Subject: Re: Possible git blame bug?

From: Domagoj Stolfa <[email-masked]>
Date: Tue, 14 Mar 2017 00:08:10 +0100

Hello,

> > For example, saying:
> >
> > $ git blame time.h --since=2017
> > ^e19f2a27ed8 (Domagoj Stolfa 2017-03-12 20:43:01 +0100  33) #ifndef _SYS_TIME_H_
> >
> > $ git blame time.h --since=2016
> > ^21613a57af9 (bz  2016-03-13 21:26:18 +0000  33) #ifndef _SYS_TIME_H_
> >
> > $ git blame time.h --since=2015
> > ^48507f436f0 (mav 2015-03-13 21:01:25 +0000  33) #ifndef _SYS_TIME_H_
> >
> > and so on, with different hashes.
> 
> The output lines "^deadbeef" does *NOT* mean that commit deadbeef
> changed the revision.  It just is telling you that the hisory was
> dug down to that revision and it was found that since that revision
> there is no change (and you told the command not to bother looking
> beyond that time range, so we do not know what happened before that
> time).
> 
> It is understandable, when your history has a lot of merges, the
> history traversal may stop at commits on different branches.
> 
> Imagine a case where the line in question never changed throughout
> the history:
> 
>           o---o---B
>          /         \
>     O---o---o---A---C---o---o
> 
> Imagine A is from 2015, B is from 2016 and C is from 2017.  C's
> first parent, i.e. C^1, is A and C^2 is B.
> 
> If you ask the command to stop digging when you hit a commit on or
> before 2017-03-13 (03-13 is because today's date is appended to your
> 2017), your traversal will stop at C and you get a line that begins
> with ^C.
> 
> If you ask it to stop at 2016, A won't be even looked at because it
> is older.  The command will keep digging from C to find B.  If B's
> parent is also newer than the cutoff, but its parent is older, then
> the line will be shown with ^ and commit object name of B's parent.
> 
> If you ask it to stop at 2015, the command will first consider A
> (C's earlier parent) and pass blame to the lines common between
> these two commits.  In this illustration, we are pretending that the
> file did not change throughout the hsitory, so blame for all lines
> are passed to A and we don't even look at B.  Then we keep digging
> through A to find the culprit, or hit a commit older than the
> specified cut-off time.  The line will be shown with ^A or perhaps
> its ancestor.
> 
> So it is entirely sane if you saw three boundary commits named with
> three different time ranges.

Thanks for clearing this up. Is this documented somewhere, so that if it happens
again I can point people to the docs that explain this behaviour?

-- 
Best regards,
Domagoj Stolfa