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

On Tue, 5 Jun 2018 16:16:34 -0700
Jonathan Nieder [off-list ref] wrote:

Hi,

Jonathan Tan wrote:

quoted

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.

quoted

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/

quoted

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.

quoted

@@ -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.