MercurialSquashCommit

version control

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I've recently had a bit of a fiddle squashing some commits with Mercurial, so thought it was worth a post in case anyone else is looking to do this. I don't know whether this is the best procedure, but it seemed to work pretty well for me.

hg clone base working
# tip of base is revision 73
cd working
# do work, committing on the way
cd ..
hg clone working squash
cd squash
hg qimport -r 74:tip
hg qgoto 74.diff
hg qfold $(hg qunapp)
hg qfinish -a
cd ../base
hg pull ../squash

The basic task I was doing was some fairly severe moving around of files and folders. I wanted to do this in several steps to checkpoint my work as I went, but I wanted a single commit in the version history. (I gather git does this more easily with rebase.) Making a single commit makes it easier to understand what happened - particularly since moving files tends to complicate looking at repository logs. Moving files also complicates the process - a couple of times I ended up with a procedure that didn't work because it lost the ability to track the moves - I want to be able to go hg log -f and see when and what the original commits were before the move.

To begin I needed to enable the mq extension (mercurial queues) and set my diffs to git style. Git style diffs help to track file moves properly.

# in ~/.hgrc
[extensions]
mq=

[diff]
git=true 

When using Mercurial in this way, it seems the general way of working is to have multiple repositories. Mercurial encourages different repositories where other systems, eg git or svn, would use different branches. People argue about this, but it's the Mercurial way of working. For this example I had 'base' as my original repos.

My first step was to clone base into a working repos.

hg clone base working

At this point the tip of base (and working) was revision 73. I did the file moves, with several checkpoint revisions as I went.

cd working
hg mv foo1 newdir/foo1
.. more hg mv ..
hg ci -m "moving around"
.. more hg mv ..
hg ci -m "moving around"
.. more hg mv and hg ci..
cd ..

By the time I was done the last revision was 80.

To squash them down into a single commit I cloned another repos.

hg clone working squash

It's important to clone at this point because I was about to edit history, so wanted to keep the original history handy until I knew it had worked. I now moved into there.

cd squash

Now I turned all the commits I'd done for the revisions into patches for the mercurial patch queue mechanism.

hg qimport -r 74:tip

I made the first change the current patch

hg qgoto 74.diff

I squashed all the patches together into a single patch

hg qfold $(hg qunapp)

The commit message for this folded patch would be all the individual commit messages linked together. I wanted a single message for my clean commit.

hg qrefresh -m "reorganized files"

I then turned the patch into a regular commit.

hg qfinish -a

I now had a single commit with all that work. I looked through it to see that everything was sane, in particular testing hg log -f on some moved files to ensure the history was still there. Once I was convinced all was well, I pulled the single changeset into the base repos.

 
cd ../base
hg pull ../squash
  

It's interesting to see how the attention on version control system has changed over the years. Early on the primary and only purpose was audit - to be able to safely go back to older revision - mainly to diagnose problems. Then attention switched to how they enabled collaboration between people. This didn't replace the need for audit, but built on top of it. Now there's more attention to using them to provide a narrative of how a code base changes - hence the desire for history rewriting commands like this. Again this need is built on top of the other two, but introduces new capabilities and new tensions.

My thanks to my colleague Chris Turner for his help and I also found this page very useful.

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