common: add amount_msat_deduct / amount_msat_deduct_sub.

diff --git a/channeld/full_channel.c b/channeld/full_channel.c
index eca0ee52..c3df6ee6 100644
--- a/channeld/full_channel.c
+++ b/channeld/full_channel.c
@@ -416,7 +416,7 @@ static bool get_room_above_reserve(const struct channel *channel,
 		return false;
 	}
 
-	if (!amount_msat_sub_sat(remainder, *remainder, reserve)) {
+	if (!amount_msat_deduct_sat(remainder, reserve)) {
 		status_debug("%s cannot afford htlc: would make balance %s"
 			     " below reserve %s",
 			     side_to_str(side),
@@ -504,7 +504,7 @@ static bool htlc_dust(const struct channel *channel,
 				      side, &trim_rmvd))
 		return false;
 
-	return amount_msat_sub(trim_total, *trim_total, trim_rmvd);
+	return amount_msat_deduct(trim_total, trim_rmvd);
 }
 
 /*
@@ -773,7 +773,7 @@ static enum channel_add_err add_htlc(struct channel *channel,
 		 */
 		if ((option_anchor_outputs || option_anchors_zero_fee_htlc_tx)
 		    && channel->opener == sender
-		    && !amount_msat_sub_sat(&remainder, remainder, AMOUNT_SAT(660)))
+		    && !amount_msat_deduct_sat(&remainder, AMOUNT_SAT(660)))
 			return CHANNEL_ERR_CHANNEL_CAPACITY_EXCEEDED;
 
 		if (channel->opener== sender) {
@@ -805,7 +805,7 @@ static enum channel_add_err add_htlc(struct channel *channel,
 
 			if ((option_anchor_outputs || option_anchors_zero_fee_htlc_tx)
 			    && channel->opener != sender
-			    && !amount_msat_sub_sat(&remainder, remainder, AMOUNT_SAT(660)))
+			    && !amount_msat_deduct_sat(&remainder, AMOUNT_SAT(660)))
 				return CHANNEL_ERR_CHANNEL_CAPACITY_EXCEEDED;
 
 			/* Should be able to afford both their own commit tx
@@ -1257,12 +1257,12 @@ u32 approx_max_feerate(const struct channel *channel)
 	 * `to_remote`).
 	 */
 	if ((option_anchor_outputs || option_anchors_zero_fee_htlc_tx)
-	    && !amount_msat_sub_sat(&avail, avail, AMOUNT_SAT(660))) {
+	    && !amount_msat_deduct_sat(&avail, AMOUNT_SAT(660))) {
 		avail = AMOUNT_MSAT(0);
 	} else {
 		/* We should never go below reserve. */
-		if (!amount_msat_sub_sat(&avail, avail,
-					 channel->config[!channel->opener].channel_reserve))
+		if (!amount_msat_deduct_sat(&avail,
+					    channel->config[!channel->opener].channel_reserve))
 		avail = AMOUNT_MSAT(0);
 	}
 
diff --git a/common/amount.c b/common/amount.c
index 0f186736..cf872d0c 100644
--- a/common/amount.c
+++ b/common/amount.c
@@ -263,6 +263,18 @@ WARN_UNUSED_RESULT bool amount_msat_accumulate(struct amount_msat *a,
 	return amount_msat_add(a, *a, b);
 }
 
+WARN_UNUSED_RESULT bool amount_msat_deduct(struct amount_msat *a,
+					       struct amount_msat b)
+{
+	return amount_msat_sub(a, *a, b);
+}
+
+WARN_UNUSED_RESULT bool amount_msat_deduct_sat(struct amount_msat *a,
+					       struct amount_sat b)
+{
+	return amount_msat_sub_sat(a, *a, b);
+}
+
 WARN_UNUSED_RESULT bool amount_msat_sub(struct amount_msat *val,
 					struct amount_msat a,
 					struct amount_msat b)
diff --git a/common/amount.h b/common/amount.h
index 11551b91..04965889 100644
--- a/common/amount.h
+++ b/common/amount.h
@@ -105,6 +105,14 @@ WARN_UNUSED_RESULT bool amount_sat_add_sat_s64(struct amount_sat *val,
 WARN_UNUSED_RESULT bool amount_msat_accumulate(struct amount_msat *a,
 					       struct amount_msat b);
 
