

    qeth: remove header_ops bug

    Remove qeth bug caused by commit:
    [NET]: Move hardware header operations out of netdevice.

    Signed-off-by: Ursula Braun <braunu@de.ibm.com>
    Signed-off-by: Jeff Garzik <jeff@garzik.org>


Thanks,

Eirik.

Re: Invalid dates in git log

[Jeff King]

On Wed, Dec 12, 2007 at 10:11:12AM +0100, Eirik Bjørsnøs wrote:

> My questions are:
> 
> 1) Is this a problem in the Git software?

No. Whoever made the commit probably just didn't have their clock set
right. Git doesn't generally care about the timestamp for its
operations; it just records it as a historical note.

> 2) Or is it a data corruption issue in the repository?

No. You can check for corruption with git-fsck, but these commits were
actually created with bad dates.

> 3) Can it be fixed and should I contact anyone to get it fixed?

Changing the date will change the commit id (since the id is the sha1 of
the commit contents). Which would mean rewriting all of the history that
follows it. You could do it in your own repository, but then you might
have some trouble merging with Linus later on. Linus could do it, but I
doubt he will think it is worth the trouble.

> $ git  log  a27ac38efd6dc6dccebfc9bcc475ab4aa5fc4a56 -1
> commit a27ac38efd6dc6dccebfc9bcc475ab4aa5fc4a56
> Author: Len Brown <len.brown@intel.com>
> Date:   Fri Apr 5 00:07:45 2019 -0500

Your best guess is probably the committer information. Try this:

  git log a27ac38 -1 --pretty=format:'Author: %an %ad%nCommitter: %cn %cd'

-Peff

Re: Invalid dates in git log

[Junio C Hamano]

Jeff King <peff@peff.net> writes:

> On Wed, Dec 12, 2007 at 10:11:12AM +0100, Eirik Bjørsnøs wrote:
>
>> My questions are:
>> 
>> 1) Is this a problem in the Git software?
>
> No. Whoever made the commit probably just didn't have their clock set
> right. Git doesn't generally care about the timestamp for its
> operations; it just records it as a historical note.

"git show -s" with --pretty=fuller and --pretty=raw on these two
commiits reveal:

    Author:     Ursula Braun <braunu@de.ibm.com>
    AuthorDate: Thu Jan 1 01:00:01 1970 +0100
    Commit:     Jeff Garzik <jeff@garzik.org>
    CommitDate: Fri Oct 19 23:00:02 2007 -0400

    author Ursula Braun <braunu@de.ibm.com> 1 +0100
    committer Jeff Garzik <jeff@garzik.org> 1192849202 -0400

    Author:     Len Brown <len.brown@intel.com>
    AuthorDate: Fri Apr 5 00:07:45 2019 -0500
    Commit:     Len Brown <len.brown@intel.com>
    CommitDate: Tue Jul 12 00:12:09 2005 -0400

    author Len Brown <len.brown@intel.com> 1554440865 -0500
    committer Len Brown <len.brown@intel.com> 1121141529 -0400

So the former commit was made while seting GIT_AUTHOR_DATE explicitly to
1, which is quite likely a bug in some script Jeff used to create this
commit about two months ago.

I have no idea about the latter, though.  It looks like quite a random
timestamp, and committer timestamp look reasonable, relative to the
other commits around it.

For a short while, between Nov 11th to Dec 8th this year on 'next' (and
between Dec 4th and Dec 8th on 'master'), git-commit-tree accepted an
empty GIT_AUTHOR_DATE and recorded a bogus "0" time in the commit by
mistake, but such a commit would have shown something like:

    author A U Thor <au.thor@example.com>

without any timestamp, and both commits predate the gotcha, so I do not
think they are caused by that recent breakage.

Re: Invalid dates in git log

[Eirik Bjørsnøs]

> Your best guess is probably the committer information. Try this:

Thanks Jeff, Junio,

I'll just use the committer date instead.

Being a Git newbie (only started looking at it yesterday) I'm not sure
my understanding of "author" and "committer" and how they releate to
dates is correct:

* author: Original source of the change. This person may typically
have sent a committer an email with a patch. It's the commiter's
responsibility to supply this information.
* author date: The commiter is free to specify a date for the contribution.

* committer: First committer to actually add this change to a repository
* committer date: Date of the actual commit, added by the git client
during the commit. Typically the system clock at the time of the
commit.

* A transfer of a commit across repositories (pull, push) will not
change the author, commit or date information

If I got some of this wrong, I'd be happy if someone would correct me.

Thanks,

Eirik.

Re: Invalid dates in git log

[Johannes Schindelin]

Hi,

On Wed, 12 Dec 2007, Junio C Hamano wrote:

>     Author:     Len Brown <len.brown@intel.com>
>     AuthorDate: Fri Apr 5 00:07:45 2019 -0500
>     Commit:     Len Brown <len.brown@intel.com>
>     CommitDate: Tue Jul 12 00:12:09 2005 -0400
> 
>     author Len Brown <len.brown@intel.com> 1554440865 -0500
>     committer Len Brown <len.brown@intel.com> 1121141529 -0400
> 
> [...] It looks like quite a random timestamp, and committer timestamp 
> look reasonable, relative to the other commits around it.

