Rebase and retag, automatically


Moving tags manually sucks. Git hooks help.

In previous post Rebase and retag, we saw that doing post patching on the spot (i.e. in the master branch, which is also the published one) is a bit at odds with the automatic publishing system (as described by ETOOBUSY automated publishing).

We also saw that, if there is a need for quick patching a post, the best option is to apply the patch on the master branch and then rebase the devel branch onto it, and then move all date-shaped tags onto the newly created commits (matching with the old ones). Totally uncool.

Git hooks

When Git does… what it does, it goes through a series of steps and, in some cases, you can insert some execution between two of these steps. You do this using git hooks.

Each hook is a program with the right name (depending on the specific command/steps you want to plug into), the right execution bits (i.e. it can be executed), living in the right place (<GIT-DIR>/hooks).

The hook that we are interested into in this case is named post-rewrite. Here is an extract from the docs:

This hook is invoked by commands that rewrite commits ([…] git-rebase […]). Its first argument denotes the command it was invoked by: currently one of amend or rebase.

The hook receives a list of the rewritten commits on stdin, in the format:

<old-sha1> SP <new-sha1> [ SP <extra-info> ] LF

This is interesting because it is triggered by the rebase and gives us the exact mapping between the old commits and the new commits. With them, we can:

  • look at the old commit and see if it has a date-shaped tag attached to it
  • if it has one, move that tag onto the new commit


Let’s code the hook

The code for the hook is the following:

Local version here.

Line 5 makes sure that we only act upon a rebase action (i.e. we avoid doing stuff for an amend action, or whatever else might come in the future).

The loop in lines 7 to 15 goes over all the input lines, storing the old-sha1 in variable pre and new-sha1 in variable post. Variable rest collects anything else in the line, should the optional extra-info be present (it’s not as of this post).

Line 9 turns the SHA1 digest of the previous commit into a list of tags that are attached to it. Finding this command proved to be very challenging for me - it seems that my websearching-fu was quite weak to this extent!

In case of errors, we just don’t do anything - this is the reason of the || printf \\n in line 9.

Tags are printed one per line, which is where grep shines. For this reason, line 10 allows us to only keep tags that look like a date in YYYY-MM-DD.

At this point, we have everything we need: the ŧag (read in at line 11) and the new commit that it should be attached to (in variable post), so we just have to force the tag on it (line 12).

It’s also useful to print out the list of tags that were moved (line 13 and, later line 16), so that we can later use it for pushing the tags to the remote repository.

Time to use it

Now, let’s use it:

$ git rebase master
First, rewinding head to replay your work on top of it...
Applying: Another commit
Applying: Another prime (5 was not!)
Applying: Now I put 7
 2020-06-05 2020-06-06

Now we can copy that last line and easily build a command to push these changed tags towards the private repository:

$ git push -f private-repository 2020-06-05 2020-06-06

And this is, finally, it!

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