Merge pull request #6502 from bhandras/musig-fix

signer: fix crash in `MuSig2Combine` when the final signature isn't ready

docs/release-notes/release-notes-0.15.0.md

@@ -86,6 +86,8 @@ to Bitcoin nodes that advertise a Tor v3 onion service address.
   arbitrator relying on htlcswitch to be started
   first](https://github.com/lightningnetwork/lnd/pull/6214).
 
+* [Fixed crash in MuSig2Combine](https://github.com/lightningnetwork/lnd/pull/6502)
+  
 ## Neutrino
 
 * [New neutrino sub-server](https://github.com/lightningnetwork/lnd/pull/5652)

lnrpc/signrpc/signer_server.go

@@ -938,10 +938,15 @@ func (s *Server) MuSig2CombineSig(_ context.Context,
 		return nil, fmt.Errorf("error combining signatures: %v", err)
 	}
 
-	return &MuSig2CombineSigResponse{
+	resp := &MuSig2CombineSigResponse{
 		HaveAllSignatures: haveAllSigs,
-		FinalSignature:    finalSig.Serialize(),
+	}
-	}, nil
+
+	if haveAllSigs {
+		resp.FinalSignature = finalSig.Serialize()
+	}
+
+	return resp, err
 }
 
 // MuSig2Cleanup removes a session from memory to free up resources.

lntest/itest/lnd_taproot_test.go

@@ -902,6 +902,20 @@ func testTaprootMuSig2CombinedLeafKeySpend(ctxt context.Context, t *harnessTest,
 	)
 	require.NoError(t.t, err)
 
+	// Before we have all partial signatures, we shouldn't get a final
+	// signature back.
+	combineSigResp, err := alice.SignerClient.MuSig2CombineSig(
+		ctxt, &signrpc.MuSig2CombineSigRequest{
+			SessionId: sessResp1.SessionId,
+			OtherPartialSignatures: [][]byte{
+				signResp2.LocalPartialSignature,
+			},
+		},
+	)
+	require.NoError(t.t, err)
+	require.False(t.t, combineSigResp.HaveAllSignatures)
+	require.Empty(t.t, combineSigResp.FinalSignature)
+
 	signResp3, err := alice.SignerClient.MuSig2Sign(
 		ctxt, &signrpc.MuSig2SignRequest{
 			SessionId:     sessResp3.SessionId,
@@ -935,7 +949,6 @@ func testTaprootMuSig2CombinedLeafKeySpend(ctxt context.Context, t *harnessTest,
 		ctxt, &signrpc.MuSig2CombineSigRequest{
 			SessionId: sessResp1.SessionId,
 			OtherPartialSignatures: [][]byte{
-				signResp2.LocalPartialSignature,
 				signResp3.LocalPartialSignature,
 			},
 		},