Re: [gentoo-dev] Re: Portage Git migration - clean cut or git-cvsserver

[Kent Fredric]

On 2 June 2012 00:42, Rich Freeman <rich0@g.o> wrote:
> On Fri, Jun 1, 2012 at 7:23 AM, Kent Fredric <kentfredric@×××××.com> wrote:
>>
>> Nope, at least not as far as I can tell, and I just implemented commit
>> signature verification >_>
>
> I've been trying to find an example of a signed commit, but can't find
> one anywhere, so it is hard to tell what it is doing without going
> through the git source carefully.  If you have an example of a signed
> commit feel free to send it to me.
>
> Note that I am NOT interested in the output of any git operation (such
> as git log --show-signature, git show, etc) - these are all processed
> and do not reveal what is actually going on.  I want a copy of the
> actual file containing the signature.  If the signature is embedded in
> the commit then I want the file in the objects directory tree that
> contains the commit.  They're just compressed text files (though it is
> a bit of a pain to decompress them).

"git cat-file -p $sha" is as close as you can get to commit objects
without needing to write your own decompressing wrapper.  But it gives
the same results.

Here is a sample signed git commit ( sans compression ) :

https://gist.github.com/2852363

Line 5 to 21 are where the GPG signature is stored. ( Git uses the
leading space on all the lines of the GPG sig to indicate that its
part of the signature, line 22 does not lead with a space, so that
line is after the GPG signature.  Line 22 also is empty, which ends
the "header" section of the commit, and there starts the "comment"
section.

If you don't believe me that "git cat-file -p $sha" is reliable, here
is a simple script that has no git internals in it , its just Zlib
decompression : https://gist.github.com/2852411

perl /tmp/deflate.pl .git/objects/66/50c09ad7b2a62e28476e639654443015ac5945

And that re-emits exactly the same thing : https://gist.github.com/2852363

And if you want to do that yourself, here's that object in compressed
form base64 encoded https://gist.github.com/2852436


> The filesystem WON'T look the same after a rebase.
>
> quick example (operations done in this order):
>
> file in commit A in master:
> 1
> 2
> 3
> 4
> 5
>
> file in commit B in a branch off of master:
> 1
> 2a
> 3
> 4
> 5
>
> file in commit C in master:
> 1
> 2
> 3
> 4a
> 5
>
> if you merge master into the branch you'll end up with a new commit D
> whose parent is B that looks like:
> 1
> 2a
> 3
> 4a
> 5
>
> if instead you rebase master into the branch you'll end up with a new
> commit D whose parent is C that looks like:
> 1
> 2a
> 3
> 4a
> 5
>
> The history for the branch will no longer contain B.  If there were 14
> commits B1..14 you'd end up with 14 commits D1.14 that each contain
> the line 4a.  None of them would use the same trees as B1..14, and
> they can't use the same signatures as a result, even if only the tree
> was signed.   As far as the new history was concerned, line 4a was
> there before the branch was started.

It all depends really on how you do the rebase, if the changes in the
rebase shadow all the competing changes , or the changes are shadowed
out before the rebase, the rebase will be the same afterwards, just
with a different history.

ie:

  root -> a -> b -> c -> d( backout of c )

If you had started a branch off "b", and then rebased that branch on
top of d, the tree would be in the exact same state, but the history
would have changed.

And likewise, if the change was :

  root -> a -> b -> c -> d -> e

And you started a branch at b, with commits m, n, o, p , changing the
same lines c, d and e changed,
and when you rebased, you opted to let the changes in m, n, and o
replace the changes in c, d and e,
( which has the same effect as if you had deleted c , d and e, except
they're still there in the history ), when you move the branch from b
to e, the last commit of that branch (p) will still be the same.

The rebased commits m, n, and o will be different to what they were
when they were on "b", because now they're not only making changes vs
"b", but they're also replacing code added in c, d, and e.

Sure, this only happens in certain circumstances, and usually when it
happens, its because you want it to happen , but it does happen, and
its quite useful sometimes.

( essentially, x + y + z == x can be true, as long as z = -y  )


> At least, that is my understanding of rebasing.  Again, I'm a bit of a
> git novice, but I never really grokked git until I saw a presentation
> that didn't start with commands, but instead started out with just
> walking through the actual structure of the repository.  Once you grok
> the COW model I find it all clicks into place.

Yeah, chapter 9 of the Pro Git book is where I recommend everyone with
even a sliver of comp-sci knowledge to start.
http://www.git-scm.com/book/en/Git-Internals

Once you see its basically a bunch of text files, that refer to each
other via sha1 , and are stored in files named after their sha1, (
after a bit of compression ), all that graph theory just clicks and
you wonder how they managed to make the UI so complicated =p




-- 
Kent

perl -e  "print substr( \"edrgmaM  SPA NOcomil.ic\\@tfrken\", \$_ * 3,
3 ) for ( 9,8,0,7,1,6,5,4,3,2 );"

http://kent-fredric.fox.geek.nz