It is quite possible that Len Brown had a similar problem to what I 
experienced yesterday: my clock was set one hour and 22 years into the 
future, but I have no idea how that happened.  My only guess is a 
half-succeeded ntpdate call, but somehow I doubt that.

Ciao,
Dscho

Re: Invalid dates in git log

[Andreas Ericsson]

Eirik Bjørsnøs wrote:
>> Your best guess is probably the committer information. Try this:
> 
> Thanks Jeff, Junio,
> 
> I'll just use the committer date instead.
> 
> Being a Git newbie (only started looking at it yesterday) I'm not sure
> my understanding of "author" and "committer" and how they releate to
> dates is correct:
> 

They're often the same. They will end up being different if
* you cherry-pick a commit made by someone else.
   ("someone else" is author, you are the committer)
* you rebase a series of commit containing changes from others
   ("others" are the authors, you are the committer)
* you apply a patch using "git am" from someone
   ("someone" is the author, you are the committer)
* you "git commit --amend" a commit from someone else
   ("someone else" is author, you are the committer)

There are probably other cases, but those were the ones I could
think of right now.

In short, whenever a commit is modified in some way, it gets a
new committer. It might help if you think of author as "contributor"
and "committer" as "integrator", where various integrators merge
between each other. A merge obviously doesn't fiddle with commits,
so once a commit has entered an integrators repository, both author
and committer stays intact (that's not strictly true, but for the
sake of this argument, which concerns the linux kernel, it will
suffice and be mostly correct insofar as I understand the kernel
workflow).

> * author: Original source of the change. This person may typically
> have sent a committer an email with a patch. It's the commiter's
> responsibility to supply this information.

No, it's the author's responsibility to supply this information.
Author is hardly ever changed.

> * author date: The commiter is free to specify a date for the contribution.
> 

No, the author does that too :)

> * committer: First committer to actually add this change to a repository

No, committer is the person who added the commit to the repository by
some other means than merging from another repository.

> * committer date: Date of the actual commit, added by the git client
> during the commit. Typically the system clock at the time of the
> commit.
> 

Sort of, yes, although commits can be created by other means than
just running "git commit". The operations listed in bullets at the
top of this mail all create new commits, in the sense that they
can't have the same SHA1 as their original ones. Hence, committer
and commitdate must be updated.

> * A transfer of a commit across repositories (pull, push) will not
> change the author, commit or date information
> 

True. Since neither ancestry nor content is allowed to change, the
commit will be exactly the same in the new repository as it was in
the old one.

> If I got some of this wrong, I'd be happy if someone would correct me.
> 

I think I just did. Perhaps I missed something, but I'm sure someone
will correct me if that's the case.

-- 
Andreas Ericsson                   andreas.ericsson@op5.se
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231

Re: Invalid dates in git log

[Junio C Hamano]

Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:

> On Wed, 12 Dec 2007, Junio C Hamano wrote:
>
>>     Author:     Len Brown <len.brown@intel.com>
>>     AuthorDate: Fri Apr 5 00:07:45 2019 -0500
>>     Commit:     Len Brown <len.brown@intel.com>
>>     CommitDate: Tue Jul 12 00:12:09 2005 -0400
>> 
>>     author Len Brown <len.brown@intel.com> 1554440865 -0500
>>     committer Len Brown <len.brown@intel.com> 1121141529 -0400
>> 
>> [...] It looks like quite a random timestamp, and committer timestamp 
>> look reasonable, relative to the other commits around it.
>
> It is quite possible that Len Brown had a similar problem to what I 
> experienced yesterday: my clock was set one hour and 22 years into the 
> future, but I have no idea how that happened.  My only guess is a 
> half-succeeded ntpdate call, but somehow I doubt that.

But how would you explain the more reasonable committer date?

I am guessing that this particular commit was rebased (using rebase,
rebase -i, stgit, guilt or something else), and whatever tool that was
used had some thinko that broke the author timestamp (and zone).

Nah, exonerate guilt and rebase -i from the list.  The commit is from
July 2005 which means there wasn't much usable tool for rebasing in the
core distribution.  Interactive rebase did not exist back then, and
rebase was done with "git-commit-script -m", which took an existing
commit and used its metainformation when creating a commit.  This was
almost totally different codepath from what we have now.  Not even
format-patch nor applymbox existed.

StGIT 0.4 (initial) was Jul 10, 2005, so it is _possible_ (I do not know
about plausibility) that it had an early bug that caused this, but if we
really want to figure it out we need to ask Len what was used, and I
suspect the most likely answer is "are you crazy enough to expect me to
remember?".

I am personally more interested in the other one, which was much more
recent incident, though.