How do git grafts and replace differ? (Are grafts

Question

There are very few Q&A's on git grafts versus replace. The search [git] +grafts +replace only found two that felt relevant of the 5. what-are-git-info-grafts-for and git-what-is-a-graftcommit-or-a-graft-id. There is also a note on git.wiki.kernel.org:GraftPoint

Are grafts now completely overtaken by the replace and filter-branch, or do they still needed for some special corner cases (and backward compatibility) ?

In general, how do they differ (e.g. which are transported between repos), and how are they generically the same? I've seen that Linus doesn't appear to care about grafts at present in the discussion on commit generation numbers (of the max parents back to any root variety) "Grafts are already unreliable."

EDIT: more info found.
A search of www.kernel.org/pub/software/scm/git/docs for graft only found 3 results:

1. git-filter-branch(1),
2. v1.5.4.7/git-filter-branch(1),
3. v1.5.0.7/git-svn(1).

A slightly broader search found RelNotes/1.6.5.txt which contains:

• refs/replace/ hierarchy is designed to be usable as a replacement of the "grafts" mechanism, with the added advantage that it can be transferred across repositories.

Unfortunately, the gitrepository-layout(5) isn't yet up to date with the refs/replace/ repository layout info (and notes), nor any deprecation note of info/grafts.

This gets closer to supporting what I was thinking but I'd welcome any confirmation or clarification.

Answer 1

EDIT: git replace --graft <commit> [<parent>…​] does the same thing as grafts and it can add or remove parents. The documentation says:

Create a graft commit. A new commit is created with the same content as <commit> except that its parents will be [<parent> …​] instead of <commit>'s parents. A replacement ref is then created to replace with the newly created commit.

(I'm leaving the old answer below as a reference.)

---

AFAIK, there is one use case that grafts can handle but replace cannot: adding or removing parents. It's a power tool for refactoring histories.

For example, if you're importing the history from an old SVN repository into Git, there is no merge information. What you can do (and I've done it lots of times) is to read through the commit messages to find out where a SVN "merge" was done, and then use Git grafts to add a parent to the merge commit.

IIRC, I've also had some cases where I've removed the parent of a commit, in order to make it the first commit in the history. Creating a clean history based on multiple chaotic legacy repositories sometimes requires drastic measures (there are some experiences of migrating projects to Git at my blog).

Then after you've cleaned up the whole history, you would do a git filter-branch before publishing the new Git repository.

Answer 2

In the same discussion about Commit Generation Number that you mention, Jakub Narębski does confirm that grafts are more aproblem than a solution:

grafts are so horrible hack that I would be not against turning off generation numbers if they are used.
In the case of replace objects you need both non-replaced and replaced DAG generation numbers.
[...] Grafts are non-transferable, and if you use them to cull rather than add history they are unsafe against garbage collection... I think.

(publishing has always been taken care of with git filter-branch, as illustrated by this 2008 thread on grafts workflow.)

The difference between grafts and git replace is best illustrated by this SO question "Setting git parent pointer to a different parent", and the comments of (Jakub's again) answer.

It does include the reference to Git1.6.5

From what I understand (from GraftPoints), git replace has superseded git grafts (assuming you have git 1.6.5 or later)

(Jakub:)

• if you want to rewrite history then grafts + git-filter-branch (or interactive rebase, or fast-export + e.g. reposurgeon) is the way to do it.
• If you want/need to preserve history, then git-replace is far superior to graft

Answer 3

If you need to rewrite a parent commit using git replace, this is how to do it.

As Philip Oakley mentioned, git replace simply replaces one commit with another. To graft on a parent to an existing commit, you need to first create a fake commit with the correct parent.

Say you have two git branchs you want to graft:

(a)-(b)-(c) (d)-(e)-(f)

Now we want (d) to be the parent of (c). So we create a replacement for (c) with the correct parent (let's call this c1), then git replace (c) with (c1). In these steps each of the letters refers to the SHA1 hash representing that commit.

To create the new commit:

git checkout d
git rm -rf * # remove all files from working direcotry
git checkout c -- . # commit everything from c over top of it
GIT_AUTHOR_DATE="..." GIT_COMMITTER_DATE="..." git commit -m "..." # create replacement commit with date author

Now you have commit (c1) which has the correct parent (d). So all we need to do is replace the existing (c) with (c1):

git replace c c1

Now your history looks like this:

(a)-(b)-(c1)-(d)-(e)-(f)

Bingo!