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

[Jeff King <peff@peff.net>]

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.

> --- 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).

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.

-Peff