On rules versus hooks, or, abstraction shock

I need to add a bit of business logic to my Commerce site: a boolean field on product nodes marks that the corresponding products can't be delivered outside the UK.

And I know the way to do this in Commerce is to create a rule: react to the cart completing checkout, iterate over line items, check the corresponding products, and block the completion if the field in question is set.

Rules is great: with Rules, site builders can change site functionality and cause it to react to events. When non-techy people ask if my job involves designing websites, to put them right I say, 'I make websites go "bing!"'; and now, site builders can make them go 'bing!' too.

But I have a confession: I'm reluctant about using Rules. It's partly that I find the UI confusing, and it feels time-consuming to test them, but deeper than that I think it's just that I feel too far removed from the actual thing I'm trying to make.

And that makes me wonder: am I becoming a Drupal dinosaur?

Because I can imagine when Views first came along, developers who were used to writing their own query and formatting the result themselves, looking at the Views UI and thinking 'I don't feel in control of my lists of stuff any more'.

Or before CCK, developers wrote exactly the form elements they needed in the node form and saved it themselves in the database. I still sometimes speak to non-Drupal developers who want to be able to dump data into the node table directly (or pull it out) and when I tell them they can't, because the data that actually makes a node is spread out over the node table, the node_revision table, and then a multitude of field tables. And their feeling of disconnection at not being able to get their hands on 'the node' as a solid lump of database stuff must surely be akin to what I feel with Rules. And I'm going to call this feeling 'abstraction shock'.

I want to write a hook. I want to write the code for it, for it to feel like a solid thing. I know that my rule can (indeed, should) be exported to code, but I want code that I can read and see exactly what it says, rather than code that Rules will consume and understand. And most of all, I want to be able to put in debug statements to understand what I'm getting as I write it, and after I've written it when it's going wrong or when the site functionality has to change.

If that makes me a dinosaur, save me a seat next to the brontosaurus.

Git tricks: repatching for an issue branch

My workflow for making patches is to use a feature branch for a single issue. Whether you're a contributor or a maintainer it lets you advance the fixing of the problem in small increments, and safely experiment knowing you can roll back.

But where it goes wrong is when your patch is superseded by a newer one in the issue queue, and you want to work on it some more. How do you update your branch for the ongoing work? As ever, with git there's a way.

Let's start with the basics first: you're making a feature branch to work on an issue. I tend to follow the naming pattern '123456-fix-all-the-bugs', but for this example I'll call it 'issue'.

// Make a new branch and switch to it.
$ git co -b issue
// Make lots of commits.
// Ready to make a patch:
$ git diff > 123456.project.issue.patch

(Note that if you can make your patch to show all your commits one by one, which can sometimes aid in making it clear what you're changing, but that's for another day.)

You've now got a patch which you're uploading to the issue queue, and your tree looks something like this:

* [issue] Last commit, ready to roll a patch!
* Fixed the foobar.
* Added a bizbax.
/
* [master]

Now someone else comes along to the issue queue, reviews your patch, and posts a new patch of their own. You in turn look at patch 2, and while it's an improvement, you think it needs still more work.

The problem is how to apply the patch to your repository. It won't apply to the tip of the issue branch, and if you checkout master, you can't get back to your issue branch. You can of course just discard your original issue branch, and create a branch issue2 for patch 2.

Or you can do this:

// Start on the issue branch.
// Stash any work in progress!
$ git stash
// Checkout just the *files* of master, while keeping the HEAD pointer on the
// issue branch.
$ git checkout master -- .
// This puts the files from master into the working tree, but keeps the index
// on the issue branch. In simpler terms, the reverse of patch 1 will appear
// staged (as git believes that your files *ought* to look like patch 1, but
// actually look like master).
// We want the index clean, so unstage everything:
$ git reset HEAD .
// Now apply the new patch.
$ patch -p1 < patch-2.patch
// Now commit this as patch 2.
// Remember to stash pop when you're done!

Because the working tree files (that is, the actual files on your system) look like the master branch, the patch applies cleanly. But because git still believes its on the tip of the issue branch, the commit you make goes on the tip of that branch, and the diff it records is effectively the interdiff between your patch-1 and the other contributor's patch-2. Your tree looks like this:

* [issue] Applied patch 2 from Ada Lovelace.
* Last commit, ready to roll a patch!
* Fixed the foobar.
* Added a bizbax.
/
* [master]

Result: you can now do more work on this branch, and make more commits, and when you're ready, diff against master to make patch-3, ready to upload to the issue queue.

Git tricks: being on the wrong branch

I often find that I'm in the middle of one thing when I have to do another. Whether it's hotfixes for a client, or just finding a minor bug that blocks my current work, or needing to add components to a feature before I can add custom functionality.

The best way is to stash your current work, checkout the master branch, commit, then go back. If you're working on a feature branch (and you should be), then rebase that afterwards so you have access to the new work there. So that's:

$ git stash
$ git checkout master
// do commits
$ git checkout feature
$ git rebase master
$ git stash pop

But that's not always feasible. Sometimes I'm sloppy, and I've already made code changes before stashing. And lately, I've got one instance of Party module that's got a feature branch that's made database changes, but I don't want to hold that up ongoing commits (and I'm too lazy to set up a new local site!).

If your fix is just one commit, you can make it on the feature branch, then cherrypick it to the master branch like this:

// make your commit and note its SHA
$ git stash
$ git checkout master
$ git cherry-pick COMMIT
$ git checkout feature
$ git rebase master

The rebase should be smart enough to figure out the same commit exists on both branches, and will silently drop it from the feature branch.

