Tutorial - merging conflicting changes

We learned how to deal with simple ["Merge"]s in TutorialMerge.

["Mercurial"] handles more complex ["Merge"] cases, too. It is not all that uncommon for two people to edit the exact same lines of a file, and then have to figure out what to do. These cases are called conflicts; figuring out what to do about a conflict is called resolving it.

Let's first create an artificial conflict situation. As we did previously, let's start by making a ["Clone"] of my-hello: 

$ cd ..
$ hg clone my-hello my-hello-not-cvs

Now let's add a new line of output to hello.c: 

$ cd my-hello-not-cvs
$ vi hello.c

We change main to read like this: 

int main(int argc, char **argv)
{
        printf("hello, world!\n");
        printf("sure am glad I'm not using CVS!\n");
        return 0;
}

And we ["Commit"] the change: 

$ hg commit -m 'Give thanks for dodging bullet'

Recall that in TutorialFirstChange, we created a ChangeSet in my-hello-new-output that also added a second line of output. What happens when we try to ["Pull"] that change in here? 

$ hg pull ../my-hello-new-output
pulling from ../my-hello-new-output
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg update' to get a working copy)

So far, so good. Let's try an ["Update"]. 

$ hg update
this update spans a branch affecting the following files:
 hello.c (resolve)
aborting update spanning branches!
(use update -m to merge across branches or -C to lose changes)

As in TutorialMerge, we have to run update -m. But here's where things change. ["Mercurial"] cannot automatically ["Merge"], because the same line of the same source file has been modified in a different way by each ChangeSet (the one we just ["Commit"]ted, and the one we just ["Pull"]ed). Instead, ["Mercurial"] will launch a program to help you do the merge manually. 

$ hg update -m

At this point, what happens depends on what programs are installed on our computer. If we are provident or lucky, and have a MergeProgram installed, we will be dropped into the MergeProgram. Here, we will be able to see what conflicts there are between the two changes, and decide how to resolve them.

["Mercurial"] is using a per-file 3-way merge. This means that there are 3 input files for merging process, and those are:

For more information about 3-way, see [http://revctrl.org/ThreeWayMerge ThreeWayMerge] on the [http://revctrl.org/ Revctrl wiki].

Else, if we do not have a MergeProgram installed, we will be dropped into a text editor, which will be visiting the file we need to ["Merge"]. Doing this by hand is highly error-prone and tedious. It is best to exit the editor and use the hg undo command to reverse the effect of the ["Pull"], then install a MergeProgram, and then try again.

Note: ["Mercurial"] looks at the HGMERGE environment variable and the config file to see which program it must run. If none is set, it runs the hgmerge script, which tries several popular programs before giving up.

Now let's continue and finish on to TutorialConclusion.