Re: git filter-branch --subdirectory-filter not working as expected, history of other folders is preserved

[Jeff King <peff@peff.net>]

On Mon, Oct 10, 2016 at 05:12:25PM +0100, Seaders Oloinsigh wrote:

> Due to the structure of this repo, it looks like there are some
> branches that never had anything to do with the android/ subdirectory,
> so they're not getting wiped out.  My branch is in a better state to
> how I want it, but still, if I run your suggestion,
> [...]

Hmm. Yeah, I think this is an artifact of the way that filter-branch
works with pathspec limiting. It keeps a mapping of commits that it has
rewritten (including ones that were rewritten only because their
ancestors were), and realizes that a branch ref needs updated when the
commit it points to was rewritten.

But if we don't touch _any_ commits in the history reachable from a
branch (because they didn't even show up in our pathspec-limited
rev-list), then it doesn't realize we touched the branch's history at
all.

I agree that the right outcome is for it to delete those branches
entirely. I suspect the fix would be pretty tricky, though.

In the meantime, I think you can work around it by either:

  1. Make a pass beforehand for refs that do not touch your desired
     paths at all, like:

       path=android ;# or whatever
       git for-each-ref --format='%(refname)' |
       while read ref; do
         if test "$(git rev-list --count "$ref" -- "$path")" = 0; then
	   echo "delete $ref"
	 fi
       done |
       git update-ref --stdin

     and then filter what's left:

       git filter-branch --subdirectory-filter $path -- --all

or

  2. Do the filter-branch, and because you know you specified --all and
     that your filters would touch all histories, any ref which _wasn't_
     touched can be deleted. That list is anything which didn't get a
     backup entry in refs/original. So something like:

       git for-each-ref --format='%(refname)' |
       perl -lne 'print $1 if m{^refs/original/(.*)}' >backups

       git for-each-ref --format='%(refname)' |
       grep -v ^refs/original >refs

       comm -23 refs backups |
       sed "s/^/delete /" |
       git update-ref --stdin

-Peff