Subject: Re: [PATCH] commit-graph: don't show progress percentages while expanding reachable commits

From: SZEDER Gábor <[email-masked]>
Date: Fri, 22 Mar 2019 15:55:50 +0100

On Fri, Mar 22, 2019 at 03:28:34PM +0100, Ævar Arnfjörð Bjarmason wrote:
> 
> On Fri, Mar 22 2019, SZEDER Gábor wrote:
> 
> > On Fri, Mar 22, 2019 at 12:11:26PM +0100, Ævar Arnfjörð Bjarmason wrote:
> >>
> >> On Fri, Mar 22 2019, SZEDER Gábor wrote:
> >>
> >> > Commit 49bbc57a57 (commit-graph write: emit a percentage for all
> >> > progress, 2019-01-19) was a bit overeager when it added progress
> >> > percentages to the "Expanding reachable commits in commit graph" phase
> >> > as well, because most of the time the number of commits that phase has
> >> > to iterate over is not known in advance and grows significantly, and,
> >> > consequently, we end up with nonsensical numbers:
> >> >
> >> >   $ git commit-graph write --reachable
> >> >   Expanding reachable commits in commit graph: 138606% (824706/595), done.
> >> >   [...]
> >> >
> >> >   $ git rev-parse v5.0 | git commit-graph write --stdin-commits
> >> >   Expanding reachable commits in commit graph: 81264400% (812644/1), done.
> >> >   [...]
> >> >
> >> > Therefore, don't show progress percentages in the "Expanding reachable
> >> > commits in commit graph" phase.
> >>
> >> There's indeed a bug here as your examples show, but there *are* cases
> >> where it's correct, as the commit message for my patch on "master" shows
> >> there's cases where we correctly.
> >>
> >> So this "fixes" things by always removing the progress, why not instead
> >> pass down the state to close_reachable() about what we're walking over,
> >> so we can always show progress, or at least in some cases?
> >
> > The cases where it does display correct percentages are exceptional,
> > and doesn't worth the effort to try to find out whether ther current
> > operation happens to be such a case.
> 
> It's the "write" entry point without arguments that displays the correct
> progress. So not exceptional, but yeah, it's not what we use on "gc".

Bit it displays the correct number only if all the reachable commits
are in packfiles, which is not necessarily the case (e.g. unpacked
small packs during 'git fetch').

> In any case, the problem is that sometimes we've walked the full set of
> commits already, and some other times we haven't.

... and that we can't really be sure whether we've walked the full set
of commits until after this loop.

> So in cases where we have we can show progress, and as a TODO (I think
> this came up in previous discussions), we could do better if we had a
> approximate_commit_count().
> 
> In any case, the below fix seems correct to me, but I haven't poked it
> much. It *does* suffer from a theoretical race with the progress bar
> similar to d9b1b309cf ("commit-graph write: show progress for object
> search", 2019-01-19), but I work around it in the same way:
> 
> diff --git a/commit-graph.c b/commit-graph.c
> index 47e9be0a3a..0fab3d8b2b 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -693,7 +693,8 @@ static void add_missing_parents(struct packed_oid_list *oids, struct commit *com
>  	}
>  }
>  
> -static void close_reachable(struct packed_oid_list *oids, int report_progress)
> +static void close_reachable(struct packed_oid_list *oids, int report_progress,
> +			    uint64_t oids_count_for_progress)
>  {
>  	int i;
>  	struct commit *commit;
> @@ -717,7 +718,8 @@ static void close_reachable(struct packed_oid_list *oids, int report_progress)
>  	 */
>  	if (report_progress)
>  		progress = start_delayed_progress(
> -			_("Expanding reachable commits in commit graph"), oids->nr);
> +			_("Expanding reachable commits in commit graph"),
> +			oids_count_for_progress);
>  	for (i = 0; i < oids->nr; i++) {
>  		display_progress(progress, i + 1);
>  		commit = lookup_commit(the_repository, &oids->list[i]);
> @@ -725,6 +727,8 @@ static void close_reachable(struct packed_oid_list *oids, int report_progress)
>  		if (commit && !parse_commit(commit))
>  			add_missing_parents(oids, commit);
>  	}
> +	if (oids->nr < oids_count_for_progress)
> +		display_progress(progress, oids_count_for_progress);
>  	stop_progress(&progress);
>  
>  	if (report_progress)
> @@ -829,6 +833,7 @@ void write_commit_graph(const char *obj_dir,
>  	uint64_t progress_cnt = 0;
>  	struct strbuf progress_title = STRBUF_INIT;
>  	unsigned long approx_nr_objects;
> +	uint64_t oids_count_for_progress = 0;
>  
>  	if (!commit_graph_compatible(the_repository))
>  		return;
> @@ -934,9 +939,10 @@ void write_commit_graph(const char *obj_dir,
>  		if (oids.progress_done < approx_nr_objects)
>  			display_progress(oids.progress, approx_nr_objects);
>  		stop_progress(&oids.progress);
> +		oids_count_for_progress = oids.nr;
>  	}
>  
> -	close_reachable(&oids, report_progress);
> +	close_reachable(&oids, report_progress, oids_count_for_progress);
>  
>  	if (report_progress)
>  		progress = start_delayed_progress(
>