TutorialConflict

[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. We call these cases conflicts; figuring out what to do about a conflict is called resolving it.

We need to create an artificial conflict situation. 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

We'll 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 ["Commit"] the change:

 $ hg commit -t'Give thanks for dodging bullet'

Recall that in TutorialFirstChange, we created a ChangeSet in my-hello-new-output thatalso 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 revisions
 modified 1 files, added 1 changesets and 1 new revisions
 (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 perform a branch merge)

As in TutorialMerge, we have to run update -m. But here's where things change. ["Mercurial"] cannot now 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).

 ~/hg/my-hello-tmp $ hg update -m

At this point, what happens may be a matter of luck. If we are 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.

If we are not lucky, and do not have a MergeProgram installed, we will be dropped into our text editor, which will be visiting the file we need to ["Merge"]. Doing this by hand ishighly 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 try again.

Now let us continue on to TutorialConclusion.