onchaind: don't create zero-output txs if fees overwhelm us.

They're illegal.  Instead do OP_RETURN so we don't pollute the UTXO.

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

bitcoin/script.c

@@ -22,6 +22,7 @@
 #define OP_NOTIF	0x64
 #define OP_ELSE		0x67
 #define OP_ENDIF	0x68
+#define OP_RETURN	0x6a
 #define OP_2DROP	0x6d
 #define OP_DEPTH	0x74
 #define OP_DROP		0x75
@@ -214,6 +215,14 @@ u8 *scriptpubkey_p2pkh(const tal_t *ctx, const struct bitcoin_address *addr)
 	return script;
 }
 
+u8 *scriptpubkey_opreturn(const tal_t *ctx)
+{
+	u8 *script = tal_arr(ctx, u8, 0);
+
+	add_op(&script, OP_RETURN);
+	return script;
+}
+
 /* Create an input script which spends p2pkh */
 u8 *bitcoin_redeem_p2pkh(const tal_t *ctx, const struct pubkey *pubkey,
 			 const secp256k1_ecdsa_signature *sig)

bitcoin/script.h

@@ -29,6 +29,9 @@ u8 *scriptpubkey_p2sh_hash(const tal_t *ctx, const struct ripemd160 *redeemhash)
 /* Create an output script using p2pkh */
 u8 *scriptpubkey_p2pkh(const tal_t *ctx, const struct bitcoin_address *addr);
 
+/* Create a prunable output script */
+u8 *scriptpubkey_opreturn(const tal_t *ctx);
+
 /* Create an input script which spends p2pkh */
 u8 *bitcoin_redeem_p2pkh(const tal_t *ctx, const struct pubkey *pubkey,
 			 const secp256k1_ecdsa_signature *sig);

onchaind/onchain.c

@@ -243,10 +243,11 @@ static struct bitcoin_tx *tx_to_us(const tal_t *ctx,
 			 + 1 + 3 + 73 + 0 + tal_len(wscript))
 		/ 1000;
 
-	/* Result is trivial?  Just eliminate output. */
-	if (tx->output[0].amount < dust_limit_satoshis + fee)
-		tal_resize(&tx->output, 0);
-	else
+	/* Result is trivial?  Spent to OP_RETURN to avoid leaving dust. */
+	if (tx->output[0].amount < dust_limit_satoshis + fee) {
+		tx->output[0].amount = 0;
+		tx->output[0].script = scriptpubkey_opreturn(tx->output);
+	} else
 		tx->output[0].amount -= fee;
 
 	sign_tx_input(tx, 0, NULL, wscript, privkey, pubkey, &sig);