[U-Boot-Users] Revised custodian git writeup

[Wolfgang Denk <wd@denx.de>]

In message <47F3F98010FF784EBEE6526EAAB078D10635DDB0@tq-mailsrv.tq-net.de> you wrote:
>
> Huh? Normally the commit IDs are the most important thing I care about
> when working with other git repositories. If I try to compare my
> git repositroy with another one (mostly the u-boot master branch at
> denx) then the commit ID is the only way (I know) to make sure that
> my respository contains the exactly same patches than the other
> repository. If the topmost patch (or any other) in my respository has

But this does not work.

Assume you and me start with absolutely identical copies of the same
git repository - say we copied a disk.

Now we both check in the very same patch from the mailing list using
for example git-am. You will see that we get two different commit IDs.

Even if you try to do this twice to identical copies of the git repo
you will get different IDs:

-> rm -fr foo
-> tar zxf foo.tgz ; cd foo ; echo x >xxx ; git-add xxx; git-commit -m test
Created commit 8c934ae: test
 1 files changed, 1 insertions(+), 1 deletions(-)
-> cd ..
-> rm -fr foo
-> tar zxf foo.tgz ; cd foo ; echo x >xxx ; git-add xxx; git-commit -m test
Created commit b5ae18e: test
 1 files changed, 1 insertions(+), 1 deletions(-)
-> cd ..
-> rm -fr foo
-> tar zxf foo.tgz ; cd foo ; echo x >xxx ; git-add xxx; git-commit -m test
Created commit 56eb723: test
 1 files changed, 1 insertions(+), 1 deletions(-)

You see? Three identical commits, three different commit IDs.

> As far as I understand the git philosophy, using SHA1 signatures as
> commit IDs was one of the most important things on git. IMO the goal

Yes, of course. And the SHA1's of the *objects*  are  of  course  the
same.  But the commit IDs may be different - they include for example
the commiter's name and time etc., so they  will  be  different  even
when  you  do  the  very same action again in the very same copy of a
repository. See above.

Note that git is tracking content, not history.

> behind that was to be able to refere to one particular state in a
> git repository by one single commit ID. If I have two git repositories
> with de facto the same content, but with different commit IDs, what's
> the worth of the SHA1 commit ID?

You can still compare the commits. Just not the commit IDs.

> I often use for example the (partial) commit ID in the U-Boot boot
> message to identify from wich particular source state a U-Boot was

This is OK, as long as you are referring to some repository for which
you have a guarantee that the history will not be rewritten.

> built. This only works, if the order of patches is the same in all
> git repositories resulting in the same commit IDs. Otherwise I need

No, this has nothing to do with that.

> to know from wich git repository the U-Boot was built, to identify

Yes, of course you need to know that. As soon as you  are  working  n
your own tree it may have commits that are present nowhere else. Only
as  long  as you are referring to a known common history you can make
references to another repo.

> the source state it was built from. And then I can't compare this
> repository to another (e. g. the denx master), because the same
> patches could have different commit IDs. 

But they will result in the very same  content  (objcts  in  the  git
repo), and git is tracking the content, not the commit ID.

> I tried this already some time ago (merging my u-boot-tq-group
> master branch, instead of rebasing) and get totally lost in a
> screwed up history. But maybe I did something wrong.

Most probably (or this was with older versions of the tools - as
mentioned before, I screwed up more than one repo before ;-)

Best regards,

Wolfgang Denk

-- 
DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
e-credibility: the non-guaranteeable likelihood that  the  electronic
data you're seeing is genuine rather than somebody's made-up crap.
- Karl Lehenbauer