Re: rebase parents, or tracking upstream but removing non-distributable bits

On Thu, Dec 30, 2010 at 03:54:29PM -0200, Alexandre Oliva wrote:

Given this problem statement, I started looking for solutions that
didn't require modifying git.

This is a problem I have come to think a bit about already.  Although
I do not have a proper solution either, let's share those ideas.

I first looked into rewriting history, removing the unwanted bits and
replaying subsequent changes, but quickly discarded it, for it would
make my local repository incompatible with upstream both ways: I
wouldn't be able to pull from it; upstream wouldn't be able to pull to
it; third parties would run into ugly situations trying to carry patches
from either one to the other.

Now, it looks like I might be able to pull from upstream if I maintain
manually a graft file that named each upstream commit as an additional
parent of the corresponding local rebase commit that brought it into my
rewritten tree.  Workable, maybe, but this wouldn't help third parties
that used my public repository.

As a side note: I fear grafts won't scale very nicely performance-wise
if you graft every commit.

My use-case was similar to yours, about "how could Debian distribute a
git tree of the kernel ?", but my focus was on distributing
well-defined snapshots, and I ended up with the idea of grafting only
the set of successive revisions that get published - and not
necessarily have a mirror of all upstream revisions in the filtered
tree: such grafts do represent points in history where we would merge
from upstream, and without the grafts the filtered tree would look
like successive imports of well-defined revisions (notably makes it
realistic to adhere to the "all commits have been tested" paradigm).

Besides, I'm concerned that pushing from the local repository (with the
graft file) to the public repository would end up publishing the changes
I'm not supposed to distribute, because they'd be taken as parents of
the local commits.

Grafts cannot be pushed/pulled.  For this, as Jonathan suggests in
another reply, "git replace" looks like a better choice, esp. in that
by default the replacement commits don't get pulled: you need to
explicitely request fetching refs/replace/commits (note: you may need
to handle merges there when needed).  OTOH, those replacement commits
would pull the whole upstream history, so they cannot be part of the
filtered repository: it must come from an unfiltered kernel repo, that
may be a real problem if you cannot redistribute some upstream parts.

OTOH, a grafts file can be distributed out-of-band, and would only
pull the problematic contents when put in place, so it might indeed be
more adequate.

Are there any other ways to support the desired features with git as-is?
AFAICT, there isn't, so I've been thinking of how to introduce this.  I
suppose the simplest way to accomplish this is to introduce the notion
of a ???weak parent???: one that is taken into account for purposes of
checking whether a commit is present in a branch being merged- or
rebased into, but that is not transmitted over pushes, and that is not
retained over purges, and not complained about when missing.

That sounds like heavy surgery with too many implications I can think
of.

When it comes to "modifying commit metadata", it rings "git notes" in
my ear - but then, you cannot add/change commit metadata from a note,
maybe that could be a better direction to dig into, so we can add
parents when a specific notes namespace is activated ?  But then we
would get back to the problem I mentionned for refs/replace/

I'm under the impression that this could not just work, but also make
rebasing in general (especially the hard case) far less problematic, for
git would be able to relate a rebased commit with an original commit.

I suppose that by "hard case" you mean forking off a branch that gets
rebased later ?  Note that "git pull" seems to be able to cope with
this using reflogs already (although I did not test that feature).  A
less volatile place than reflogs could again using notes, without a
need for core changes.

I'm not sure this would enable upstream to easily integrate
my changes.  Or would it?

This problem suggests a more generic one: how to "merge back" most
changes from a branch while still not merging some specific changes ?
It would also help when a maint branch gets some short-term
workarounds that we don't want in the master branch, but the very idea
has a serious flaw: it implies that the "merge back" commit contains
also the commits we don't want (here, the "filtering commits").  So I
guess cherry-pick will be the way here.  Anyway, I doubt Linus would
like the idea of merging from such a filtered repo, sending patches is
probably prefered.

Best regards,
-- 
Yann