+/* a -= b */
+WARN_UNUSED_RESULT bool amount_msat_deduct(struct amount_msat *a,
+					   struct amount_msat b);
+
+/* a -= b */
+WARN_UNUSED_RESULT bool amount_msat_deduct_sat(struct amount_msat *a,
+					       struct amount_sat b);
+
 /* returns floor(msat/div) */
 struct amount_msat amount_msat_div(struct amount_msat msat, u64 div);
 
diff --git a/common/initial_commit_tx.c b/common/initial_commit_tx.c
index 07e3c890..fb585513 100644
--- a/common/initial_commit_tx.c
+++ b/common/initial_commit_tx.c
@@ -40,7 +40,7 @@ bool try_subtract_fee(enum side opener, enum side side,
 	else
 		opener_amount = other;
 
-	if (amount_msat_sub_sat(opener_amount, *opener_amount, base_fee))
+	if (amount_msat_deduct_sat(opener_amount, base_fee))
 		return true;
 
 	*opener_amount = AMOUNT_MSAT(0);
diff --git a/devtools/mkclose.c b/devtools/mkclose.c
index de51c7f6..d32aa2a6 100644
--- a/devtools/mkclose.c
+++ b/devtools/mkclose.c
@@ -121,7 +121,7 @@ int main(int argc, char *argv[])
 	if (option_anchor_outputs && !amount_sat_add(&fee, fee, AMOUNT_SAT(660)))
 		errx(1, "Can't afford anchors");
 
-	if (!amount_msat_sub_sat(&local_msat, local_msat, fee))
+	if (!amount_msat_deduct_sat(&local_msat, fee))
 		errx(1, "Can't afford fee %s",
 		     fmt_amount_sat(NULL, fee));
 	if (!amount_sat_sub_msat(&remote_msat, funding_amount, local_msat))
diff --git a/lightningd/invoice.c b/lightningd/invoice.c
index dc823919..aa954685 100644
--- a/lightningd/invoice.c
+++ b/lightningd/invoice.c
@@ -571,7 +571,7 @@ static struct route_info **select_inchan(const tal_t *ctx,
 				    candidates[i].c->our_config.channel_reserve,
 				    candidates[i].c->channel_info.their_config.channel_reserve)
 		    || !amount_sat_to_msat(&capacity, candidates[i].c->funding_sats)
-		    || !amount_msat_sub_sat(&capacity, capacity, cumulative_reserve)) {
+		    || !amount_msat_deduct_sat(&capacity, cumulative_reserve)) {
 			log_broken(ld->log, "Channel %s capacity overflow!",
 					fmt_short_channel_id(tmpctx, *candidates[i].c->scid));
 			continue;
diff --git a/lightningd/peer_control.c b/lightningd/peer_control.c
index 4b44384d..e5fbc74a 100644
--- a/lightningd/peer_control.c
+++ b/lightningd/peer_control.c
@@ -721,7 +721,7 @@ static void subtract_offered_htlcs(const struct channel *channel,
 	     hout = htlc_out_map_next(ld->htlcs_out, &outi)) {
 		if (hout->key.channel != channel)
 			continue;
-		if (!amount_msat_sub(amount, *amount, hout->msat))
+		if (!amount_msat_deduct(amount, hout->msat))
 			*amount = AMOUNT_MSAT(0);
 	}
 }
@@ -738,7 +738,7 @@ static void subtract_received_htlcs(const struct channel *channel,
 	     hin = htlc_in_map_next(ld->htlcs_in, &ini)) {
 		if (hin->key.channel != channel)
 			continue;
-		if (!amount_msat_sub(amount, *amount, hin->msat))
+		if (!amount_msat_deduct(amount, hin->msat))
 			*amount = AMOUNT_MSAT(0);
 	}
 }
@@ -759,9 +759,9 @@ struct amount_msat channel_amount_spendable(const struct channel *channel)
 
 	/* If we're opener, subtract txfees we'll need to spend this */
 	if (channel->opener == LOCAL) {
-		if (!amount_msat_sub_sat(&spendable, spendable,
-					 commit_txfee(channel, spendable,
-						      LOCAL)))
+		if (!amount_msat_deduct_sat(&spendable,
+					    commit_txfee(channel, spendable,
+							 LOCAL)))
 			return AMOUNT_MSAT(0);
 	}
 
