How do criss-cross merges arise in Git?

Question

I have been going through the, "git merge-base", man page and I can't understand how multiple merge bases develop. Specifically, I'm hung up on the following illustration in the man page:

When the history involves criss-cross merges, there can be more than one best common
ancestor for two commits. For example, with this topology:
  ---1---o---A
      \ /
       X
      / \
  ---2---o---o---B
both 1 and 2 are merge-bases of A and B. Neither one is better than the other (both
are best merge bases). When the --all option is not given, it is unspecified which
best one is output.

I just can't understand how such a situation could be created. I have attempted to recreate this criss-cross merge situation between branches using a test repository but I cannot replicate it. In all cases, I always wind up with 1 merge commit to which both A and B point to (instead of A and B pointing to independent merge commits as the diagram illustrates).

Can anyone illustrate how this situation can arise? Is this a common situation or an error situation?

Answer 1

I just can't understand how such a situation could be created. I have attempted to recreate this criss-cross merge situation between branches using a test repository but I cannot replicate it. In all cases, I always wind up with 1 merge commit to which both A and B point to (instead of A and B pointing to independent merge commits as the diagram illustrates).

Can anyone illustrate how this situation can arise?

Criss-cross merges can arise in different ways. On example that springs to mind is when you have two branch references pointing to the same merge commit (one of those branches being checked out) and you run git commit --amend. The following toy example does just that:

# set things up
cd ~/Desktop
mkdir crisscross
cd crisscross
git init

# make an initial commit
printf "bar\n" > README.md
git add README.md
git commit -m "add README"

# add a line at the top of the file and commit
sed -i '' '1s/^/foo\n/' README.md
git commit -am "add foo line"

# create and checkout a branch pointing at the initial commit
git checkout -b other master^

# add a line at the bottom of the file and commit
printf "baz\n" >> README.md
git commit -am "add baz line"

# do a merge and make both branches point to this merge commit
git merge master
git checkout master
git merge other

At this stage, the output of git log --graph --oneline --decorate --all is

*   30b9175 (HEAD, master, other) Merge branch 'master' into other
|\  
| * f318ead add foo line
* | 31875d9 add baz line
|/  
* 8b313a0 add README

Now amend the last commit (see How does git commit --amend work, exactly? for more details):

git commit --amend

After that, the output of git log --graph --oneline --decorate --all will be

*   69bbcbe (HEAD, master) Amended merge commit
|\  
| | *   30b9175 (other) Merge branch 'master' into other
| | |\  
| |/ /  
|/| /   
| |/    
| * f318ead add foo line
* | 31875d9 add baz line
|/  
* 8b313a0 add README

There you go: the history now contains criss-cross merges.

Is this a common situation or an error situation?

As illustrated above, the situation can arise. It may be undesirable, but it shouldn't be considered an "error state".

Answer 2

Here is a sequence of commits that would generate a criss-cross merge. More steps than the one involving the --amend, but only uses the basics. Run this script to see that you do indeed get a criss-cross merge.

#!/bin/sh
# Initialize 
git init .                                                                                                                       
echo date > file.txt; git add file.txt
git commit -m "Initial commit"

# Make branches                                                                                                                     
git branch A; git branch B

# Make change in A                                                                                                                  
git checkout A
echo date > fileA.txt; git add fileA.txt
git commit -m "first commit along branch A"

# Make change in B; add tag for later                                                                                                               
git checkout B
echo date > fileB.txt; git add fileB.txt
git commit -m "first commit along branch B"
git tag "tag-B"

# Merge A into B (still on B)                                                                                                       
git merge --no-edit A; git tag "M"

# Add another commit on B whose parent is M                                                                                         
echo date > fileB2.txt; git add fileB2.txt
git commit -m "second commit along B"

# Switch to A and add a commit; note that
# M is not an ancestor of this commit                                                                          
git checkout A
echo date > fileA2.txt; git add fileA2.txt
git commit -m "second commit along A"

echo "Best common ancestors (before criss-cross):"
# Should be only one
git merge-base --all A B

# Merge the commit tagged "tag-B" 
# into A generating the cris-cross-merge                                                            
git merge --no-edit tag-B

echo "Best common ancestors (after criss-cross):"
# Should be two
git merge-base --all A B