Re: [PATCH v2] peel_onion(): add support for <rev>^{tag}

[Jeff King <peff@peff.net>]

On Mon, Sep 02, 2013 at 01:42:31AM -0400, Richard Hansen wrote:

> Complete the <rev>^{<type>} family of object specifiers by having
> <rev>^{tag} dereference <rev> until a tag object is found (or fail if
> unable).
> 
> At first glance this may not seem very useful, as commits, trees, and
> blobs cannot be peeled to a tag, and a tag would just peel to itself.
> However, this can be used to ensure that <rev> names a tag object:
> 
>     $ git rev-parse --verify v1.8.4^{tag}
>     04f013dc38d7512eadb915eba22efc414f18b869
>     $ git rev-parse --verify master^{tag}
>     error: master^{tag}: expected tag type, but the object dereferences to tree type
>     fatal: Needed a single revision
> 
> Signed-off-by: Richard Hansen <rhansen@bbn.com>
> ---

FWIW, this makes sense to me. You can already accomplish the same thing
by checking the output of $(git cat-file -t $name), but this is a
natural extension of the other ^{} rules, and I can see making some
callers more natural.

>  Documentation/revisions.txt | 3 +++
>  sha1_name.c                 | 2 ++

Can you please add a test (probably in t1511) that checks the behavior,
similar to what you wrote in the commit message?

> diff --git a/sha1_name.c b/sha1_name.c
> index 65ad066..6dc496d 100644
> --- a/sha1_name.c
> +++ b/sha1_name.c
> @@ -679,6 +679,8 @@ static int peel_onion(const char *name, int len, unsigned char *sha1)
>  	sp++; /* beginning of type name, or closing brace for empty */
>  	if (!strncmp(commit_type, sp, 6) && sp[6] == '}')
>  		expected_type = OBJ_COMMIT;
> +	else if (!strncmp(tag_type, sp, 3) && sp[3] == '}')
> +		expected_type = OBJ_TAG;

This is not a problem you are introducing to this code, but the use of
opaque constants like commit_type along with the magic number "6"
assuming that it contains "commit" seems like a maintenance nightmare
(the only thing saving us is that it will almost certainly never change
from "commit"; but then why do we have the opaque type in the first
place?).

I wonder if we could do better with:

  #define COMMIT_TYPE "commit"
  ...
  if (!strncmp(COMMIT_TYPE, sp, strlen(COMMIT_TYPE))
      && sp[strlen(COMMIT_TYPE)] == '}')

Any compiler worth its salt will optimize the strlen on a string
constant into a constant itself. The length makes it a bit less
readable, though.

I wonder if we could do even better with:

  const char *x;
  ...
  if ((x = skip_prefix(sp, commit_type)) && *x == '}')

which avoids the magic lengths altogether (though the compiler cannot
optimize out the strlen call inside skip_prefix, because we declare
commit_type and friends as an extern. It probably doesn't matter in
peel_onion, though, which should not generally be performance critical
anyway).

-Peff