multi: fix inclusion proof size

The inclusion proof field in the TapscriptPartialReveal function was
incorrect. An inclusion proof can be zero or more elements of 32-byte
slices. So an empty inclusion proof can be valid too for a tree that
only consists of a single leaf.

input/taproot.go

@@ -75,12 +75,12 @@ func TapscriptFullTree(internalKey *btcec.PublicKey,
 // key and revealed script.
 func TapscriptPartialReveal(internalKey *btcec.PublicKey,
 	revealedLeaf txscript.TapLeaf,
-	inclusionProof [32]byte) *waddrmgr.Tapscript {
+	inclusionProof []byte) *waddrmgr.Tapscript {
 
 	controlBlock := &txscript.ControlBlock{
 		InternalKey:    internalKey,
 		LeafVersion:    txscript.BaseLeafVersion,
-		InclusionProof: inclusionProof[:],
+		InclusionProof: inclusionProof,
 	}
 	rootHash := controlBlock.RootHash(revealedLeaf.Script)
 	tapKey := txscript.ComputeTaprootOutputKey(internalKey, rootHash)

lntest/itest/lnd_taproot_test.go

@@ -131,8 +131,9 @@ func testTaprootSignOutputRawScriptSpend(ctxt context.Context, t *harnessTest,
 	// Let's add a second script output as well to test the partial reveal.
 	leaf2 := testScriptSchnorrSig(t.t, leafSigningKey)
 
+	inclusionProof := leaf1.TapHash()
 	tapscript := input.TapscriptPartialReveal(
-		dummyInternalKey, leaf2, leaf1.TapHash(),
+		dummyInternalKey, leaf2, inclusionProof[:],
 	)
 	taprootKey, err := tapscript.TaprootKey()
 	require.NoError(t.t, err)

lnwallet/btcwallet/signer_test.go

@@ -273,8 +273,9 @@ func TestScriptImport(t *testing.T) {
 	// Now, as a last test, make sure that when we try adding an address
 	// with partial script reveal, we get an error that the address already
 	// exists.
+	inclusionProof := leaf2.TapHash()
 	tapscript2 := input.TapscriptPartialReveal(
-		testPubKey, leaf1, leaf2.TapHash(),
+		testPubKey, leaf1, inclusionProof[:],
 	)
 	_, err = w.ImportTaprootScript(scope, tapscript2)
 	require.Error(t, err)