Subject: Regression in: [PATCH on sb/more-repo-in-api] revision: use commit graph in get_reference()

From: SZEDER Gábor <[email-masked]>
Date: Fri, 25 Jan 2019 16:33:48 +0100

On Tue, Dec 04, 2018 at 02:42:38PM -0800, Jonathan Tan wrote:
> When fetching into a repository, a connectivity check is first made by
> check_exist_and_connected() in builtin/fetch.c that runs:
> 
>   git rev-list --objects --stdin --not --all --quiet <(list of objects)
> 
> If the client repository has many refs, this command can be slow,
> regardless of the nature of the server repository or what is being
> fetched. A profiler reveals that most of the time is spent in
> setup_revisions() (approx. 60/63), and of the time spent in
> setup_revisions(), most of it is spent in parse_object() (approx.
> 49/60). This is because setup_revisions() parses the target of every ref
> (from "--all"), and parse_object() reads the buffer of the object.
> 
> Reading the buffer is unnecessary if the repository has a commit graph
> and if the ref points to a commit (which is typically the case). This
> patch uses the commit graph wherever possible; on my computer, when I
> run the above command with a list of 1 object on a many-ref repository,
> I get a speedup from 1.8s to 1.0s.
> 
> Another way to accomplish this effect would be to modify parse_object()
> to use the commit graph if possible; however, I did not want to change
> parse_object()'s current behavior of always checking the object
> signature of the returned object.
> 
> Signed-off-by: Jonathan Tan <[email-masked]>
> ---
> This is on sb/more-repo-in-api because I'm using the repo_parse_commit()
> function.
> 
> A colleague noticed this issue when handling a mirror clone.
> 
> Looking at the bigger picture, the speed of the connectivity check
> during a fetch might be further improved by passing only the negotiation
> tips (obtained through --negotiation-tip) instead of "--all". This patch
> just handles the low-hanging fruit first.
> ---

I stumbled upon a regression that bisects down to this commit
ec0c5798ee (revision: use commit graph in get_reference(),
2018-12-04):

  $ ~/src/git/bin-wrappers/git version
  git version 2.19.1.566.gec0c5798ee
  $ ~/src/git/bin-wrappers/git commit-graph write --reachable
  Computing commit graph generation numbers: 100% (58994/58994), done.
  $ ~/src/git/bin-wrappers/git status
  HEAD detached at origin/pu
  nothing to commit, working tree clean
  $ ~/src/git/bin-wrappers/git -c core.commitGraph=false describe --dirty
  v2.20.1-833-gcb3b9e7ee3
  $ ~/src/git/bin-wrappers/git -c core.commitGraph=true describe --dirty
  v2.20.1-833-gcb3b9e7ee3

It's all good with only '--dirty', but watch this with '--all
--dirty':

  $ ~/src/git/bin-wrappers/git -c core.commitGraph=false describe --all --dirty
  remotes/origin/pu
  $ ~/src/git/bin-wrappers/git -c core.commitGraph=true describe --all --dirty
  remotes/origin/pu-dirty

IOW if the commit-graph is enabled, then my clean worktree is reported
as dirty.

And to add a cherry on top of my confusion:

  $ git checkout v2.20.0
  Previous HEAD position was cb3b9e7ee3 Merge branch 'jh/trace2' into pu
  HEAD is now at 5d826e9729 Git 2.20
  $ ~/src/git/bin-wrappers/git -c core.commitGraph=true describe --all --dirty
  tags/v2.20.0

It's clean even with '--all' and commit-graph enabled, but watch this:

  $ git branch this-will-screw-it-up
  $ ~/src/git/bin-wrappers/git -c core.commitGraph=true describe --all --dirty
  tags/v2.20.0-dirty

Have fun! :)

>  revision.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/revision.c b/revision.c
> index b5108b75ab..e7da2c57ab 100644
> --- a/revision.c
> +++ b/revision.c
> @@ -212,7 +212,20 @@ static struct object *get_reference(struct rev_info *revs, const char *name,
>  {
>  	struct object *object;
>  
> -	object = parse_object(revs->repo, oid);
> +	/*
> +	 * If the repository has commit graphs, repo_parse_commit() avoids
> +	 * reading the object buffer, so use it whenever possible.
> +	 */
> +	if (oid_object_info(revs->repo, oid, NULL) == OBJ_COMMIT) {
> +		struct commit *c = lookup_commit(revs->repo, oid);
> +		if (!repo_parse_commit(revs->repo, c))
> +			object = (struct object *) c;
> +		else
> +			object = NULL;
> +	} else {
> +		object = parse_object(revs->repo, oid);
> +	}
> +
>  	if (!object) {
>  		if (revs->ignore_missing)
>  			return object;
> -- 
> 2.19.0.271.gfe8321ec05.dirty
>