How to split a Mercurial chain into independent changes merging at the tip

I started working on a commit, let's call it C

master - C (tip)

However, after a series of splits, I ended up with something like

master - A - B - C (tip)

As it turns out, A and B are independent of each other but C depends on both. Is there a way to split this chain in such a way that I can work on A and B independently:

master - A - merge - C (tip)
      \     /
        - B

Solution

You can do this as long as you haven't pushed your changes yet. If you've pushed and other people have replicated the original series of A-B-C changesets you can't easily undo that.

There are in fact several ways to do this... this is what I think is the most straightforward.

STEP 1:

hg up master

hg revert -r B

hg commit (consider this B2)

So now B2 looks like the original B but follows master instead of A. The original B still exists for now:

master - A - B - C
       \
         B2

STEP 2:

hg merge -r A

hg revert -r C

hg commit (consider this C2)

And now you have:

master - A  - B - C
       \    \
         B2 - C2

STEP 3:

Finally clean up your original unwanted B & C:

hg strip B

Note that strip is technically an extension so you might have to enable it in your hgrc.

Comment - in step 1 you could use also graft instead of revert to replicate B to B2. I tend to use revert because it is more versatile but it does require an explicit commit that graft would avoid.