Re: [PATCH 1/6] fetch-pack: clear marks before everything_local()

[Jonathan Nieder <jrnieder@gmail.com>]

Hi,

Jonathan Tan wrote:

> If tag following is required when using a transport that does not
> support tag following, fetch_pack() will be invoked twice in the same
> process, necessitating a clearing of the object flags used by
> fetch_pack() sometime during the second invocation. This is currently
> done in find_common(), which means that the work done by
> everything_local() in marking complete remote refs as COMMON_REF is
> wasted.
>
> To avoid this wastage, move this clearing from find_common() to its
> parent function do_fetch_pack(), right before it calls
> everything_local().

I had to read this a few times and didn't end up understanding it.

Is the idea that this will speed something up?  Can you provide e.g.
"perf stat" output (or even a new perf test in t/perf) demonstrating
the improvement?  Or is it a cleanup?

[...]
> --- a/fetch-pack.c
> +++ b/fetch-pack.c
> @@ -336,9 +336,6 @@ static int find_common(struct fetch_pack_args *args,
>  
>  	if (args->stateless_rpc && multi_ack == 1)
>  		die(_("--stateless-rpc requires multi_ack_detailed"));
> -	if (marked)
> -		for_each_ref(clear_marks, NULL);
> -	marked = 1;
>  
>  	for_each_ref(rev_list_insert_ref_oid, NULL);
>  	for_each_cached_alternate(insert_one_alternate_object);
> @@ -1053,6 +1050,9 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
>  	if (!server_supports("deepen-relative") && args->deepen_relative)
>  		die(_("Server does not support --deepen"));
>  
> +	if (marked)
> +		for_each_ref(clear_marks, NULL);
> +	marked = 1;
>  	if (everything_local(args, &ref, sought, nr_sought)) {

As an experiment, I tried applying the '-' part of the change without
the '+' part to get confidence that tests cover this well.  To my
chagrin, all tests still passed. :/

In the preimage, we call clear_marks in find_common.  This is right
before we start setting up the revision walk, e.g. by inserting
revisions for each ref.  In the postimage, we call clear_marks in
do_fetch_pack.  This is right before we call everything_local.

I end up feeling that I don't understand the code well, neither before
nor after the patch.  Ideas for making it clearer?

Thanks,
Jonathan