@@ -803,9 +803,9 @@ struct amount_msat channel_amount_receivable(const struct channel *channel)
 
 	/* If they're opener, subtract txfees they'll need to spend this */
 	if (channel->opener == REMOTE) {
-		if (!amount_msat_sub_sat(&receivable, receivable,
-					 commit_txfee(channel,
-						      receivable, REMOTE)))
+		if (!amount_msat_deduct_sat(&receivable,
+					    commit_txfee(channel,
+							 receivable, REMOTE)))
 			return AMOUNT_MSAT(0);
 	}
 
diff --git a/lightningd/peer_htlcs.c b/lightningd/peer_htlcs.c
index 548a15c8..b6433750 100644
--- a/lightningd/peer_htlcs.c
+++ b/lightningd/peer_htlcs.c
@@ -2006,8 +2006,7 @@ static void remove_htlc_out(struct channel *channel, struct htlc_out *hout)
 		const struct channel_coin_mvt *mvt;
 		struct amount_msat oldamt = channel->our_msat;
 		/* We paid for this HTLC, so deduct balance. */
-		if (!amount_msat_sub(&channel->our_msat, channel->our_msat,
-				     hout->msat)) {
+		if (!amount_msat_deduct(&channel->our_msat, hout->msat)) {
 			channel_internal_error(channel,
 					       "Underflow our_msat %s - HTLC %s",
 					       fmt_amount_msat(tmpctx,
diff --git a/plugins/askrene/askrene.c b/plugins/askrene/askrene.c
index d25fc4ae..8ed0ceab 100644
--- a/plugins/askrene/askrene.c
+++ b/plugins/askrene/askrene.c
@@ -1057,7 +1057,7 @@ static struct command_result *json_askrene_inform_channel(struct command *cmd,
 		if (!reserve_accumulate(askrene->reserved, scidd, amount))
 			return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
 					    "Amount overflow with reserves");
-		if (!amount_msat_sub(amount, *amount, AMOUNT_MSAT(1)))
+		if (!amount_msat_deduct(amount, AMOUNT_MSAT(1)))
 			*amount = AMOUNT_MSAT(0);
 		if (command_check_only(cmd))
 			return command_check_done(cmd);
diff --git a/plugins/askrene/mcf.c b/plugins/askrene/mcf.c
index e41bfb05..3acf0a33 100644
--- a/plugins/askrene/mcf.c
+++ b/plugins/askrene/mcf.c
@@ -345,7 +345,7 @@ static double pickhardt_richter_probability(struct amount_msat low,
 	struct amount_msat all_states, good_states;
 	if (amount_msat_greater_eq(amount, high))
 		return 0.0;
-	if (!amount_msat_sub(&amount, amount, low))
+	if (!amount_msat_deduct(&amount, low))
 		return 1.0;
 	if (!amount_msat_sub(&all_states, high, low))
 		abort(); // we expect high > low
@@ -1587,9 +1587,8 @@ linear_routes(const tal_t *ctx, struct route_query *rq,
 			}
 		}
 
-		if (!amount_msat_sub(&feebudget, feebudget, all_fees) ||
-		    !amount_msat_sub(&amount_to_deliver, amount_to_deliver,
-				     all_deliver)) {
+		if (!amount_msat_deduct(&feebudget, all_fees) ||
+		    !amount_msat_deduct(&amount_to_deliver, all_deliver)) {
 			error_message =
 			    rq_log(ctx, rq, LOG_BROKEN,
 				   "%s: unexpected arithmetic operation "
diff --git a/plugins/askrene/refine.c b/plugins/askrene/refine.c
index 00ee4f75..12743ebb 100644
--- a/plugins/askrene/refine.c
+++ b/plugins/askrene/refine.c
@@ -372,10 +372,10 @@ static struct amount_msat remove_excess(struct flow **flows,
 	/* Remove the smaller parts if they deliver less than the
 	 * excess.  */
 	for (int i = tal_count(*flows_index) - 1; i >= 0; i--) {
-		if (!amount_msat_sub(&excess, excess,
+		if (!amount_msat_deduct(&excess,
 				     flows[(*flows_index)[i]]->delivers))
 			break;
-		if (!amount_msat_sub(&all_deliver, all_deliver,
+		if (!amount_msat_deduct(&all_deliver,
 				     flows[(*flows_index)[i]]->delivers))
 			abort();
 		tal_arr_remove(flows_index, i);
@@ -397,21 +397,19 @@ static struct amount_msat remove_excess(struct flow **flows,
 		/* rounding errors: don't remove more than excess */
 		remove = amount_msat_min(remove, excess);
 
