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 'hg merge' to merge across branches or 'hg update -C' to lose changes)

As in TutorialMerge, we have to run hg merge. As before, the MergeProgram will be started under cover. It will usually not be able to ["Merge"] automatically, 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). 

$ hg merge

At this point, what happens depends on what programs are installed on our computer. If we are provident or lucky, and have a graphical MergeProgram installed, 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 graphical 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 rollback command to reverse the effect of the ["Pull"], then install a MergeProgram, and then try again.

(Note: before Mercurial version 0.9, hg update -m should have been used in place of hg merge and hg undo should have been used instead of hg rollback).

Now let's continue and finish on to TutorialConclusion.