Re: pre-rebase safety hook

Junio C Hamano [off-list ref] writes:

If you want to prevent a branch whose tip commit is on more than one
branches from being rebased, I think something like this would suffice.

    #!/bin/sh
    LF='
    '
    in_branches=$(git branch -a --with "${2-HEAD}")
    case "$in_branches" in
    *"$LF"*)
	: this commit is on more than two branches
        exit 1
        ;;
    esac
    exit 0

But I didn't test it.

Actually, the above cannot possibly be right.  To decide whether to allow
rebasing of a branch or not, you need to also give it from which commit
the rebase will rewrite.

For example, suppose you have a branch "topic", that was forked from
"master" and built two commits, then another branch "side" was forked from
that, and you have three more commits on "topic" since then:

               o "side"
              /  
         A---B---C---D---E "topic"
        /
    ---o---o---o---o "master"

Now, can I allow you to rebase "topic"?  It depends.  These should be
allowed:

	git rebase B "topic"
	git rebase C "topic"
	git rebase D "topic"

but rebasing "topic" on top of "master", or anything that changes the fact
that "topic" contains commits A and B, should be prohibited, because it
will interfere with "side".  For example,

	git rebase A "topic"

would make this history:

           B---o "side"
          /
         A---B'--C'--D'--E' "topic"
        /
    ---o---o---o---o "master"

where B' and B are different commits.

So you need to check all the commits that will be affected by the rebase
to see if any of them is on a branch other than the one that is being
rebased.  The set of commits that needs to be checked are:

        git rev-list "$1..${2-HEAD}"

so a naive implementation that is based on brnach --with would probably
look like:

	#!/bin/sh

	: allow rebasing a detached HEAD
	git symbolic-ref -q HEAD || exit 0

        LF='
        '
        git rev-list "$1..${2-HEAD}" |
        while read commit
        do
        	case "$(git branch -a --with $commit)" in
                *"$LF"*)
                	: this is on two or more branches
                        exit 1
                        ;;
		esac
	done