-		if (!amount_msat_sub(&excess, excess, remove))
+		if (!amount_msat_deduct(&excess, remove))
 			abort();
 
-		if (!amount_msat_sub(&all_deliver, all_deliver, remove) ||
-		    !amount_msat_sub(&flows[(*flows_index)[i]]->delivers,
-				     flows[(*flows_index)[i]]->delivers,
-				     remove))
+		if (!amount_msat_deduct(&all_deliver, remove) ||
+		    !amount_msat_deduct(&flows[(*flows_index)[i]]->delivers,
+					remove))
 			abort();
 	}
 
 	/* any rounding error left, take it from the first */
 	assert(tal_count(*flows_index) > 0);
-	if (!amount_msat_sub(&all_deliver, all_deliver, excess) ||
-	    !amount_msat_sub(&flows[(*flows_index)[0]]->delivers,
-			     flows[(*flows_index)[0]]->delivers, excess))
+	if (!amount_msat_deduct(&all_deliver, excess) ||
+	    !amount_msat_deduct(&flows[(*flows_index)[0]]->delivers, excess))
 		abort();
 	return all_deliver;
 }
diff --git a/plugins/askrene/reserve.c b/plugins/askrene/reserve.c
index 738577fc..de88091c 100644
--- a/plugins/askrene/reserve.c
+++ b/plugins/askrene/reserve.c
@@ -103,7 +103,7 @@ void reserve_sub(const struct reserve_htable *reserved,
 	for (r = reserve_htable_getfirst(reserved, scidd, &rit);
 	     r;
 	     r = reserve_htable_getnext(reserved, scidd, &rit)) {
-		if (!amount_msat_sub(amount, *amount, r->rhop.amount))
+		if (!amount_msat_deduct(amount, r->rhop.amount))
 			*amount = AMOUNT_MSAT(0);
 	}
 }
diff --git a/plugins/bkpr/channelsapy.c b/plugins/bkpr/channelsapy.c
index f4fa55a0..898b3175 100644
--- a/plugins/bkpr/channelsapy.c
+++ b/plugins/bkpr/channelsapy.c
@@ -126,11 +126,9 @@ static void fillin_apy_acct_details(const struct bkpr *bkpr,
 
 	/* If there is any push_out or lease_fees_out, we subtract
 	 * from starting balance */
-	ok = amount_msat_sub(&apy->our_start_bal, apy->our_start_bal,
-			     apy->push_out);
+	ok = amount_msat_deduct(&apy->our_start_bal, apy->push_out);
 	assert(ok);
-	ok = amount_msat_sub(&apy->our_start_bal, apy->our_start_bal,
-			     apy->lease_out);
+	ok = amount_msat_deduct(&apy->our_start_bal, apy->lease_out);
 	assert(ok);
 
 	/* we add values in to starting balance */
diff --git a/plugins/bkpr/onchain_fee.c b/plugins/bkpr/onchain_fee.c
index 6cdce78a..8cfc0c13 100644
--- a/plugins/bkpr/onchain_fee.c
+++ b/plugins/bkpr/onchain_fee.c
@@ -288,7 +288,7 @@ static void insert_chain_fees_diff(struct command *cmd,
 		if (!amount_msat_accumulate(&current_amt, ofs[i]->credit))
 			plugin_err(cmd->plugin, "Overflow when adding onchain fees");
 
-		if (!amount_msat_sub(&current_amt, current_amt, ofs[i]->debit))
+		if (!amount_msat_deduct(&current_amt, ofs[i]->debit))
 			plugin_err(cmd->plugin, "Underflow when subtracting onchain fees");
 		if (ofs[i]->update_count > max_update_count)
 			max_update_count = ofs[i]->update_count;
diff --git a/plugins/libplugin-pay.c b/plugins/libplugin-pay.c
index 58deb4d0..3c8b9d60 100644
--- a/plugins/libplugin-pay.c
+++ b/plugins/libplugin-pay.c
@@ -483,7 +483,7 @@ payment_constraints_update(struct payment_constraints *cons,
 		return false;
 
