surprising behavior from merge

Hi folks, I just ran in to a strange behavior with git merge.

Things start out with two branches (let's call them 'master' and 'other') pointing at a particular commit.  In master I commit a small one-line change, then make a second commit that adds some stuff just after the line changed in the previous commit.  In the other branch, i cherry-pick the second commit from master (the one that adds the new stuff).  The cherry-pick succeeds, despite the fuzzy context.  So far, so good.

Next I try to merge other into master.  I expected it to notice there was nothing to do and leave the master tree unchanged, but it applied the "add new stuff" patch to master (even though that patch is already in master) and made a commit from that.  So it silently did the wrong thing, and now the file contains two copies of stuff I added.

That is a simplified version of what happened, in my real repo there were several (unrelated and unimportant) commits on both master and the other branch.  When the surprising double-add happened, i simplified the repo to remove distractions.  The simplified repo is here if anyone wants to inspect it:  https://github.com/SebKuzminsky/merge-problem

On Fri, 11 May 2012 16:25:29 -0600, Sebastian Kuzminsky wrote:

It would obviously be helpful to supply:

  * Explicit commands that anyone on the list can try and discuss.
    For example:

      git init repo
      cd repo
      echo a  > a; git add a; git commit -m 'small one-line change'
      echo b >> a; git commit -am 'adds some stuff after'
      ...

  * Expected behavior from those commands.

  * Actual behavior from those commands.

In other words, rather than burdening people with the task of constructing a mental picture of what you have done, you should show them as directly and precisely as possible; in this way, people can go about the business of discussing your issue much more quickly and, most importantly, PRECISELY.

Sincerely, Michael Witten

On 05/11/2012 05:57 PM, Michael Witten wrote:

...

Ah, I had intended the extremely tiny git repo I linked to to provide the info in the most concise way possible. The surprising behaviour happened at the final commit in the repo, which was made by 'git merge other'.

I can email a list of commands to reproduce the issue later tonight if that would make anything clearer.

On 5/11/2012 5:05 PM, Sebastian Kuzminsky wrote:

The repository you provided is actually quite simple and clear, though I have no idea why this might be happening. Or if it is an expected behavior.

At the same time if you provide a list of command if someone will be fixing this they may server as an automated test. Git has a lot of them.

-- Illia Bobyr

Here's a little script that reproduces the problem:

----- #!/bin/bash set -e

if [ -e git-merge-problem ]; then
    echo "'git-merge-problem' exists, not running test!"
    exit 1
fi

mkdir git-merge-problem cd git-merge-problem git init

echo "first line" > myfile echo "second line" >> myfile echo "" >> myfile echo "next to last line" >> myfile echo "final line" >> myfile git add myfile git commit -m 'initial commit of myfile'

git branch other

sed -i -e 's/second line/this line is in the context of the crucial patch/' myfile git add myfile git commit -m 'change context for crucial patch'

sed -i -e 's/^$/\nthis line is added only once, but in two branches\n/' myfile git add myfile git commit -m 'add a line'

git tag add-a-line

git checkout other git cherry-pick -x add-a-line

git checkout master

git merge other

NUM_LINES=$(grep 'added only once' myfile | wc -l)
if [ $NUM_LINES -eq 2 ]; then
    echo "FAIL!  the merge added the line twice"
    cat myfile
    exit 1
elif [ $NUM_LINES -ne 1 ]; then
    echo "FAIL!  the merge added the line a strange number of times ($NUM_LINES)"
    exit 1
fi

echo "PASS! the merge added the line just once!" exit 0 -----

On May 11, 2012, at 20:25 , Illia Bobyr wrote: