psbt: implement psbt_txid.

You can't just take the txid of the global tx, if some are p2sh-wrapped.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

bitcoin/psbt.c

@@ -463,3 +463,34 @@ struct wally_psbt *fromwire_wally_psbt(const tal_t *ctx,
 	return psbt;
 }
 
+/* This only works on a non-final psbt because we're ALL SEGWIT! */
+void psbt_txid(const struct wally_psbt *psbt, struct bitcoin_txid *txid,
+	       struct wally_tx **wtx)
+{
+	struct wally_tx *tx;
+
+	/* You can *almost* take txid of global tx.  But @niftynei thought
+	 * about this far more than me and pointed out that P2SH
+	 * inputs would not be represented, so here we go. */
+
+	wally_tx_clone_alloc(psbt->tx, 0, &tx);
+
+	for (size_t i = 0; i < tx->num_inputs; i++) {
+		u8 *script;
+		if (!psbt->inputs[i].redeem_script)
+			continue;
+
+		/* P2SH requires push of the redeemscript, from libwally src */
+		script = tal_arr(tmpctx, u8, 0);
+		script_push_bytes(&script,
+				  psbt->inputs[i].redeem_script,
+				  psbt->inputs[i].redeem_script_len);
+		wally_tx_set_input_script(tx, i, script, tal_bytelen(script));
+	}
+
+	wally_txid(tx, txid);
+	if (wtx)
+		*wtx = tx;
+	else
+		wally_tx_free(tx);
+}

bitcoin/psbt.h

@@ -13,6 +13,7 @@ struct wally_tx;
 struct amount_asset;
 struct amount_sat;
 struct bitcoin_signature;
+struct bitcoin_txid;
 struct pubkey;
 
 void psbt_destroy(struct wally_psbt *psbt);
@@ -31,6 +32,15 @@ struct wally_psbt *new_psbt(const tal_t *ctx,
  */
 bool psbt_is_finalized(const struct wally_psbt *psbt);
 
+/**
+ * psbt_txid - get the txid of the psbt (what it would be after finalization)
+ * @psbt: the psbt.
+ * @txid: the transaction id (output)
+ * @wtx: if non-NULL, returns a copy of the transaction (caller must wally_tx_free).
+ */
+void psbt_txid(const struct wally_psbt *psbt, struct bitcoin_txid *txid,
+	       struct wally_tx **wtx);
+
 struct wally_tx *psbt_finalize(struct wally_psbt *psbt, bool finalize_in_place);
 
 struct wally_psbt_input *psbt_add_input(struct wally_psbt *psbt,

bitcoin/test/run-bitcoin_block_from_hex.c

@@ -78,6 +78,9 @@ void pubkey_to_der(u8 der[PUBKEY_CMPR_LEN] UNNEEDED, const struct pubkey *key UN
 /* Generated stub for pubkey_to_hash160 */
 void pubkey_to_hash160(const struct pubkey *pk UNNEEDED, struct ripemd160 *hash UNNEEDED)
 { fprintf(stderr, "pubkey_to_hash160 called!\n"); abort(); }
+/* Generated stub for script_push_bytes */
+void script_push_bytes(u8 **scriptp UNNEEDED, const void *mem UNNEEDED, size_t len UNNEEDED)
+{ fprintf(stderr, "script_push_bytes called!\n"); abort(); }
 /* Generated stub for scriptpubkey_p2wsh */
 u8 *scriptpubkey_p2wsh(const tal_t *ctx UNNEEDED, const u8 *witnessscript UNNEEDED)
 { fprintf(stderr, "scriptpubkey_p2wsh called!\n"); abort(); }

bitcoin/test/run-tx-encode.c

@@ -79,6 +79,9 @@ void pubkey_to_der(u8 der[PUBKEY_CMPR_LEN] UNNEEDED, const struct pubkey *key UN
 /* Generated stub for pubkey_to_hash160 */
 void pubkey_to_hash160(const struct pubkey *pk UNNEEDED, struct ripemd160 *hash UNNEEDED)
 { fprintf(stderr, "pubkey_to_hash160 called!\n"); abort(); }
+/* Generated stub for script_push_bytes */
+void script_push_bytes(u8 **scriptp UNNEEDED, const void *mem UNNEEDED, size_t len UNNEEDED)
+{ fprintf(stderr, "script_push_bytes called!\n"); abort(); }
 /* Generated stub for scriptpubkey_p2wsh */
 u8 *scriptpubkey_p2wsh(const tal_t *ctx UNNEEDED, const u8 *witnessscript UNNEEDED)
 { fprintf(stderr, "scriptpubkey_p2wsh called!\n"); abort(); }