 	/* amount_msat_sub performs a check before actually subtracting. */
-	if (!amount_msat_sub(&cons->fee_budget, cons->fee_budget, delta_fee))
+	if (!amount_msat_deduct(&cons->fee_budget, delta_fee))
 		return false;
 
 	cons->cltv_budget -= delta_cltv;
@@ -572,9 +572,8 @@ static bool payment_chanhints_apply_route(struct payment *p)
 			curhint->local->htlc_budget--;
 		}
 
-		if (!amount_msat_sub(&curhint->estimated_capacity,
-				     curhint->estimated_capacity,
-				     curhop->amount)) {
+		if (!amount_msat_deduct(&curhint->estimated_capacity,
+					curhop->amount)) {
 			/* Given our preemptive test
 			 * above, this should never
 			 * happen either. */
@@ -1448,7 +1447,7 @@ handle_intermediate_failure(struct command *cmd,
 		 * work. This allows us to then allow on equality, this is for
 		 * example necessary for local channels where exact matches
 		 * should be allowed. */
-		if (!amount_msat_sub(&estimated, estimated, AMOUNT_MSAT(1)))
+		if (!amount_msat_deduct(&estimated, AMOUNT_MSAT(1)))
 			abort();
 
 		/* These are an indication that the capacity was insufficient,
@@ -3408,10 +3407,8 @@ static struct command_result *shadow_route_listchannels(struct command *cmd,
 		 * destination could otherwise cause the route to be too
 		 * expensive, while really being ok. If any of these fail then
 		 * the above checks are insufficient. */
-		if (!amount_msat_sub(&d->constraints.fee_budget,
-				     d->constraints.fee_budget, best_fee) ||
-		    !amount_msat_sub(&p->constraints.fee_budget,
-				     p->constraints.fee_budget, best_fee))
+		if (!amount_msat_deduct(&d->constraints.fee_budget, best_fee) ||
+		    !amount_msat_deduct(&p->constraints.fee_budget, best_fee))
 			paymod_err(p,
 				   "Could not update fee constraints "
 				   "for shadow route extension. "
diff --git a/plugins/renepay/chan_extra.c b/plugins/renepay/chan_extra.c
index 931463ca..d4ca3019 100644
--- a/plugins/renepay/chan_extra.c
+++ b/plugins/renepay/chan_extra.c
@@ -128,7 +128,7 @@ enum renepay_errorcode channel_liquidity(struct amount_msat *liquidity,
 	if (!h)
 		return RENEPAY_CHANNEL_NOT_FOUND;
 	struct amount_msat value_liquidity = h->known_max;
-	if (!amount_msat_sub(&value_liquidity, value_liquidity, h->htlc_total))
+	if (!amount_msat_deduct(&value_liquidity, h->htlc_total))
 		return RENEPAY_AMOUNT_OVERFLOW;
 	*liquidity = value_liquidity;
 	return RENEPAY_NOERROR;
@@ -616,12 +616,12 @@ double edge_probability(struct amount_msat min, struct amount_msat max,
 		goto function_fail;
 
 	// in_flight cannot be greater than max
-	if (!amount_msat_sub(&B, B, in_flight))
+	if (!amount_msat_deduct(&B, in_flight))
 		goto function_fail;
 
 	struct amount_msat A = min; // = MAX(0,min-in_flight);
 
-	if (!amount_msat_sub(&A, A, in_flight))
+	if (!amount_msat_deduct(&A, in_flight))
 		A = AMOUNT_MSAT(0);
 
 	struct amount_msat denominator; // = B-A
@@ -655,7 +655,7 @@ chan_extra_remove_htlc(struct chan_extra_map *chan_extra_map,
 	if (h->num_htlcs <= 0)
 		return RENEPAY_PRECONDITION_ERROR;
 
