Re: [RFC] cherry-pick using multiple parents to implement -x

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"Stephen R. van den Berg" <srb@xxxxxxx> writes:
> The questions now are:
> - Would there be good reason not to record the backport/forwardport
>   relationship in the additional parents of a commit?

In general, I do not think what you did is a good idea.  The _only_ case
you can do what you did and keep your sanity is if you cherry-picked every
single commit that matters from one branch to the other.

If something is not "parent", you shouldn't be recording it as such.

Remember, when you are making a commit on top of one or more parents, you
are making this statement:

 * I have considered all histories leading to these parent commits, and
   based on that I decided that the tree I am recording as a child of
   these parents suits the purpose of my branch better than any of them.

This applies to one-parent case as well.

Imagine you have two histories, forked long time ago, and have side-port
of one commit:

             o---...o---B---A
            /
        ---o---o---...o---X---A'

What side-porting A from the top history to create A' in the bottom
history means is that the change between B and A in the top history, and
no other change from the top history, is applied on top of X to produce
state A' in the bottom history.  What B did is not included in the bottom
history.

If you recorded A' with parents A and X.  Here is what you would get:

             o---...o---B---A
            /                \ (wrong)
        ---o---o---...o---X---A'

But that is not what you did.  The tree state A' lacks what B did, which
could be a critical security fix, and you didn't consider all history that
leads to A when you cherry-picked it to create A'.

To put it another way, having the parent link from A' to A is a statement
that A' is a superset of A.  Because A contains B, you are claiming A'
also contains B, which is not the case in your cherry-picked history.
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