lnwallet/channel: fix crash on receiving too few HTLC sigs

This commit fixes an out of bounds error that would occur in the case
where we received a new commitment where the accompanying HTLC sigs were
too few. Now we'll just reject such an commitment.

A test exercising the behavior is also added.

lnwallet/channel.go

@@ -3532,6 +3532,12 @@ func genHtlcSigValidationJobs(localCommitmentView *commitment,
 				return sigHash, nil
 			}
 
+			// Make sure there are more signatures left.
+			if i >= len(htlcSigs) {
+				return nil, fmt.Errorf("not enough HTLC " +
+					"signatures.")
+			}
+
 			// With the sighash generated, we'll also store the
 			// signature so it can be written to disk if this state
 			// is valid.
@@ -3578,6 +3584,12 @@ func genHtlcSigValidationJobs(localCommitmentView *commitment,
 				return sigHash, nil
 			}
 
+			// Make sure there are more signatures left.
+			if i >= len(htlcSigs) {
+				return nil, fmt.Errorf("not enough HTLC " +
+					"signatures.")
+			}
+
 			// With the sighash generated, we'll also store the
 			// signature so it can be written to disk if this state
 			// is valid.

lnwallet/channel_test.go

@@ -1461,6 +1461,77 @@ func TestHTLCDustLimit(t *testing.T) {
 	}
 }
 
+// TestHTLCSigNumber tests that a received commitment is only accepted if it
+// comes with the exact number of valid HTLC signatures.
+func TestHTLCSigNumber(t *testing.T) {
+	t.Parallel()
+
+	// createChanWithHTLC is a helper method that sets ut two channels, and
+	// adds HTLCs with the passed values to the channels.
+	createChanWithHTLC := func(htlcValues ...btcutil.Amount) (
+		*LightningChannel, *LightningChannel, func()) {
+
+		// Create a test channel funded evenly with Alice having 5 BTC,
+		// and Bob having 5 BTC. Alice's dustlimit is 200 sat, while
+		// Bob has 1300 sat.
+		aliceChannel, bobChannel, cleanUp, err := createTestChannels(3)
+		if err != nil {
+			t.Fatalf("unable to create test channels: %v", err)
+		}
+
+		for i, htlcSat := range htlcValues {
+			htlcMsat := lnwire.NewMSatFromSatoshis(htlcSat)
+			htlc, _ := createHTLC(i, htlcMsat)
+			_, err := aliceChannel.AddHTLC(htlc, nil)
+			if err != nil {
+				t.Fatalf("alice unable to add htlc: %v", err)
+			}
+			_, err = bobChannel.ReceiveHTLC(htlc)
+			if err != nil {
+				t.Fatalf("bob unable to receive htlc: %v", err)
+			}
+		}
+
+		return aliceChannel, bobChannel, cleanUp
+	}
+
+	// Calculate two values that will be below and above Bob's dust limit.
+	estimator := &StaticFeeEstimator{24}
+	feePerVSize, err := estimator.EstimateFeePerVSize(1)
+	if err != nil {
+		t.Fatalf("unable to get fee: %v", err)
+	}
+	feePerKw := feePerVSize.FeePerKWeight()
+
+	aboveDust := btcutil.Amount(1400) + htlcSuccessFee(feePerKw)
+
+	// ===================================================================
+	// Test that Bob will reject a commitment if Alice doesn't send enough
+	// HTLC signatures.
+	// ===================================================================
+	aliceChannel, bobChannel, cleanUp := createChanWithHTLC(aboveDust,
+		aboveDust)
+	defer cleanUp()
+
+	aliceSig, aliceHtlcSigs, err := aliceChannel.SignNextCommitment()
+	if err != nil {
+		t.Fatalf("Error signing next commitment: %v", err)
+	}
+
+	if len(aliceHtlcSigs) != 2 {
+		t.Fatalf("expected 2 htlc sig, instead got %v",
+			len(aliceHtlcSigs))
+	}
+
+	// Now discard one signature from the htlcSig slice. Bob should reject
+	// the commitment because of this.
+	err = bobChannel.ReceiveNewCommitment(aliceSig, aliceHtlcSigs[1:])
+	if err == nil {
+		t.Fatalf("Expected Bob to reject signatures")
+	}
+
+}
+
 // TestChannelBalanceDustLimit tests the condition when the remaining balance
 // for one of the channel participants is so small as to be considered dust. In
 // this case, the output for that participant is removed and all funds (minus