Mercurial: an incident with "hg pull --rebase"

Our team uses Mercurial. Some time ago we started using rebase instead of merges to have clean linear timeline. The rebase approach seemed to be straightforward and almost automatic. But recently we run into an issue: a code from the development branch leaked into the stable branch, the branch that is indended for bugfixes only.

Below is a reconstruction of the incident. While it might appear to be a lot of text, it is actually just a couple of commits, so please don't be scared and read on.

Consider a team of two developers - Leo and Penny - working on a product. The product's repository has two branches: 'default' and 'stable'. New features are being developed on the 'default' branch. Bugs are fixed on the 'stable' branch. From time to time the 'stable' branch is merged back into the 'default'.


hg init repo
cd repo
@echo > product.cs
hg add
hg commit -u leo -m "initial version"
hg branch stable
hg commit -u leo -m "creating 'stable' branch"

1. Leo fixes a bug and pushes his changes:

hg clone repo repo-leo

cd repo-leo

hg up stable

@echo > bugfix-01.cs

hg add

hg commit -u leo -m "fixed: bug-01"

hg push

2. Penny fixes a bug and merges the 'stable' into the 'default':

hg clone repo repo-penny
cd repo-penny
hg up stable
@echo > bugfix-02.cs
hg add
hg commit -u penny -m "fixed: bug-02"
hg up default
hg merge stable
hg commit -u penny -m "merged bugfixes into the default branch"

3. Penny writes a new feature:

hg up default
@echo > new-feature.cs
hg add
hg commit -u penny -m "new cool feature"

Penny's repository: bug fixed, feature implemented.

4. Now Penny wants to push her changes:

hg push
pushing to C:\Temp\repo
searching for changes
abort: push creates new remote head 4fa702a0c723 on branch 'stable'!
(you should pull and merge or use push -f to force)

But the server aborts the push. Penny understands that someone has already pushed changes, so she does a regular routine: pull --rebase

hg up stable
hg pull --rebase

Damage is done!

Penny's repository before rebase

Penny's repository after rebase. A code from the development branch is now on the stable branch.

Look at the screenshots illustrating Penny's repository before and after rebase. Notice the "new cool feature" changeset that should be on the development branch. Now it is on the branch 'stable' and that should have never happen...

(to be continued)