bitcoin: Split pull_bitcoin_tx

The deserialization of bitcoin transactions in wire/ is rather
annoying in that we first allocate a new bitcoin_tx, then copy it's
contents onto the destination and then still carry the newly allocated
one around due to the tal-tree. This splits `pull_bitcoin_tx` into
two: one part that does the allocation and another one that proceeds
to parse.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

bitcoin/tx.c

@@ -383,10 +383,9 @@ static void pull_witness(struct bitcoin_tx_input *inputs, size_t i,
 	}
 }
 
-struct bitcoin_tx *pull_bitcoin_tx(const tal_t *ctx,
+struct bitcoin_tx *pull_bitcoin_tx_onto(const tal_t *ctx, const u8 **cursor,
-				   const u8 **cursor, size_t *max)
+					size_t *max, struct bitcoin_tx *tx)
 {
-	struct bitcoin_tx *tx = tal(ctx, struct bitcoin_tx);
 	size_t i;
 	u64 count;
 	u8 flag = 0;
@@ -425,6 +424,13 @@ struct bitcoin_tx *pull_bitcoin_tx(const tal_t *ctx,
 	return tx;
 }
 
+struct bitcoin_tx *pull_bitcoin_tx(const tal_t *ctx,
+				   const u8 **cursor, size_t *max)
+{
+	struct bitcoin_tx *tx = tal(ctx, struct bitcoin_tx);
+	return pull_bitcoin_tx_onto(ctx, cursor, max, tx);
+}
+
 struct bitcoin_tx *bitcoin_tx_from_hex(const tal_t *ctx, const char *hex,
 				       size_t hexlen)
 {

bitcoin/tx.h

@@ -71,4 +71,18 @@ bool bitcoin_txid_to_hex(const struct bitcoin_txid *txid,
 struct bitcoin_tx *pull_bitcoin_tx(const tal_t *ctx,
 				   const u8 **cursor, size_t *max);
 
+/**
+ * pull_bitcoin_tx_onto - De-serialize a bitcoin tx into tx
+ *
+ * Like pull_bitcoin_tx, but skips the allocation of tx. Used by the
+ * wire implementation where the caller allocates, and the callee only
+ * fills in values.
+ *
+ * @ctx: Allocation context
+ * @cursor: buffer to read from
+ * @max: Buffer size left to read
+ * @tx (out): Destination transaction
+ */
+struct bitcoin_tx *pull_bitcoin_tx_onto(const tal_t *ctx, const u8 **cursor,
+					size_t *max, struct bitcoin_tx *tx);
 #endif /* LIGHTNING_BITCOIN_TX_H */