reuse commit->buffer when parsing signaturesjk/show-sig-reuse-commit-buffer

When we call show_signature or show_mergetag, we read the
commit object fresh via read_sha1_file and reparse its
headers. However, in most cases we already have the object
data available as commit->buffer.  This is partially
laziness in dealing with the memory allocation issues, but
partially defensive programming, in that we would always
want to verify a clean version of the buffer (not one that
might have been munged by other users of the commit).

However, we do not currently ever munge commit->buffer, and
not using the already-available buffer carries a fairly big
performance penalty when we are looking at a large number of
commits. Here are timings on linux.git:

  [baseline, no signatures]
  $ time git log >/dev/null
  real    0m4.902s
  user    0m4.784s
  sys     0m0.120s

  [before]
  $ time git log --show-signature >/dev/null
  real    0m14.735s
  user    0m9.964s
  sys     0m0.944s

  [after]
  $ time git log --show-signature >/dev/null
  real    0m9.981s
  user    0m5.260s
  sys     0m0.936s

Note that our user CPU time drops almost in half, close to
the non-signature case, but we do still spend more
wall-clock and system time, presumably from dealing with
gpg.

An alternative to this is to note that most commits do not
have signatures (less than 1% in this repo), yet we pay the
re-parsing cost for every commit just to find out if it has
a mergetag or signature. If we checked that when parsing the
commit initially, we could avoid re-examining most commits
later on. Even if we did pursue that direction, however,
this would still speed up the cases where we _do_ have
signatures. So it's probably worth doing either way.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/commit.h b/commit.h
index bd841f4d0c..8cde84e4fa 100644
--- a/commit.h
+++ b/commit.h
@@ -257,7 +257,7 @@ struct merge_remote_desc {
  */
 struct commit *get_merge_parent(const char *name);
 
-extern int parse_signed_commit(const unsigned char *sha1,
+extern int parse_signed_commit(const struct commit *commit,
 			       struct strbuf *message, struct strbuf *signature);
 extern void print_commit_list(struct commit_list *list,
 			      const char *format_cur,