onchaind: allow multiple candidate HTLCs for output match

When we have multiple HTLCs with the same preimage and the same CLTV,
it doesn't matter what order we treat them (they're literally
identical).  But when we offer HTLCs with the same preimage but
different CLTVs, the commitment tx outputs look identical, but the
HTLC txs are different: if we simply take the first HTLC which matches
(and that's not the right one), the HTLC signature we got from them
won't match.  As we rely on the signature matching to detect the fee
paid, we get:

	onchaind: STATUS_FAIL_INTERNAL_ERROR: grind_fee failed

So we alter match_htlc_output() to return an array of all matching
HTLC indices, which can have more than one entry for offered HTLCs.
If it's our commitment, we loop through until one of the HTLC
signatures matches.  If it's their commitment, we choose the HTLC with
the largest CLTV: we're going to ignore it once that hits anyway, so
this is the most conservative approach.  If it's a penalty, it doesn't
matter since we steal all HTLC outputs the same independent of CLTV.

For accepted HTLCs, the CLTV value is encoded in the witness script,
so this confusion isn't possible.  We nonetheless assert that the
CLTVs all match in that case.

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

CHANGELOG.md

@@ -52,6 +52,7 @@ changes.
 - Protocol: fix occasional deadlock when both peers flood with gossip.
 - Protocol: fix occasional long delay on sending `reply_short_channel_ids_end`.
 - Protocol: re-send `node_announcement` when address/alias/color etc change.
+- Protocol: multiple HTLCs with the same payment_hash are handled correctly.
 - Options: 'autotor' defaults to port 9051 if not specified.
 
 ### Security

onchaind/onchaind.c

@@ -144,18 +144,10 @@ static u64 grind_htlc_tx_fee(struct bitcoin_tx *tx,
 
 		return fee;
 	}
-	status_failed(STATUS_FAIL_INTERNAL_ERROR,
-		      "grind_fee failed from %u - %u"
-		      " for tx %s, inputamount %"PRIu64", signature %s, wscript %s, multiplier %"PRIu64,
-		      min_possible_feerate, max_possible_feerate,
-		      type_to_string(tmpctx, struct bitcoin_tx, tx),
-		      input_amount,
-		      type_to_string(tmpctx, secp256k1_ecdsa_signature, remotesig),
-		      tal_hex(tmpctx, wscript),
-		      multiplier);
+	return UINT64_MAX;
 }
 
-static void set_htlc_timeout_fee(struct bitcoin_tx *tx,
+static bool set_htlc_timeout_fee(struct bitcoin_tx *tx,
 				 const secp256k1_ecdsa_signature *remotesig,
 				 const u8 *wscript)
 {
@@ -170,21 +162,12 @@ static void set_htlc_timeout_fee(struct bitcoin_tx *tx,
 	 */
 	if (fee == UINT64_MAX) {
 		fee = grind_htlc_tx_fee(tx, remotesig, wscript, 663);
-		return;
+		return fee != UINT64_MAX;
 	}
 
 	tx->output[0].amount = *tx->input[0].amount - fee;
-	if (check_tx_sig(tx, 0, NULL, wscript,
-			 &keyset->other_htlc_key, remotesig))
-		return;
-
-	status_failed(STATUS_FAIL_INTERNAL_ERROR,
-		      "htlc_timeout_fee %"PRIu64" failed sigcheck "
-		      " for tx %s, signature %s, wscript %s",
-		      fee,
-		      type_to_string(tmpctx, struct bitcoin_tx, tx),
-		      type_to_string(tmpctx, secp256k1_ecdsa_signature, remotesig),
-		      tal_hex(tmpctx, wscript));
+	return check_tx_sig(tx, 0, NULL, wscript,
+			    &keyset->other_htlc_key, remotesig);
 }
 
 static void set_htlc_success_fee(struct bitcoin_tx *tx,
@@ -1305,42 +1288,105 @@ static u8 **derive_htlc_scripts(const struct htlc_stub *htlcs, enum side side)
 	return htlc_scripts;
 }
 
