Return owned Strings for onion message message types

Returning a reference from a trait method is relatively difficult
to map in bindings and is currently handled by storing the object
in the trait instance, returning a reference to the local field.

This is fine when the object we're returning only needs to live as
long as the trait, but when it needs to be `'static` (as is the
case for onion message `msg_type`s), there's not really a good way
to map them at all.

Instead, here, condition on `#[cfg(c_bindings)]` we return a fully
owned `String`. This is obviously relatively less effecient, but
the extra allocation and `memcpy` isn't the end of the world,
especially given it should be released relatively quickly.

Author: TheBlueMatt

lightning/src/ln/peer_handler.rs

@@ -170,6 +170,9 @@ impl CustomOnionMessageHandler for IgnoringMessageHandler {
 
 impl OnionMessageContents for Infallible {
 	fn tlv_type(&self) -> u64 { unreachable!(); }
+	#[cfg(c_bindings)]
+	fn msg_type(&self) -> String { unreachable!(); }
+	#[cfg(not(c_bindings))]
 	fn msg_type(&self) -> &'static str { unreachable!(); }
 }
 

lightning/src/onion_message/async_payments.rs

@@ -77,6 +77,11 @@ impl OnionMessageContents for ReleaseHeldHtlc {
 	fn tlv_type(&self) -> u64 {
 		RELEASE_HELD_HTLC_TLV_TYPE
 	}
+	#[cfg(c_bindings)]
+	fn msg_type(&self) -> String {
+		"Release Held HTLC".to_string()
+	}
+	#[cfg(not(c_bindings))]
 	fn msg_type(&self) -> &'static str {
 		"Release Held HTLC"
 	}
@@ -107,6 +112,14 @@ impl OnionMessageContents for AsyncPaymentsMessage {
 			Self::ReleaseHeldHtlc(msg) => msg.tlv_type(),
 		}
 	}
+	#[cfg(c_bindings)]
+	fn msg_type(&self) -> String {
+		match &self {
+			Self::HeldHtlcAvailable(_) => "Held HTLC Available".to_string(),
+			Self::ReleaseHeldHtlc(msg) => msg.msg_type(),
+		}
+	}
+	#[cfg(not(c_bindings))]
 	fn msg_type(&self) -> &'static str {
 		match &self {
 			Self::HeldHtlcAvailable(_) => "Held HTLC Available",

lightning/src/onion_message/offers.rs

@@ -99,6 +99,16 @@ impl OffersMessage {
 			_ => Err(Bolt12ParseError::Decode(DecodeError::InvalidValue)),
 		}
 	}
+
+	fn get_msg_type(&self) -> &'static str {
+		match &self {
+			OffersMessage::InvoiceRequest(_) => "Invoice Request",
+			OffersMessage::Invoice(_) => "Invoice",
+			#[cfg(async_payments)]
+			OffersMessage::StaticInvoice(_) => "Static Invoice",
+			OffersMessage::InvoiceError(_) => "Invoice Error",
+		}
+	}
 }
 
 impl fmt::Debug for OffersMessage {
@@ -131,14 +141,13 @@ impl OnionMessageContents for OffersMessage {
 			OffersMessage::InvoiceError(_) => INVOICE_ERROR_TLV_TYPE,
 		}
 	}
+	#[cfg(c_bindings)]
+	fn msg_type(&self) -> String {
+		self.get_msg_type().to_string()
+	}
+	#[cfg(not(c_bindings))]
 	fn msg_type(&self) -> &'static str {
-		match &self {
-			OffersMessage::InvoiceRequest(_) => "Invoice Request",
-			OffersMessage::Invoice(_) => "Invoice",
-			#[cfg(async_payments)]
-			OffersMessage::StaticInvoice(_) => "Static Invoice",
-			OffersMessage::InvoiceError(_) => "Invoice Error",
-		}
+		self.get_msg_type()
 	}
 }
 

lightning/src/onion_message/packet.rs

@@ -148,6 +148,16 @@ impl<T: OnionMessageContents> OnionMessageContents for ParsedOnionMessageContent
 			&ParsedOnionMessageContents::Custom(ref msg) => msg.tlv_type(),
 		}
 	}
+	#[cfg(c_bindings)]
+	fn msg_type(&self) -> String {
+		match self {
+			ParsedOnionMessageContents::Offers(ref msg) => msg.msg_type(),
+			#[cfg(async_payments)]
+			ParsedOnionMessageContents::AsyncPayments(ref msg) => msg.msg_type(),
+			ParsedOnionMessageContents::Custom(ref msg) => msg.msg_type(),
+		}
+	}
+	#[cfg(not(c_bindings))]
 	fn msg_type(&self) -> &'static str {
 		match self {
 			ParsedOnionMessageContents::Offers(ref msg) => msg.msg_type(),
@@ -174,6 +184,11 @@ pub trait OnionMessageContents: Writeable + core::fmt::Debug {
 	/// Returns the TLV type identifying the message contents. MUST be >= 64.
 	fn tlv_type(&self) -> u64;
 
+	#[cfg(c_bindings)]
+	/// Returns the message type
+	fn msg_type(&self) -> String;
+
+	#[cfg(not(c_bindings))]
 	/// Returns the message type
 	fn msg_type(&self) -> &'static str;
 }