sphinx: Decode payload and place shortcuts in the route-step

We'll need to pass them around anyway, so just make them easier to access by
doing a bit more to `process_onionpacket`.

common/sphinx.c

@@ -683,6 +683,52 @@ struct onionpacket *create_onionpacket(
 	return packet;
 }
 
+/**
+ * Helper to extract fields from the legacy or tlv payload into the top-level
+ * struct.
+ */
+static void route_step_decode(struct route_step *rs)
+{
+	switch (rs->type) {
+	case SPHINX_V0_PAYLOAD:
+		rs->amt_to_forward = &rs->payload.v0.amt_forward;
+		rs->outgoing_cltv = &rs->payload.v0.outgoing_cltv;
+		if (rs->nextcase == ONION_FORWARD) {
+			rs->forward_channel = &rs->payload.v0.channel_id;
+		} else {
+			rs->forward_channel = NULL;
+		}
+		break;
+	case SPHINX_TLV_PAYLOAD:
+		if (rs->payload.tlv->amt_to_forward) {
+			rs->amt_to_forward = tal(rs, struct amount_msat);
+			amount_msat_from_u64(
+			    rs->amt_to_forward,
+			    rs->payload.tlv->amt_to_forward->amt_to_forward);
+		} else {
+			rs->amt_to_forward = NULL;
+		}
+
+		if (rs->payload.tlv->outgoing_cltv_value) {
+			rs->outgoing_cltv =
+			    &rs->payload.tlv->outgoing_cltv_value
+				 ->outgoing_cltv_value;
+		} else {
+			rs->outgoing_cltv = NULL;
+		}
+
+		if (rs->payload.tlv->short_channel_id)
+			rs->forward_channel = &rs->payload.tlv->short_channel_id
+						   ->short_channel_id;
+		else
+			rs->forward_channel = NULL;
+		break;
+	case SPHINX_INVALID_PAYLOAD:
+	case SPHINX_RAW_PAYLOAD:
+		abort();
+	}
+}
+
 /*
  * Given an onionpacket msg extract the information for the current
  * node and unwrap the remainder so that the node can forward it.
@@ -754,6 +800,8 @@ struct route_step *process_onionpacket(
 		step->nextcase = ONION_FORWARD;
 	}
 
+	route_step_decode(step);
+
 	return step;
 }
 

common/sphinx.h

@@ -84,6 +84,11 @@ struct route_step {
 		struct tlv_tlv_payload *tlv;
 	} payload;
 	u8 *raw_payload;
+
+	/* Quick access for internal use. */
+	struct amount_msat *amt_to_forward;
+	u32 *outgoing_cltv;
+	struct short_channel_id *forward_channel;
 };
 
 /**

common/test/run-sphinx.c

@@ -18,6 +18,9 @@ bool amount_asset_is_main(struct amount_asset *asset UNNEEDED)
 /* Generated stub for amount_asset_to_sat */
 struct amount_sat amount_asset_to_sat(struct amount_asset *asset UNNEEDED)
 { fprintf(stderr, "amount_asset_to_sat called!\n"); abort(); }
+/* Generated stub for amount_msat_from_u64 */
+void amount_msat_from_u64(struct amount_msat *msat UNNEEDED, u64 millisatoshis UNNEEDED)
+{ fprintf(stderr, "amount_msat_from_u64 called!\n"); abort(); }
 /* Generated stub for amount_sat_add */
  bool amount_sat_add(struct amount_sat *val UNNEEDED,
 				       struct amount_sat a UNNEEDED,