Re: [PATCH 09/10] fast-export: add a --show-original-ids option to show original names

[Elijah Newren <newren@gmail.com>]

On Sat, Nov 10, 2018 at 11:20 PM Jeff King <peff@peff.net> wrote:
>
> On Sat, Nov 10, 2018 at 10:23:11PM -0800, Elijah Newren wrote:
>
> > Knowing the original names (hashes) of commits, blobs, and tags can
> > sometimes enable post-filtering that would otherwise be difficult or
> > impossible.  In particular, the desire to rewrite commit messages which
> > refer to other prior commits (on top of whatever other filtering is
> > being done) is very difficult without knowing the original names of each
> > commit.
> >
> > This commit teaches a new --show-original-ids option to fast-export
> > which will make it add a 'originally <hash>' line to blob, commits, and
> > tags.  It also teaches fast-import to parse (and ignore) such lines.
>
> Makes sense as a feature; I think filter-branch can make its mappings
> available, too.
>
> Do we need to worry about compatibility with other fast-import programs?
> I think no, because this is not enabled by default (so if sending the
> extra lines to another importer hurts, the answer is "don't do that").
>
> I have a vague feeling that there might be some way to combine this with
> --export-marks or --no-data, but I can't really think of a way. They
> seem related, but not quite.
>
> > ---
> >  Documentation/git-fast-export.txt |  7 +++++++
> >  builtin/fast-export.c             | 20 +++++++++++++++-----
> >  fast-import.c                     | 17 +++++++++++++++++
> >  t/t9350-fast-export.sh            | 17 +++++++++++++++++
> >  4 files changed, 56 insertions(+), 5 deletions(-)
>
> The fast-import format is documented in Documentation/git-fast-import.txt.
> It might need an update to cover the new format.

We document the format in both fast-import.c and
Documentation/git-fast-import.txt?  Maybe we should delete the long
comments in fast-import.c so this isn't duplicated?

> > --- a/Documentation/git-fast-export.txt
> > +++ b/Documentation/git-fast-export.txt
> > @@ -121,6 +121,13 @@ marks the same across runs.
> >       used by a repository which already contains the necessary
> >       parent commits.
> >
> > +--show-original-ids::
> > +     Add an extra directive to the output for commits and blobs,
> > +     `originally <SHA1SUM>`.  While such directives will likely be
> > +     ignored by importers such as git-fast-import, it may be useful
> > +     for intermediary filters (e.g. for rewriting commit messages
> > +     which refer to older commits, or for stripping blobs by id).
>
> I'm not quite sure how a blob ends up being rewritten by fast-export (I
> get that commits may change due to dropping parents).

It doesn't get rewritten by fast-export; it gets rewritten by other
intermediary filters, e.g. in something like this:

   git fast-export --show-original-ids --all | intermediary_filter |
git fast-import

The intermediary_filter program may want to strip out blobs by id, or
remove filemodify and filedelete directives unless they touch certain
paths, etc.

> The name "originally" doesn't seem great to me. Probably because I would
> continually wonder if it has one "l" or two. ;) Perhaps something like
> "original-oid" might be better. That's well into bikeshed territory,
> though.

I wasn't a huge fan of "originally" either, but I just couldn't come
up with anything else that wasn't really long.  I'd be happy to switch
to original-oid.