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Re: [PATCH 2/6] fetch-pack: truly stop negotiation upon ACK ready

On Tue, 5 Jun 2018 16:16:34 -0700
Jonathan Nieder <jrnieder@xxxxxxxxx> wrote:

> Hi,
> Jonathan Tan wrote:
> > When "ACK %s ready" is received, find_common() clears rev_list in an
> > attempt to stop further "have" lines from being sent [1]. This appears
> > to work, despite the invocation to mark_common() in the "while" loop.
> Does "appears to work" mean "works" or "doesn't work but does an okay
> job of faking"?

"Appears to work" means I think that it works, but I don't think I can
conclusively prove it.

> > Though it is possible for mark_common() to invoke rev_list_push() (thus
> > making rev_list non-empty once more), it is more likely that the commits
> nit: s/more likely/most likely/
> or s/it is more likely that/usually/
> > in rev_list that have un-SEEN parents are also unparsed, meaning that
> > mark_common() is not invoked on them.
> >
> > To avoid all this uncertainty, it is better to explicitly end the loop
> > when "ACK %s ready" is received instead of clearing rev_list. Remove the
> > clearing of rev_list and write "if (got_ready) break;" instead.
> I'm still a little curious about whether this can happen in practice
> or whether it's just about readability (or whether you didn't figure
> out which, which is perfectly fine), but either way it's a good
> change.

I tried to figure out which, but concluded that I can't.

I think that in v2's commit message, I'll start with describing the
readability aspect.

> > @@ -1281,7 +1281,6 @@ static int process_acks(struct packet_reader *reader, struct oidset *common)
> >  		}
> >  
> >  		if (!strcmp(reader->line, "ready")) {
> > -			clear_prio_queue(&rev_list);
> >  			received_ready = 1;
> >  			continue;
> I'm curious about the lifetime of &rev_list.  Does the priority queue
> get freed eventually?

No (which potentially causes a problem in the case that fetch-pack is
invoked twice), but I fix that in patch 4/6, so I didn't bother
addressing it here. I'll add a note about the lifetime of this priority
queue in v2.