-	if (!amount_msat_sub(&h->htlc_total, h->htlc_total, amount))
+	if (!amount_msat_deduct(&h->htlc_total, amount))
 		return RENEPAY_AMOUNT_OVERFLOW;
 	h->num_htlcs--;
 	return RENEPAY_NOERROR;
diff --git a/plugins/renepay/mcf.c b/plugins/renepay/mcf.c
index 7d2f1490..8fdd067b 100644
--- a/plugins/renepay/mcf.c
+++ b/plugins/renepay/mcf.c
@@ -1440,7 +1440,7 @@ get_flow_paths(const tal_t *ctx, const struct gossmap *gossmap,
 			// substract the excess of msats for not having msat
 			// accuracy
 			struct amount_msat delivered = amount_msat(delta*1000);
-			if (!amount_msat_sub(&delivered, delivered, excess)) {
+			if (!amount_msat_deduct(&delivered, excess)) {
 				if (fail)
 				*fail = tal_fmt(
 				    ctx, "unable to substract excess");
diff --git a/plugins/renepay/mods.c b/plugins/renepay/mods.c
index 30d6314a..e6da2d10 100644
--- a/plugins/renepay/mods.c
+++ b/plugins/renepay/mods.c
@@ -680,7 +680,7 @@ static struct command_result *compute_routes_cb(struct payment *payment)
 			   __func__);
 
 	/* Remaining fee budget. */
-	if (!amount_msat_sub(&feebudget, feebudget, fees_spent))
+	if (!amount_msat_deduct(&feebudget, fees_spent))
 		feebudget = AMOUNT_MSAT(0);
 
 	/* How much are we still trying to send? */
diff --git a/plugins/renepay/routebuilder.c b/plugins/renepay/routebuilder.c
index 4e9d1514..d4c940f7 100644
--- a/plugins/renepay/routebuilder.c
+++ b/plugins/renepay/routebuilder.c
@@ -364,7 +364,7 @@ struct route **get_routes(const tal_t *ctx,
 			}
 
 			// update the fee budget
-			if (!amount_msat_sub(&feebudget, feebudget, fee)) {
+			if (!amount_msat_deduct(&feebudget, fee)) {
 				// should never happen
 				tal_report_error(
 				    ctx, ecode, fail, PLUGIN_ERROR,
@@ -377,8 +377,7 @@ struct route **get_routes(const tal_t *ctx,
 			}
 
 			// update the amount that we deliver
-			if (!amount_msat_sub(&amount_to_deliver,
-					     amount_to_deliver, delivering)) {
+			if (!amount_msat_deduct(&amount_to_deliver, delivering)) {
 				// should never happen
 				tal_report_error(
 				    ctx, ecode, fail, PLUGIN_ERROR,
diff --git a/plugins/renepay/routetracker.c b/plugins/renepay/routetracker.c
index 2e8b9ba2..b30a4a54 100644
--- a/plugins/renepay/routetracker.c
+++ b/plugins/renepay/routetracker.c
@@ -330,11 +330,10 @@ void payment_collect_results(struct payment *payment,
 				*final_msg = tal_strdup(tmpctx, r->final_msg);
 		}
 
-		if (!amount_msat_sub(&payment->total_delivering,
-				     payment->total_delivering,
-				     route_delivers(r)) ||
-		    !amount_msat_sub(&payment->total_sent, payment->total_sent,
-				     route_sends(r))) {
+		if (!amount_msat_deduct(&payment->total_delivering,
+					route_delivers(r)) ||
+		    !amount_msat_deduct(&payment->total_sent,
+					route_sends(r))) {
 			plugin_err(pay_plugin->plugin,
 				   "%s: routes do not add up to "
 				   "payment total amount.",
diff --git a/tests/plugins/channeld_fakenet.c b/tests/plugins/channeld_fakenet.c
index d56c5824..c8281671 100644
--- a/tests/plugins/channeld_fakenet.c
+++ b/tests/plugins/channeld_fakenet.c
@@ -657,9 +657,8 @@ static struct amount_msat calc_capacity(struct info *info,
 		if (!short_channel_id_dir_eq(&info->reservations[i]->scidd, scidd))
 			continue;
 		/* We should never use more that we have! */
-		if (!amount_msat_sub(&dynamic_capacity,
-				     dynamic_capacity,
-				     info->reservations[i]->amount))
+		if (!amount_msat_deduct(&dynamic_capacity,
+					info->reservations[i]->amount))
 			abort();
 		status_debug("... minus reservation %s",
 			     fmt_amount_msat(tmpctx, info->reservations[i]->amount));