Re: git merge deletes my changes

From: Paul Smith <paul@mad-scientist.net>
Date: Oct 10, 2016, 17:52 UTC
Message-ID: <1476121942.15746.99.camel@mad-scientist.net>
In-Reply-To: <AM4PR03MB1636EA0DEB4C8095F04AB92ADBDB0@AM4PR03MB1636.eurprd03.prod.outlook.com>

On Mon, 2016-10-10 at 10:19 +0000, Eduard Egorov wrote:

> # ~/gitbuild/git-2.10.1/git merge -s subtree --squash ceph_ansible
> 
> Can somebody confirm this please? Doesn't "merge -s subtree" really
> merges branches?

I think possibly you're not fully understanding what the --squash flag does... that's what's causing your issue here, not the "-s" option.

A squash merge takes the commits that would be merged from the origin branch and squashes them into a single patch and applies them to the current branch as a new commit... but this new commit is not a merge commit (that is, when you look at it with "git show" etc. the commit will have only one parent, not two--or more--parents like a normal merge commit).

Basically, it's syntactic sugar for a diff plus patch operation plus some Git goodness wrapped around it to make it easier to use.

But ultimately once you're done, Git has no idea that this new commit has any relationship whatsoever to the origin branch. So the next time you merge, Git doesn't know that there was a previous merge and it will try to merge everything from scratch rather than starting at the previous common merge point.

So either you'll have to use a normal, non-squash merge, or else you'll have to tell Git by hand what the previous common merge point was (as Jeff King's excellent email suggests). Or else, you'll have to live with this behavior.