Purging stacked branches in Bazaar

Stacked branches are awesome!

Shared repositories go so far, but don't work so well if the parent and child branches are far away from each other in the file system (nor if they are on different volumes), and shared repositories have the weakness that if you create a revision, it lives on forever, even if you later delete the branch associated with that revision. (You can't actually get the revision back, not by any way I've found (UPDATE : "bzr heads --all" looks like it lets you find "lost" revisions.), but the shared repository's size never goes down - it just keeps accruing more and more data, never letting any of it go. (UPDATE : I'm no longer entirely sure when the repository's size changes - "bzr pack --clean-obsolete-packs" does wonders))

In contrast, stacked branches can be used at any time both the parent and child branch are simultaneously accessible (even if they're on different hard disks or even one on a URL), and best of all, if you make an experimental branch and decide to kill it, bam! - its history is gone forever and your trunk repository isn't forever bloated by the revisions you decided to nuke.

And they're extremely useful if you want the same library to be in multiple apps (in different Bazaar repositories) and want to be able to edit the source code in each copy of the library independently but have them all closely associated.

And did I mention they save a lot of storage space?

But thence cometh the problem : stacked branches start out tiny, because they aren't carrying the five decades of history that the trunk contains, but after that they grow.

And grow.

What if you just want the stacked branch repositories to stay nice and trim, like they were when you made them?

There doesn't seem to be any built-in feature in Bazaar to do that.

push, pull, merge, do whatever you want - the stacked branch's repository only grows.

So we resort to a little bit of - very effective - skullduggery.

FIRST UP, ENSURE YOU TRY THIS EXPERIMENTALLY FIRST. It worked for me, but might destroy you and your world and your company's beautiful source code and get you fired. THIS USES UNDOCUMENTED TRICKS. So it could stop working when new versions of Bazaar roll out. I have and accept no responsibility for what happens to you if you try this yourself!

1) Purging the stacked branch history obviously needs to be done at times that the stacked branch is in-sync with the trunk. So make sure you've merged or pushed the branch into the trunk.

2) In the branch, delete all files in these two folders :
.bzr\repository\indices
.bzr\repository\packs

3) Still in the branch, locate this file :
.bzr\repository\pack-names
... and change its content to the following five lines :

B+Tree Graph Index 2
node_ref_lists=0
key_elements=1
len=0
row_lengths=

Voila! Do a bzr status or bzr log and the history is all there - its just now coming from the stacked-on branch like you wanted all along. You have successfully purged the stacked branch's history.