Git repository unique id

Question

I need to find out if a commit belongs to a particular git repository.

The idea is to generate some unique id for every repository I need to test. Then I can compa

Answer 1

Compare with Mercurial, where is checks mercurial/treediscovery.py (Mercurial repository identification):

base = list(base)
if base == [nullid]:
    if force:
        repo.ui.warn(_("warning: repository is unrelated\n"))
    else:
        raise util.Abort(_("repository is unrelated"))

base variable store last common parts of two repositories.

Git have same assumptions when emit warning: no common commits on fetch/push. I just didn't grep Git sources, that require time.

By giving this idea of Mercurial push/pull checks we may assume that repositories are related if they have common roots. For mercurial this means that hashes from command:

$ hg log -r "roots(all())"

for both repositories must have non-empty interjection.

You may not trick roots checking by carefully crafting repositories because building two repositories looks like these (with common parts but different roots):

0 <--- SHA-256-XXX <--- SHA-256-YYY <--- SHA-256-ZZZ
0 <--- SHA-256-YYY <--- SHA-256-ZZZ

impossible because that mean you reverse SHA-256 as each subsequent hash depends on previous values. That is true both for Mercurial and Git.

Corresponding command to see roots in Git is:

$ git log --format=oneline --all --max-parents=0

You can toy yourself with:

bash# md git
/home/user/tmp/git

bash# md one
/home/user/tmp/git/one

bash# git init
Initialized empty Git repository in /home/user/tmp/git/one/.git/

bash# echo x1 > x1
bash# git add x1
bash# git ci -m x1
[master (root-commit) 1208fb0] x1

bash# echo x2 > x2
bash# git add x2
bash# git ci -m x2
[master 1c3fe86] x2

bash# cd ..

bash# md two
/home/user/tmp/git/two

bash# git init
Initialized empty Git repository in /home/user/tmp/git/two/.git/

bash# echo y1 > y1
bash# git add y1
bash# git ci -m y1
[master (root-commit) ff56a8e] y1

bash# echo y2 > y2
bash# git add y2
bash# git ci -m y2
[master 18adff5] y2

bash# git fetch ../one/
warning: no common commits
remote: Counting objects: 6, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 6 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (6/6), done.
From ../one
 * branch            HEAD       -> FETCH_HEAD

bash# git co --orphan one
Switched to a new branch 'one'

bash# git merge FETCH_HEAD

bash# git log --format=oneline --all
18adff541c7ce9f1a1f2be2804d6d0e5792ff086 y2
ff56a8e7e9145d2b1b5a760bbc9b12451927ab0c y1
1c3fe8665851e89d37f49633cd2478900217b91c x2
1208fb0f721005207c6afe6a549a9ed0dcc5b0a8 x1

bash# git log --format=oneline --all --max-parents=0
ff56a8e7e9145d2b1b5a760bbc9b12451927ab0c y1
1208fb0f721005207c6afe6a549a9ed0dcc5b0a8 x1

bash# git log --all --graph

* commit 18adff541c7ce9f1a1f2be2804d6d0e5792ff086
|     y2
|  
* commit ff56a8e7e9145d2b1b5a760bbc9b12451927ab0c
      y1

* commit 1c3fe8665851e89d37f49633cd2478900217b91c
|     x2
|  
* commit 1208fb0f721005207c6afe6a549a9ed0dcc5b0a8
      x1

NOTE Git allow partial checkout. I didn't check this case for --max-parents=0.