annotate docs/evolve-good-practice.rst @ 3846:f9dad99a90d5

evolve: create a new commit instead of amending one of the divergents

This patch changes the behavior of evolve command while resolving
content-divergence to create a new commit instead of amending one of the
divergent ones.

In past, I have made this change, backed out this change and now today again I
am doing this change, so let's dive in some history.

Using cmdrewrite.amend() was never a good option as that requires hack to delete
the evolvestate and also gives us less control over things. We can't make the
commit on top of different parents as that of content-divergent ones. Due to all
these, I first made this change to create a new commit instead of amending one.

But, after few days, there was flakiness observed in the tests and turned out
that we need to do some dirstate dance as repo.dirstate.setparents() does not
always fix the dirstate. That flakiness was a blocker for progress at that time
and we decided to switch to amend back so that we can have things working with
some hacks and we can later fix the implementation part.

Now, yesterday while tackling resolving content-divergence of a stack which is
as follows:

C1 C2
 | |
B1 B2
 | |
A1 A2
 \/
base

where, A1-A2, B1-B2, C1-C2 are content-divergent with each other. Now we can
resolve A1-A2 very well because they have the same parent and let's say that
resolution leads to A3.
Now, we want to resolve B1-B2 and make the new resolution commit on top of A3 so
that we can end up something like:

C3
 |
B3
 |
A3
 |
base

however, amending one of the divergent changesets, it's not possible to create a
commit on a different parent like A3 here without some relocation. We should
prevent relocation as that may leads to some conflicts and should change the
parent before committing.

So, looking ahead, we can't move with using amend as still using that we will
need some relocation hacks making code ugly and prone to bad behaviors, bugs.
Let's change back to creating a new commit so that we can move forward in a good
way.

About repo.dirstate.setparents() not setting the dirstate, I have researched
yesterday night about how we can do that and found out that we can use
cmdrewrite._uncommitdirstate() here. Expect upcoming patches to improve the
documentation of that function.

There are lot of test changes because of change in hash but there is no behavior
change. The only behavior change is in test-evolve-abort-contentdiv.t which is
nice because creating a new commit helps us in stripping that while aborting.
We have a lot of testing of content-divergence and no behavior change gives
enough confidence for making this change.

I reviewed the patch carefully to make sure there is no behavior change and I
suggest reviewer to do the same.

.. Copyright 2011 Pierre-Yves David <[e-mail]>
..                Logilab SA        <[e-mail]>

-----------------------------------------
Good practice for (early) users of evolve
-----------------------------------------

Avoid unstability
-----------------

The less unstability you have the less you need to resolve.

Evolve is not yet able to detect and solve every situation. And your mind is
not ready neither.

Branch as much as possible
--------------------------

This is not MQ; you are not constrained to linear history.

Making a branch per independent branch will help you avoid unstability
and conflict.

Rewrite your changes only
-------------------------

There is no descent conflict detection and handling right now.
Rewriting other people's changesets guarantees that you will get
conflicts. Communicate with your fellow developers before trying to
touch other people's work (which is a good practice in any case).

Using multiple branches will help you to achieve this goal.

Prefer pushing unstability to touching other people changesets
--------------------------------------------------------------


If you have children changesets from other people that you don't really care
about, prefer not altering them to risking a conflict by stabilizing them.


Do not get too confident
------------------------

This is an experimental extension and a complex concept. This is beautiful,
powerful and robust on paper, but the tool and your mind may not be prepared for
all situations yet.