-static void resolve_our_htlc_ourcommit(struct tracked_output *out)
+static size_t resolve_our_htlc_ourcommit(struct tracked_output *out,
+					 const size_t *matches,
+					 const struct htlc_stub *htlcs,
+					 u8 **htlc_scripts)
 {
 	struct bitcoin_tx *tx;
 	secp256k1_ecdsa_signature localsig;
+	size_t i;
 
-	/* BOLT #5:
-	 *
-	 * ## HTLC Output Handling: Local Commitment, Local Offers
-	 * ...
-	 *  - if the commitment transaction HTLC output has *timed out* and
-	 *  hasn't been *resolved*:
-	 *    - MUST *resolve* the output by spending it using the HTLC-timeout
-	 *    transaction.
-	 */
-	tx = htlc_timeout_tx(out, &out->txid, out->outnum, out->satoshi * 1000,
-			     out->htlc->cltv_expiry,
-			     to_self_delay[LOCAL], 0, keyset);
+	assert(tal_count(matches));
 
-	set_htlc_timeout_fee(tx, out->remote_htlc_sig, out->wscript);
+	/* These htlcs are all possibilities, but signature will only match
+	 * one with the correct cltv: check which that is. */
+	for (i = 0; i < tal_count(matches); i++) {
+		/* BOLT #5:
+		 *
+		 * ## HTLC Output Handling: Local Commitment, Local Offers
+		 * ...
+		 *  - if the commitment transaction HTLC output has *timed out*
+		 *  and hasn't been *resolved*:
+		 *    - MUST *resolve* the output by spending it using the
+		 *    HTLC-timeout transaction.
+		 */
+		tx = htlc_timeout_tx(tmpctx, &out->txid, out->outnum,
+				     out->satoshi * 1000,
+				     htlcs[matches[i]].cltv_expiry,
+				     to_self_delay[LOCAL], 0, keyset);
 
-	hsm_sign_local_htlc_tx(tx, out->wscript, &localsig);
+		if (set_htlc_timeout_fee(tx, out->remote_htlc_sig,
+					 htlc_scripts[matches[i]]))
+			break;
+	}
+
+	/* Since there's been trouble with this before, we go to some length
+	 * to give details here! */
+	if (i == tal_count(matches)) {
+		char *cltvs, *wscripts;
+
+		cltvs = tal_fmt(tmpctx, "%u", htlcs[matches[0]].cltv_expiry);
+		wscripts = tal_hex(tmpctx, htlc_scripts[matches[0]]);
+
+		for (i = 1; i < tal_count(matches); i++) {
+			tal_append_fmt(&cltvs, "/%u",
+				       htlcs[matches[i]].cltv_expiry);
+			tal_append_fmt(&wscripts, "/%s",
+				       tal_hex(tmpctx, htlc_scripts[matches[i]]));
+		}
+
+		status_failed(STATUS_FAIL_INTERNAL_ERROR,
+			      "No valid signature found for %zu htlc_timeout_txs"
+			      " feerate %u-%u,"
+			      " last tx %s, inputamount %"PRIu64", signature %s,"
+			      " cltvs %s wscripts %s",
+			      tal_count(matches),
+			      min_possible_feerate, max_possible_feerate,
+			      type_to_string(tmpctx, struct bitcoin_tx, tx),
+			      out->satoshi,
+			      type_to_string(tmpctx, secp256k1_ecdsa_signature,
+					     out->remote_htlc_sig),
+			      cltvs, wscripts);
+	}
+
+	hsm_sign_local_htlc_tx(tx, htlc_scripts[matches[i]], &localsig);
 
 	tx->input[0].witness
 		= bitcoin_witness_htlc_timeout_tx(tx->input,
 						  &localsig,
 						  out->remote_htlc_sig,
-						  out->wscript);
+						  htlc_scripts[matches[i]]);
 
