[RFC PATCH 0/2] pretty: add %I formatting for patch-id

[Michael McClimon <michael@mcclimon.org>]

I'll say up front that this patch doesn't actually work, but I am stuck
on it and interested in continuing, and am hopeful someone can point me
in something closer to the right direction. (Also, the proposed log
messages are not good, and I would not actually submit the patch for
merge consideration with these messages.)

I would like to have a single-command way to get the patch id for a
commit: the thing you'd see in a pipeline like
    git diff-tree --patch-with-raw HEAD | git patch-id

My initial thought was to add a --patch-id flag to git diff-tree, but
then I thought that maybe better would be to add a pretty specifier to
do so, so that (for instance) you could generate patch-ids for
everything in a branch by saying something like
    git log --pretty='%I %H' start..

I have taken a pass at doing so here, but it doesn't _work_, and I'm not
sure why. It seemed like the thing to do was to use commit_patch_id from
patch-ids.c, but I think that either I'm not holding it correctly, or
that it's not fit for purpose. It was added (or rather, made public) in
ded2c097 (patch-ids: make commit_patch_id() a public helper function,
2016-04-26), which was in service of fa2ab86d (format-patch: add
'--base' option to record base tree info, 2016-04-26). It seems like at
the very least, it probably won't work as expected on merge commits.

The thing that is perplexing to me is that it _does_ appear to work on
some commits, for example (where 8b3aa36f is a recent-ish commit from
master, chosen at random):

$ ./git --no-pager show -s --format='%I %H' 8b3aa36f5a7a0c923bc4a28ff19caae78644ae08
ec66952bdef82a1fc6d31c1057195af31c86da48 8b3aa36f5a7a0c923bc4a28ff19caae78644ae08

$ git diff-tree --patch-with-raw 8b3aa36f5a7a0c923bc4a28ff19caae78644ae08 | git patch-id
ec66952bdef82a1fc6d31c1057195af31c86da48 8b3aa36f5a7a0c923bc4a28ff19caae78644ae08

But for other commits, like the one in the test here, it does not. I
have done a bit of investigation, but I would not really call myself a C
programmer and I'm not super familiar with the codebase, so I'm a bit
stuck. I thought maybe at first I wasn't initializing the diff_options
correctly, but I suspect the problem is actually more fundamental than
that.

Anyway: I would be happy to hear any response at all, whether that's 
a) a pointer to some other way of implementing a %I pretty format,
b) an easier-to-implement possible solution (maybe diff-tree --patch-id
would actually be better), or c) saying the thing I want isn't feasible
and I should stick with the pipeline. Thanks!


Michael McClimon (2):
  patch-ids: add const modifier to commit
  pretty: add 'I' placeholder for patch-id

 patch-ids.c                   |  4 ++--
 patch-ids.h                   |  2 +-
 pretty.c                      | 11 +++++++++++
 t/t4205-log-pretty-formats.sh |  7 +++++++
 4 files changed, 21 insertions(+), 3 deletions(-)

-- 
2.42.0.424.gceadf0f3