Implement option_upfront_shutdown_script user-side

We use user config to decide to commit to closing script
in open_channel/accept_channel messages. We don't check
that other peer supporting the option as including
script without other peer public support is borne by
the protocol. If user opt-out, following protocol and
due to the fact we always signal, we provide a zero-length
script

Author: Antoine Riard

src/ln/channel.rs

@@ -3081,7 +3081,7 @@ impl Channel {
 			htlc_basepoint: PublicKey::from_secret_key(&self.secp_ctx, &self.local_keys.htlc_base_key),
 			first_per_commitment_point: PublicKey::from_secret_key(&self.secp_ctx, &local_commitment_secret),
 			channel_flags: if self.config.announced_channel {1} else {0},
-			shutdown_scriptpubkey: OptionalField::Absent
+			shutdown_scriptpubkey: OptionalField::Present(if self.config.commit_upfront_shutdown_pubkey { self.get_closing_scriptpubkey() } else { Builder::new().into_script() })
 		}
 	}
 
@@ -3113,7 +3113,7 @@ impl Channel {
 			delayed_payment_basepoint: PublicKey::from_secret_key(&self.secp_ctx, &self.local_keys.delayed_payment_base_key),
 			htlc_basepoint: PublicKey::from_secret_key(&self.secp_ctx, &self.local_keys.htlc_base_key),
 			first_per_commitment_point: PublicKey::from_secret_key(&self.secp_ctx, &local_commitment_secret),
-			shutdown_scriptpubkey: OptionalField::Absent
+			shutdown_scriptpubkey: OptionalField::Present(if self.config.commit_upfront_shutdown_pubkey { self.get_closing_scriptpubkey() } else { Builder::new().into_script() })
 		}
 	}
 

src/util/config.rs

@@ -129,6 +129,14 @@ pub struct ChannelConfig {
 	///
 	/// This cannot be changed after the initial channel handshake.
 	pub announced_channel: bool,
+	/// Set to commit to an upfront shutdown_pubkey at channel opening. In case of mutual
+	/// closing, the other peer will check that our closing transction output is encumbered
+	/// by the provided script.
+	///
+	/// We set it by default as this ensure greater security to the user funds.
+	///
+	/// This cannot be changed after channel opening.
+	pub commit_upfront_shutdown_pubkey: bool
 }
 
 impl ChannelConfig {
@@ -137,12 +145,14 @@ impl ChannelConfig {
 		ChannelConfig {
 			fee_proportional_millionths: 0,
 			announced_channel: false,
+			commit_upfront_shutdown_pubkey: true,
 		}
 	}
 }
 
 //Add write and readable traits to channelconfig
-impl_writeable!(ChannelConfig, 8+1, {
+impl_writeable!(ChannelConfig, 8+1+1, {
 	fee_proportional_millionths,
-	announced_channel
+	announced_channel,
+	commit_upfront_shutdown_pubkey
 });