find_p2wsh_out: new helper to find a particular output.

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

find_p2sh_out.c

@@ -5,21 +5,40 @@
 #include <ccan/tal/tal.h>
 #include <string.h>
 
-u32 find_p2sh_out(const struct bitcoin_tx *tx, u8 *redeemscript)
+static u32 find_output(const struct bitcoin_tx *tx, const u8 *scriptpubkey)
 {
-	/* This is the scriptPubKey commit tx will have */
-	u8 *p2sh = scriptpubkey_p2sh(NULL, redeemscript);
 	u32 i;
 
 	for (i = 0; i < tx->output_count; i++) {
-		if (tx->output[i].script_length != tal_count(p2sh))
+		if (tx->output[i].script_length != tal_count(scriptpubkey))
 			continue;
-		if (memcmp(tx->output[i].script, p2sh, tal_count(p2sh)) == 0)
+		if (memcmp(tx->output[i].script, scriptpubkey, tal_count(scriptpubkey)) == 0)
 			break;
 	}
 	/* FIXME: Return failure! */
 	if (i == tx->output_count)
 		errx(1, "No matching output in tx");
+	return i;
+}
+
+u32 find_p2sh_out(const struct bitcoin_tx *tx, const u8 *redeemscript)
+{
+	/* This is the scriptPubKey commit tx will have */
+	u8 *p2sh = scriptpubkey_p2sh(NULL, redeemscript);
+	u32 i;
+
+	i = find_output(tx, p2sh);
 	tal_free(p2sh);
 	return i;
 }
+
+u32 find_p2wsh_out(const struct bitcoin_tx *tx, const u8 *witnessscript)
+{
+	/* This is the scriptPubKey commit tx will have */
+	u8 *p2wsh = scriptpubkey_p2wsh(NULL, witnessscript);
+	u32 i;
+
+	i = find_output(tx, p2wsh);
+	tal_free(p2wsh);
+	return i;
+}

find_p2sh_out.h

@@ -8,5 +8,8 @@ struct bitcoin_tx;
 /* Normally we'd simply remember which output of the anchor or commit
  * tx is the one which pays to this script.  But for these examples,
  * we have to figure it out by recreating the output and matching. */
-u32 find_p2sh_out(const struct bitcoin_tx *tx, u8 *redeemscript);
+u32 find_p2sh_out(const struct bitcoin_tx *tx, const u8 *redeemscript);
+
+/* Similar routine for finding a specific p2wsh output. */
+u32 find_p2wsh_out(const struct bitcoin_tx *tx, const u8 *witnessscript);
 #endif /* LIGHTNING_FIND_P2SH_OUT_H */