-	propose_resolution_at_block(out, tx, out->htlc->cltv_expiry,
+	/* Steals tx onto out */
+	propose_resolution_at_block(out, tx, htlcs[matches[i]].cltv_expiry,
 				    OUR_HTLC_TIMEOUT_TX);
+
+	return matches[i];
+}
+
+/* wscript for *received* htlcs (ie. our htlcs in their commit tx, or their
+ * htlcs in our commit tx) includes cltv, so they must be the same for all
+ * matching htlcs.  Unless, of course, they've found a sha256 clash. */
+static u32 matches_cltv(const size_t *matches,
+			const struct htlc_stub *htlcs)
+{
+	for (size_t i = 1; i < tal_count(matches); i++) {
+		assert(matches[i] < tal_count(htlcs));
+		assert(htlcs[matches[i]].cltv_expiry
+		       == htlcs[matches[i-1]].cltv_expiry);
+	}
+	return htlcs[matches[0]].cltv_expiry;
 }
 
-static void resolve_our_htlc_theircommit(struct tracked_output *out)
+static size_t resolve_our_htlc_theircommit(struct tracked_output *out,
+					   const size_t *matches,
+					   const struct htlc_stub *htlcs,
+					   u8 **htlc_scripts)
 {
 	struct bitcoin_tx *tx;
 	enum tx_type tx_type = OUR_HTLC_TIMEOUT_TO_US;
+	u32 cltv_expiry = matches_cltv(matches, htlcs);
 
 	/* BOLT #5:
 	 *
@@ -1353,15 +1399,24 @@ static void resolve_our_htlc_theircommit(struct tracked_output *out)
 	 *       address.
 	 */
 	tx = tx_to_us(out, remote_htlc_to_us,
-		      out, 0, out->htlc->cltv_expiry, NULL, 0,
-		      out->wscript,
+		      out, 0, cltv_expiry, NULL, 0,
+		      htlc_scripts[matches[0]],
 		      &tx_type);
 
-	propose_resolution_at_block(out, tx, out->htlc->cltv_expiry, tx_type);
+	propose_resolution_at_block(out, tx, cltv_expiry, tx_type);
+
+	/* They're all equivalent: might as well use first one. */
+	return matches[0];
 }
 
-static void resolve_their_htlc(struct tracked_output *out)
+/* Returns which htlcs it chose to use of matches[] */
+static size_t resolve_their_htlc(struct tracked_output *out,
+				 const size_t *matches,
+				 const struct htlc_stub *htlcs,
+				 u8 **htlc_scripts)
 {
+	size_t which_htlc;
+
 	/* BOLT #5:
 	 *
 	 * ## HTLC Output Handling: Remote Commitment, Remote Offers
@@ -1371,18 +1426,45 @@ static void resolve_their_htlc(struct tracked_output *out)
 	 * If not otherwise resolved, once the HTLC output has expired, it is
 	 * considered *irrevocably resolved*.
 	 */
+
+	/* BOLT #5:
+	 *
+	 * ## HTLC Output Handling: Local Commitment, Remote Offers
+	 *...
+	 * ### Requirements
+	 *...
+	 * If not otherwise resolved, once the HTLC output has expired, it is
+	 * considered *irrevocably resolved*.
+	 */
+
+	/* The two cases are identical as far as default handling goes.
+	 * But in the remote commitment / remote offer (ie. caller is
+	 * handle_their_unilateral), htlcs which match may have different cltvs.
+	 * So wait until the worst case (largest HTLC). */
+	assert(tal_count(matches));
+	which_htlc = matches[0];
+	for (size_t i = 1; i < tal_count(matches); i++) {
+		if (htlcs[matches[i]].cltv_expiry > htlcs[which_htlc].cltv_expiry)
+			which_htlc = matches[i];
+	}
+
 	/* If we hit timeout depth, resolve by ignoring. */
-	propose_resolution_at_block(out, NULL, out->htlc->cltv_expiry,
+	propose_resolution_at_block(out, NULL, htlcs[which_htlc].cltv_expiry,
 				    THEIR_HTLC_TIMEOUT_TO_THEM);
+	return which_htlc;
 }
 
