Tutorial - Merging conflicting Changes

(This page is part of the Tutorial series. Previous part is TutorialMerge, next part is TutorialConclusion)

We learned how to deal with simple merges 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
updating working directory
2 files updated, 0 files merged, 0 files removed, 0 files unresolved

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 heads' to see heads, 'hg merge' to merge)

So far, so good. Let's try an update.

$ hg update
abort: crosses branches (use 'hg merge' or 'hg update -C')

As in TutorialMerge, we have to run hg merge. It will 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 committed, and the one we just pulled).

$ hg merge

At this point, what happens depends on how Mercurial is configured (see MergeToolConfiguration). Per default, Mercurial inserts a set of markers into the files to be merged in your working copy:

/*
 * hello.c
 *
 * Placed in the public domain by Bryan O'Sullivan
 *
 * This program is not covered by patents in the United States or other
 * countries.
 */

#include <stdio.h>

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

You can find these files using the 'hg status' command, as before, noting that Mercurial has saved a copy of the original file before inserting the conflict markers:

$ hg status
M hello.c
? hello.c.orig

To resolve the conflict, we open 'hello.c' in an editor, delete the conflict markers and keep the "sure am glad I'm using Mercurial!\n" line, deleting the line about CVS. We can then save and quit the editor.

Then, we let Mercurial know that we have resolved the conflict using the hg resolve command:

$ hg resolve -m hello.c

Mercurial accepts the resolution without any output.

As before, be sure to commit this change to the repository once the merge is complete:

$ hg commit -m "Merged changes from my-hello-new-output"

What we have seen here is the default behaviour of Mercurial. You can, if you want, configure Mercurial to open the editor automatically. Mercurial can also be configured to call external three-way merge tools. Information about both of these can be found at MergeToolConfiguration.

Now let's continue and finish on to TutorialConclusion.

CategoryTutorial