CODEWITHSUNDEEP
gitgit-mergecherry-pick
Paul
Paul

Reputation: 26660

How does git merge after cherry-pick work?

Let's imagine that we have a master branch.

Then we create a newbranch

git checkout -b newbranch

and make two new commits to newbranch: commit1 and commit2

Then we switch to master and make cherry-pick

git checkout master
git cherry-pick hash_of_commit1

Looking into gitk we see that commit1 and its cherry-picked version have different hashes, so technically they are two different commits.

Finally we merge newbranch into master:

git merge newbranch

and see that these two commits with different hashes were merged without problems although they imply that the same changes should be applied twice, so one of them should fail.

Does git really do a smart analysis of commit's content while merging and decide that changes shouldn't be applied twice or these commits are marked internally as linked together?

Upvotes: 246

Views: 56222

Answers (2)

ephemerr
ephemerr

Reputation: 1983

After such merge you may have cherry-picked commits in history twice.

Solution to prevent this I quote from article which recommends for branches with duplicate(cherry-picked) commits use rebase before merge:

git merge after git cherry-pick: avoiding duplicate commits

Imagine we have the master branch and a branch b: 

   o---X   <-- master
    \
     b1---b2---b3---b4   <-- b

Now we urgently need the commits b1 and b3 in master, but not the remaining commits in b. So what we do is checkout the master branch and cherry-pick commits b1 and b3:

$ git checkout master
$ git cherry-pick "b1's SHA"
$ git cherry-pick "b3's SHA"

The result would be: 

   o---X---b1'---b3'   <-- master
    \
     b1---b2---b3---b4   <-- b

Let’s say we do another commit on master and we get: 

   o---X---b1'---b3'---Y   <-- master
    \
     b1---b2---b3---b4   <-- b

If we would now merge branch b into master: 

$ git merge b

We would get the following: 

   o---X---b1'---b3'---Y--- M  <-- master
     \                     /
      b1----b2----b3----b4   <-- b

That means the changes introduced by b1 and b3 would appear twice in the history. To avoid that we can rebase instead of merge: 

$ git rebase master b

Which would yield: 

   o---X---b1'---b3'---Y   <-- master
                        \
                         b2'---b4'   <-- b

Finally: 

$ git checkout master
$ git merge b

gives us: 

   o---X---b1'---b3'---Y---b2'---b4'   <-- master, b

EDIT Corrections supposed by David Lemon's comment

Upvotes: 73

helmbert
helmbert

Reputation: 38014

Short answer

Don't worry, Git will handle it.

Long answer

Unlike e.g. SVN1, Git does not store commits in delta format, but is snapshot-based2,3. While SVN would naively try to apply each merged commit as a patch (and fail, for the exact reason you described), Git is generally able to handle this scenario.

When merging, Git will try to combine the snapshots of both HEAD commits into a new snapshot. If a portion of code or a file is identical in both snapshots (i.e. because a commit was already cherry-picked), Git won't touch it.

Sources

1 Skip-Deltas in Subversion
2 Git Basics
3 The Git object model

Upvotes: 167

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