-static int match_htlc_output(const struct bitcoin_tx *tx,
-			     unsigned int outnum,
-			     u8 **htlc_scripts)
+/* Return tal_arr of htlc indexes. */
+static const size_t *match_htlc_output(const tal_t *ctx,
+				 const struct bitcoin_tx *tx,
+				 unsigned int outnum,
+				 u8 **htlc_scripts)
 {
+	size_t *matches = tal_arr(ctx, size_t, 0);
+
 	/* Must be a p2wsh output */
 	if (!is_p2wsh(tx->output[outnum].script, NULL))
-		return -1;
+		return matches;
 
 	for (size_t i = 0; i < tal_count(htlc_scripts); i++) {
 		struct sha256 sha;
@@ -1393,9 +1475,21 @@ static int match_htlc_output(const struct bitcoin_tx *tx,
 		if (memeq(tx->output[outnum].script + 2,
 			  tal_count(tx->output[outnum].script) - 2,
 			  &sha, sizeof(sha)))
-			return i;
+			*tal_arr_expand(&matches) = i;
 	}
-	return -1;
+	return matches;
+}
+
+/* They must all be in the same direction, since the scripts are different for
+ * each dir.  Unless, of course, they've found a sha256 clash. */
+static enum side matches_direction(const size_t *matches,
+				   const struct htlc_stub *htlcs)
+{
+	for (size_t i = 1; i < tal_count(matches); i++) {
+		assert(matches[i] < tal_count(htlcs));
+		assert(htlcs[matches[i]].owner == htlcs[matches[i-1]].owner);
+	}
+	return htlcs[matches[0]].owner;
 }
 
 /* Tell master about any we didn't use, if it wants to know. */
@@ -1508,7 +1602,8 @@ static void handle_our_unilateral(const struct bitcoin_tx *tx,
 
 	for (i = 0; i < tal_count(tx->output); i++) {
 		struct tracked_output *out;
-		int j;
+		const size_t *matches;
+		size_t which_htlc;
 
 		if (script[LOCAL]
 		    && scripteq(tx->output[i].script, script[LOCAL])) {
@@ -1577,14 +1672,14 @@ static void handle_our_unilateral(const struct bitcoin_tx *tx,
 			continue;
 		}
 
+		matches = match_htlc_output(tmpctx, tx, i, htlc_scripts);
 		/* FIXME: limp along when this happens! */
-		j = match_htlc_output(tx, i, htlc_scripts);
-		if (j == -1)
+		if (tal_count(matches) == 0)
 			status_failed(STATUS_FAIL_INTERNAL_ERROR,
 				      "Could not find resolution for output %zu",
 				      i);
 
-		if (htlcs[j].owner == LOCAL) {
+		if (matches_direction(matches, htlcs) == LOCAL) {
 			/* BOLT #5:
 			 *
 			 *     - MUST handle HTLCs offered by itself as specified
@@ -1596,17 +1691,19 @@ static void handle_our_unilateral(const struct bitcoin_tx *tx,
 						 OUR_UNILATERAL, i,
 						 tx->output[i].amount,
 						 OUR_HTLC,
-						 &htlcs[j], htlc_scripts[j],
+						 NULL, NULL,
 						 remote_htlc_sigs);
-			resolve_our_htlc_ourcommit(out);
+			/* Tells us which htlc to use */
+			which_htlc = resolve_our_htlc_ourcommit(out, matches,
+								htlcs,
+								htlc_scripts);
 		} else {
 			out = new_tracked_output(&outs, txid,
 						 tx_blockheight,
 						 OUR_UNILATERAL, i,
 						 tx->output[i].amount,
 						 THEIR_HTLC,
-						 &htlcs[j],
-						 htlc_scripts[j],
+						 NULL, NULL,
 						 remote_htlc_sigs);
 			/* BOLT #5:
 			 *
@@ -1614,13 +1711,17 @@ static void handle_our_unilateral(const struct bitcoin_tx *tx,
 			 *     as specified in [HTLC Output Handling: Local
 			 *     Commitment, Remote Offers]
 			 */
-			resolve_their_htlc(out);
+			/* Tells us which htlc to use */
+			which_htlc = resolve_their_htlc(out, matches, htlcs,
+							htlc_scripts);
 		}
+		out->htlc = &htlcs[which_htlc];
+		out->wscript = htlc_scripts[which_htlc];
 