Alternatively, if you want to do a chunk of main branch work, make a temporary branch on the tip of feature, which you can then move to the master branch when you're done:

$ git checkout -b moveme
// make as many commits as you like
// Now we take everything that's between the tips of feature and moveme, and move it to the tip of master
$ git rebase --onto master feature moveme
// Now merge moveme into master: this'll just fast-foward master.
$ git checkout master
$ git merge moveme
// moveme can be deleted now
$ git branch -d moveme
// Now rebase the feature branch
$ git checkout feature
$ git rebase master

As I've become more familiar with git, I've found that temporary, throw-away branches can be useful in a variety of situations. Another one is making a backup branch prior to potentially messy rebases: just create a branch 'backup' where you are to be sure that no matter what happens with the rebase, your current chain of commits will be preserved. If there are conflicts, you can diff against it to check no code was lost. And when you're happy with the result of the rebase, just delete it. Branches being cheap, and local, opens up a whole new set of uses for them.

Get out, git!

There are lots of good reasons to have your server's codebase be an actual git checkout. But there's one potential flaw: your entire repository's history ends up in your webroot inside a .git folder.

You can block access to it in your .htaccess, but that's hacking core (until this patch lands at least).

There is however an alternative method that lets you keep the entirety of git's working folder outside the webroot completely.

Here's how to convert an existing repository to this format:

  1. Move the .git folder to another location, renaming it in the process so it's no longer hidden. The convention is to leave it with a .git ending though, so for example, 'mysite.git'. I put these inside a folder called 'git' in the user's home folder, for instance.

    $ mv .git ~/git/mysite.git

  2. In its original place in your webroot, create a new file called '.git'. Into this file place a single line thus:

    gitdir: /absolute/path/to/your/mysite.git

    This needs to be an absolute path; relative ones confuse git when you go into subfolders. Using '~' to start at the user's home folder doesn't seem to work either.

  3. Finally, we need to tell the config file where the work folder is. This step isn't completely necessary, but it allows you to invoke the git command while standing in subfolders of your webroot, which is too handy a thing to lose.

    Standing either in the webroot or in the git folder, do:

    $ git config core.worktree "/absolute/path/to/your/webroot"

    You can also edit the git config file by hand to set this, which allows you to also add a comment explaining the manoeuvre for future reference.

That's all there is to it. You now have a working git repository whose working folder is completely inaccessible from the outside world.

For creating a new repo, you can use the following finger-twister:

$ git --git-dir=/path/to/repo.git --work-tree=. init && echo "gitdir: /path/to/repo.git" > .git

There are more tips in this question on StackOverflow. And for a hands-on tutorial, come to my session on git at DrupalCamp Scotland, taking place later this month in Edinburgh.

Moving a git local branch from one local to another

You're maybe at one of the many Drupal Co-worker Friday events that are taking place around the world today. You've packed up your laptop and your lunchbox, and you're looking forward to a day out of the house with some human contact.

But yesterday you were halfway through a big piece of work on your project. And you were using a git local branch, of course. Why? Because it keeps your work isolated off the main development branch, allowing other work to continue independently. And because while your commits are only local you're free to reorder them, edit the log messages, fixup mistakes as if they never happened, and so on. In fact, if you so choose, merging your local branch in with the --squash option makes it look like you made all of the work in a single, perfect commit. Wow!

But that branch is on your desktop machine, and today you're on the laptop. How to get it over from one to the other? You don't want to push it to the remote, because then that means you can't rework it, and it doesn't really belong there anyway. You could make one big patch of your branch against the development branch, but then on your laptop you won't have the dozen or so commits you made yesterday (and you work with small, simple commits, because it makes it easier to roll back should you need to, and to see what's been changed). Furthermore, you've a bunch of changes that aren't yet committed because they're work in progress. You need those too, but not mixed in with what's already committed and reasonably stable.

The solution is the git format-patch command. At first try this is a rather weird one, which fills your folder up with cryptic mailbox files (which as far as I can tell are nigh on useless in the modern world of webmail). But it has an option which turns it into something very useful indeed: --stout. So much so that I've added it to my git global config thus:

  fp  = format-patch --stdout

So standing in your repository and doing

  git fp the-dev-branch > my-local-branch.patch

will give you one single file that comprises multiple patches, one for each commit. Copy that to your laptop by your favourite means, and over there do:

  git co -b my-local-branch
  git am  my-local-branch.patch

And all your commits from your desktop machine are reproduced on your laptop.

But what about your work in progress? I was coming to that. Before you begin, make one commit of all of them, perhaps with a log message to remind you it's the work in progress.

Then after you've done 'git am', all we have to do is kill off that final tip commit, while keeping the changes it contains in the filesystem. The command for that is git reset with the --mixed option:

  git reset --mixed HEAD^

All your work in progress changes are now 'unstaged changes', and your laptop's copy of the repository is in exactly the same stage as your desktop machine.

What about going back the other way? Well I'll figure that one out tonight, I expect ;)

PS. The return trip is easily done thus:

When you create the branch on the new machine, but before you apply the patch, add a tag: git tag mytag. Then at the end of the day, do the original process in reverse but take your diff from the tab: git fp mytag > homeward.patch

And on your home machine, remember to kill the 'work in progress' commit before you apply the patch, with 'git reset --hard HEAD^'. That's a hard reset this time, since the changes in that commit are in the new commits you're bringing back.

Hope your Drupal Coworker Friday was as productive as mine!

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