funding: record changes so we can abide by BOLT#2

BOLT#2 says we MUST NOT send out commit messages with no changes (and
we drop the connection if the peer does).  But that's harder than it
sounds: we can have changes in theory which cancel out (eg. fee
adjustments, not yet implemented) or others which don't change the
commit tx (eg. dust outputs).  Simplest is to have a generation count,
which also allows us to simply show number of pending changes in RPC.

It's 32 bit, but you can only use it to screw yourself really (each
side can only add 1500 htlcs, so the rest would have to be fee
changes; wrapping will only make us hang up on you).

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

funding.c

@@ -139,6 +139,7 @@ struct channel_state *initial_funding(const tal_t *ctx,
 	cstate->b.htlcs = tal_arr(cstate, struct channel_htlc, 0);
 	cstate->fee_rate = fee_rate;
 	cstate->anchor = anchor_satoshis;
+	cstate->changes = 0;
 
 	/* Anchor must fit in 32 bit. */
 	if (anchor_satoshis >= (1ULL << 32) / 1000)
@@ -199,6 +200,7 @@ void adjust_fee(struct channel_state *cstate, uint32_t fee_rate)
 	fee_msat = calculate_fee_msat(total_nondust_htlcs(cstate), fee_rate);
 
 	recalculate_fees(&cstate->a, &cstate->b, fee_msat);
+	cstate->changes++;
 }
 
 bool force_fee(struct channel_state *cstate, uint64_t fee)
@@ -207,6 +209,7 @@ bool force_fee(struct channel_state *cstate, uint64_t fee)
 	if (fee > 0xFFFFFFFFFFFFFFFFULL / 1000)
 		return false;
 	recalculate_fees(&cstate->a, &cstate->b, fee * 1000);
+	cstate->changes++;
 	return cstate->a.fee_msat + cstate->b.fee_msat == fee * 1000;
 }
 
@@ -220,7 +223,7 @@ void invert_cstate(struct channel_state *cstate)
 }
 
 /* Add a HTLC to @creator if it can afford it. */
-static bool add_htlc(const struct channel_state *cstate,
+static bool add_htlc(struct channel_state *cstate,
 		     struct channel_oneside *creator,
 		     struct channel_oneside *recipient,
 		     u32 msatoshis, const struct abs_locktime *expiry,
@@ -250,11 +253,12 @@ static bool add_htlc(const struct channel_state *cstate,
 	memcheck(&creator->htlcs[n].msatoshis,
 		 sizeof(creator->htlcs[n].msatoshis));
 	memcheck(&creator->htlcs[n].rhash, sizeof(creator->htlcs[n].rhash));
+	cstate->changes++;
 	return true;
 }
 
 /* Remove htlc from creator, credit it to beneficiary. */
-static void remove_htlc(const struct channel_state *cstate,
+static void remove_htlc(struct channel_state *cstate,
 			struct channel_oneside *creator,
 			struct channel_oneside *beneficiary,
 			struct channel_oneside *non_beneficiary,
@@ -285,6 +289,7 @@ static void remove_htlc(const struct channel_state *cstate,
 	memmove(creator->htlcs + i, creator->htlcs + i + 1,
 		(n - i - 1) * sizeof(*creator->htlcs));
 	tal_resize(&creator->htlcs, n-1);
+	cstate->changes++;
 }
 
 bool funding_a_add_htlc(struct channel_state *cstate,

funding.h

@@ -25,6 +25,8 @@ struct channel_state {
 	uint64_t anchor;
 	/* Satoshis per 1000 bytes. */
 	uint32_t fee_rate;
+	/* Generation counter (incremented on every change) */
+	uint32_t changes;
 	struct channel_oneside a, b;
 };