Update accept_channel flow.

1. If we had an acceptchannel earlier, we check if it matches the one
   received just now. If not we error, because the channel parameters
   have changed and that warrants renegotiating parameters again.
2. If they do match we simply skip the rest of the function because we
   had updated those values already.

Author: shaavan

lightning/src/ln/channel.rs

@@ -7525,9 +7525,16 @@ impl<SP: Deref> OutboundV1Channel<SP> where SP::Target: SignerProvider {
 		if !self.context.is_outbound() {
 			return Err(ChannelError::Close("Got an accept_channel message from an inbound peer".to_owned()));
 		}
-		if !matches!(self.context.channel_state, ChannelState::NegotiatingFunding(flags) if flags == NegotiatingFundingFlags::OUR_INIT_SENT) {
+		if !matches!(self.context.channel_state, ChannelState::NegotiatingFunding(flags) if flags == NegotiatingFundingFlags::OUR_INIT_SENT) && !self.deja_vu() {
 			return Err(ChannelError::Close("Got an accept_channel message at a strange time".to_owned()));
 		}
+		if self.outbound_context.received_accept_channel() {
+			if self.outbound_context.received_accept_channel_msg.as_ref().unwrap() == msg {
+				return Ok(());
+			} else {
+				return Err(ChannelError::Close("The new accep_channel_msg doesn't match the old one. Should renegotiating channel parameters".to_owned()));
+			}
+		}
 		if msg.common_fields.dust_limit_satoshis > 21000000 * 100000000 {
 			return Err(ChannelError::Close(format!("Peer never wants payout outputs? dust_limit_satoshis was {}", msg.common_fields.dust_limit_satoshis)));
 		}
@@ -7614,6 +7621,10 @@ impl<SP: Deref> OutboundV1Channel<SP> where SP::Target: SignerProvider {
 			}
 		} else { None };
 
+
+		// Save the accept channel msg after all the sanity checks to the outbound_context parameters.
+		self.outbound_context.received_accept_channel_msg = Some(msg.clone());
+
 		self.context.counterparty_dust_limit_satoshis = msg.common_fields.dust_limit_satoshis;
 		self.context.counterparty_max_htlc_value_in_flight_msat = cmp::min(msg.common_fields.max_htlc_value_in_flight_msat, self.context.channel_value_satoshis * 1000);
 		self.context.counterparty_selected_channel_reserve_satoshis = Some(msg.channel_reserve_satoshis);