 		/* Each of these consumes one HTLC signature */
 		remote_htlc_sigs++;
 		/* We've matched this HTLC, can't do again. */
-		htlc_scripts[j] = NULL;
+		htlc_scripts[which_htlc] = NULL;
 
 	}
 
@@ -1809,7 +1910,8 @@ static void handle_their_cheat(const struct bitcoin_tx *tx,
 
 	for (i = 0; i < tal_count(tx->output); i++) {
 		struct tracked_output *out;
-		int j;
+		const size_t *matches;
+		size_t which_htlc;
 
 		if (script[LOCAL]
 		    && scripteq(tx->output[i].script, script[LOCAL])) {
@@ -1854,13 +1956,16 @@ static void handle_their_cheat(const struct bitcoin_tx *tx,
 			continue;
 		}
 
-		j = match_htlc_output(tx, i, htlc_scripts);
-		if (j == -1)
+		matches = match_htlc_output(tmpctx, tx, i, htlc_scripts);
+		if (tal_count(matches) == 0)
 			status_failed(STATUS_FAIL_INTERNAL_ERROR,
 				      "Could not find resolution for output %zu",
 				      i);
 
-		if (htlcs[j].owner == LOCAL) {
+		/* In this case, we don't care which HTLC we choose; so pick
+		   first one */
+		which_htlc = matches[0];
+		if (matches_direction(matches, htlcs) == LOCAL) {
 			/* BOLT #5:
 			 *
 			 *  - MUST *resolve* the _local node's offered HTLCs_
@@ -1877,7 +1982,8 @@ static void handle_their_cheat(const struct bitcoin_tx *tx,
 						 THEIR_REVOKED_UNILATERAL, i,
 						 tx->output[i].amount,
 						 OUR_HTLC,
-						 &htlcs[j], htlc_scripts[j],
+						 &htlcs[which_htlc],
+						 htlc_scripts[which_htlc],
 						 NULL);
 			steal_htlc(out);
 		} else {
@@ -1886,7 +1992,8 @@ static void handle_their_cheat(const struct bitcoin_tx *tx,
 						 THEIR_REVOKED_UNILATERAL, i,
 						 tx->output[i].amount,
 						 THEIR_HTLC,
-						 &htlcs[j], htlc_scripts[j],
+						 &htlcs[which_htlc],
+						 htlc_scripts[which_htlc],
 						 NULL);
 			/* BOLT #5:
 			 *
@@ -1899,7 +2006,7 @@ static void handle_their_cheat(const struct bitcoin_tx *tx,
 			 */
 			steal_htlc(out);
 		}
-		htlc_scripts[j] = NULL;
+		htlc_scripts[which_htlc] = NULL;
 	}
 
 	note_missing_htlcs(htlc_scripts, htlcs,
@@ -2021,7 +2128,8 @@ static void handle_their_unilateral(const struct bitcoin_tx *tx,
 
 	for (i = 0; i < tal_count(tx->output); i++) {
 		struct tracked_output *out;
-		int j;
+		const size_t *matches;
+		size_t which_htlc;
 
 		if (script[LOCAL]
 		    && scripteq(tx->output[i].script, script[LOCAL])) {
@@ -2068,12 +2176,13 @@ static void handle_their_unilateral(const struct bitcoin_tx *tx,
 			continue;
 		}
 
