How can I force a manual review of all changes in a merge?

Question

Look at the following example:

git log --all --oneline --graph

*   fc89735 (HEAD -> master) Merge branch 'feature'
|\
| * 034f1de (feature) modify the calculate function
| * 57aaea2 added another calculation function
* | 382de62 just add a comment
|/
* 303a4ed initial commit

Here is what I did:

1. Initial commit (feature.php) (303a4ed):

<?php
function calculate($a) {
    return ($a + 500) * 0.4 + 10;
}

2. I've created a branch FEATURE where I just added another function, a feature... (57aaea2):

<?php
function calculate($a) {
    return ($a + 500) * 0.4 + 10;
}

function anotherCaluclation($b) {
    return ($b - 500) * 0.2 + 5;
}

3. I've modified the calculation in the calculate function to fit the requirements of the FEATURE (034f1de):

<?php
function calculate($a) {
    $a += 1243028;
    return ($a + 500) * 0.4 + 10;
}

function anotherCaluclation($b) {
    return ($b - 500) * 0.2 + 5;
}

4. In MASTER I've added a comment to the calculate function (382de62)

<?php
//a crazy calculation function calculate($a) {
    return ($a + 500) * 0.4 + 10;
}

5. The Feature is finished and I want to intergate it. At this time MASTER and FEATURE have diverged, a fast-forward merge is impossible. In FEATURE the calculation was heavily modified which is correct for the branch, but incorrect for the rest of the project, including MASTER. However, imagine, the problem is unknown and I'm doing a git merge, merging FEATURE into MASTER (fc89735):

git merge feature

I get the following log:

Auto-merging feature.php
Merge made by the 'recursive' strategy.
 feature.php | 5 +++++
 1 file changed, 5 insertions(+)

And this is the resulting content:

<?php
//a crazy calculation function
function calculate($a) {
    $a += 1243028;
    return ($a + 500) * 0.4 + 10;
}

function anotherCaluclation($b) {
    return ($b - 500) * 0.2 + 5;
}

As you can see: The added line $a += 1243028; from the FEATURE branch (which is correct for that branch but not for the rest of the project) was merged into MASTER.

That means, from this moment on there is a bug in the project, introduced by GITs auto-merge.

How do you deal with that sort of problem? What are possible strategies? From this example I'd plead: All automatic merge actions need to be reviewed manually!

Is there a way to force a manual review of all changes when doing a merge?

Answer 1

After doing further research, I guess at the moment the best solution for this problem is following:

(* HEAD is a reference to the latest MASTER commit)

1. git merge --no-commit feature Do the merge, but don't commit, so the changes can be reviewed.

Now it would be nice to do a threeway-diff with the working directory, HEAD and FEATURE. But that's unfortunatley not possible with GIT at the moment. So:

2. git difftool HEAD Compare Working Directory with the last commit of the MASTER branch, correct errors if necessary
3. git difftool FEATURE Compare Working Directory with the last commit of the FEATURE branch, correct errors if necessary
4. Commit the (corrected) merge

Answer 2

Look into creating a custom merge driver for git. Here is an example: https://stackoverflow.com/a/5091756/346912

Basically, create a custom merge script that always fails the merge and then tell git to use that script for all files, or just for a specific set of files. Then the merge will always fail, and you can manually merge everything.