Re: [PATCH v4] fetch-pack: support negotiation tip whitelist

quoted

@@ -230,7 +246,7 @@ static int find_common(struct fetch_negotiator *negotiator,
 	if (args->stateless_rpc && multi_ack == 1)
 		die(_("--stateless-rpc requires multi_ack_detailed"));

-	for_each_ref(rev_list_insert_ref_oid, negotiator);
+	mark_tips(negotiator, args->negotiation_tips);
 	for_each_cached_alternate(negotiator, insert_one_alternate_object);

 	fetching = 0;

Here we blindly add objects found in an alternate repo. I found and
debugged this with this:

    diff --git a/fetch-negotiator.h b/fetch-negotiator.h
    index 9e3967ce66..cbe71c9c8d 100644
    --- a/fetch-negotiator.h
    +++ b/fetch-negotiator.h
    @@ -33,2 +33,3 @@ struct fetch_negotiator {
            void (*add_tip)(struct fetch_negotiator *, struct commit *);
    +       int done_adding;

    diff --git a/fetch-pack.c b/fetch-pack.c
    index 3f24d0c8a6..6b43b4f8f1 100644
    --- a/fetch-pack.c
    +++ b/fetch-pack.c
    @@ -238,2 +238,3 @@ static void mark_tips(struct fetch_negotiator *negotiator,
                                        &negotiation_tips->oid[i]);
    +       negotiator->done_adding = 1;
            return;
    diff --git a/negotiator/default.c b/negotiator/default.c
    index 4b78f6bf36..4e45f05f25 100644
    --- a/negotiator/default.c
    +++ b/negotiator/default.c
    @@ -137,2 +137,4 @@ static void add_tip(struct fetch_negotiator *n, struct commit *c)
     {
    +       if (n->done_adding)
    +               return;
            n->known_common = NULL;
    @@ -166,2 +168,3 @@ void default_negotiator_init(struct fetch_negotiator *negotiator)
            negotiator->add_tip = add_tip;
    +       negotiator->done_adding = 0;
            negotiator->next = next;

Perhaps something like that with an assert() is a good idea for the
negotiation backend code in general? It seems rather fragile to depend
on there being no other codepath that calls add_tip() again after some
other code (--negotiation-tip=*) that expects it not to be called again.

Thanks for spotting this bug.

There is already some defense from add_tip() not being called
unexpectedly - see negotiator/default.c and negotiator.skipping.c, which
sets add_tip to NULL when next() is called.

I can see that this doesn't help in this case, when we want to declare
done_adding but we haven't called next() yet, but I don't think that
this API layer is the right place to prevent that.