-		j = match_htlc_output(tx, i, htlc_scripts);
-		if (j == -1)
+		matches = match_htlc_output(tmpctx, tx, i, htlc_scripts);
+		if (tal_count(matches) == 0)
 			status_failed(STATUS_FAIL_INTERNAL_ERROR,
 				      "Could not find resolution for output %zu",
 				      i);
-		if (htlcs[j].owner == LOCAL) {
+
+		if (matches_direction(matches, htlcs) == LOCAL) {
 			/* BOLT #5:
 			 *
 			 * - MUST handle HTLCs offered by itself as specified in
@@ -2085,16 +2194,19 @@ static void handle_their_unilateral(const struct bitcoin_tx *tx,
 						 THEIR_UNILATERAL, i,
 						 tx->output[i].amount,
 						 OUR_HTLC,
-						 &htlcs[j], htlc_scripts[j],
+						 NULL, NULL,
 						 NULL);
-			resolve_our_htlc_theircommit(out);
+			which_htlc = resolve_our_htlc_theircommit(out,
+								  matches,
+								  htlcs,
+								  htlc_scripts);
 		} else {
 			out = new_tracked_output(&outs, txid,
 						 tx_blockheight,
 						 THEIR_UNILATERAL, i,
 						 tx->output[i].amount,
 						 THEIR_HTLC,
-						 &htlcs[j], htlc_scripts[j],
+						 NULL, NULL,
 						 NULL);
 			/* BOLT #5:
 			 *
@@ -2102,9 +2214,12 @@ static void handle_their_unilateral(const struct bitcoin_tx *tx,
 			 *   specified in [HTLC Output Handling: Remote
 			 *   Commitment, Remote Offers]
 			 */
-			resolve_their_htlc(out);
+			which_htlc = resolve_their_htlc(out, matches, htlcs,
+							htlc_scripts);
 		}
-		htlc_scripts[j] = NULL;
+		out->htlc = &htlcs[which_htlc];
+		out->wscript = htlc_scripts[which_htlc];
+		htlc_scripts[which_htlc] = NULL;
 	}
 
 	note_missing_htlcs(htlc_scripts, htlcs,

tests/test_closing.py

@@ -630,7 +630,6 @@ def test_onchain_dust_out(node_factory, bitcoind, executor):
     assert only_one(l2.rpc.listinvoices('onchain_dust_out')['invoices'])['status'] == 'unpaid'
 
 
-@pytest.mark.xfail(strict=True)
 @unittest.skipIf(not DEVELOPER, "needs DEVELOPER=1")
 def test_onchain_timeout(node_factory, bitcoind, executor):
     """Onchain handling of outgoing failed htlcs"""
@@ -1094,7 +1093,6 @@ def setup_multihtlc_test(node_factory, bitcoind):
     return h, nodes
 
 
-@pytest.mark.xfail(strict=True)
 @unittest.skipIf(not DEVELOPER, "needs DEVELOPER=1 for dev_ignore_htlcs")
 def test_onchain_multihtlc_our_unilateral(node_factory, bitcoind):
     """Node pushes a channel onchain with multiple HTLCs with same payment_hash """
@@ -1182,7 +1180,6 @@ def test_onchain_multihtlc_our_unilateral(node_factory, bitcoind):
             assert only_one(nodes[i].rpc.listpeers(nodes[i + 1].info['id'])['peers'])['connected']
 
 
-@pytest.mark.xfail(strict=True)
 @unittest.skipIf(not DEVELOPER, "needs DEVELOPER=1 for dev_ignore_htlcs")
 def test_onchain_multihtlc_their_unilateral(node_factory, bitcoind):
     """Node pushes a channel onchain with multiple HTLCs with same payment_hash """