• TutorialConflict

Tutorial - Merging conflicting changes

(This page is part 8 of 9 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 commited, and the one we just pulled).

$ hg merge

At this point, what happens depends on how Mercurial is configured (see MergeToolConfiguration). Per default, Mercurial drops into the editor marking the conflicts in the file for manual resolution:

/*
 * 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");
<<<<<<< /home/adi/tmp/tutorial/my-hello-not-cvs/hello.c
        printf("sure am glad I'm not using CVS!\n");
=======
        printf("sure am glad I'm using Mercurial!\n");
>>>>>>> /tmp/hello.c~other.2xAVqv
        return 0;
}

In the editor, we delete the conflict markers and keep the "sure am glad I'm using Mercurial!\n" line, deleting the line about CVS. When we leave the editor, Mercurial asks:

Was the merge successful? [y/n]

Here, we answer "y". Mercurial now reports the summary of the manual merge process:

0 files updated, 1 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)

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. However, Mercurial can be configured to call external three-way merge tools. Information about configuring three-way merge tools can be found at MergeToolConfiguration.

Now let's continue and finish on to TutorialConclusion.

CategoryTutorial