Re: [PATCHv2 6/7] builtin/describe.c: describe a blob

[Junio C Hamano <gitster@pobox.com>]

Jacob Keller <jacob.keller@gmail.com> writes:

> On Thu, Nov 2, 2017 at 10:18 PM, Junio C Hamano <gitster@pobox.com> wrote:
>> ...
>> It is easy to imagine that we can restrict "git log" traversal with
>> a "--blobchange=<blob>" option instead of (or in addition to) the
>> limitation <pathspec> gives us.  Instead of treating a commit whose
>> diff against its parent commit has any filepair that is different
>> within <pathspec> as "!TREESAME", we can treat a commit whose diff
>> against its parent commit has a filepair that has the <blob> on
>> either side of the filepair as "!TREESAME" (in other words, we
>> ignore a transition that is not about introducing or forgetting the
>> <blob> we are looking for as an "interesting change").  That would
>> give you a commit history graph in which only (and all) such commits
>> that either adds or removes the <blob> in it.
>>
>> Hmm?
>
> This seems quite useful in the context of figuring out how a file got
> to such a state. This is useful to me, since if I know the state of a
> file (ie: it's exact contents) I can determine the blob name, and then
> use that to lookup where it was introduced.

This is probably a bit harder than an average #leftoverbits, but if
somebody wants to get their hands dirty, it should be reasonably
straightforward.  The needed changes would roughly go like so:

 - Add "struct oid *blob_change;" to diff_options, initialized to
   NULL.

 - Teach diff_opt_parse() a new option "--blobchange=<blob>".
   Allocate a struct oid when you parse this option and point at it
   with the blob_change field above.

 - Write diffcore-blobchange.c, modeled after diffcore-pickaxe.c,
   but this should be a lot simpler (as there is no need for an
   equivalent for "pickaxe-all" option).  It would

   - prepare an empty diff_queue_struct "outq";
   - iterate over the given diff_queue "q", and 
     - a filepair p whose p->one is blob_change and p->two is not,
       (or the other way around) is added to outq with diff_q()
     - a filepair whose p->one/p->two do not involve blob_change
       is freed with diff_free_filepair().
   - replace "q" with "outq".

 - Add a call to diffcore_blobchange() early in diffcore_std(),
   probably immediately after skip-stat-unmatch, when blob_change
   field is not NULL.

 - Arrange that blob_change is propagated to revs->pruning in
   setup_revisions().