git merge --ff creates merge commit

Question

I'm trying fast forward develop branch to the main as

git checkout main git merge --ff develop git push

but that for some reason creates merge commit which introduces a new hash

Then I have did that:

git merge --verbose --ff-only develop

which ended up with

fatal: Not possible to fast-forward, aborting.

Regular --ff came up as

Merge made by the 'recursive' strategy. pom.xml | 2 +- src/main/webapp/WEB-INF/web.xml | 2 +- 2 files changed, 2 insertions(+), 2 deletions(-)

Any thoughts on that?

Can you show us the output of the git merge run? I do not think that a merge is created there, if successful.... perhaps you are trying to merge a branch that has a merge at the tip, so you end up with a merge at the tip. — eftshift0
– eftshift0, Commented Sep 30, 2022 at 16:31
Not much to add. The message sais it all. Can't ff.... that happens because where you are standing, you have diverged from the branch you are trying to merge. A ff only happens if you are standing on an ancestor of the branch you are trying to merge. — eftshift0
– eftshift0, Commented Sep 30, 2022 at 17:27
Was reading the documentation.... so --ff alows for a merge commit to happen if a ff is not possible.... it is a default option, by the way. --ff-only breaks because... well, I explained already. — eftshift0
– eftshift0, Commented Sep 30, 2022 at 17:31

eftshift0 · Accepted Answer · 2022-10-01 09:46:08Z

ff only works if you are standing on an ancestor of the branch you are trying to merge:

A <- B <- C <- (master) ^ \ D <- E <- F <- (develop) git checkout master git merge --ff-only develop

That would be ok (because master is on an ancestor of develop)

A <- B <- C <- G <- (master) ^ \ D <- E <- F <- (develop) git checkout master git merge --ff-only develop

This would not work because now master is not on an ancestor of develop.

So..... why is G there? I would not be able to know. You should be able to list that (or those) commit with this to be able to see what they are about:

git log ^develop master

Now.... if you always intend to have master on top of develop, there's more than one way to do it. The way you are attempting (git merge --ff-only) works if you are on an ancestor..... but if you always want it to work regardless of anything that you have committed in master(you don't care losing it if there's something at all), then you can do:

git checkout develop git branch -f master # set master where we are right now git checkout master

Or

git checkout master git reset --force develop

The first approach looks more efficient because the working tree is only being adjusted once whereas in the second it is twice (unless you are in master to start with).

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