fix: require a collected signature before signing-power validation passes

The final signersWeight check counts payload-blind subdigest leaves as
satisfied even for an empty signer set, so BuildRegularSignature and
BuildNoChainIDSignature with validateSigningPower could succeed without
collecting any signature. Restore the isValid flag, set only after a
non-nil signature brings the optimistic signersWeight to the threshold,
while cancellation keeps using collectedSignersWeight.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: ScreamingHawk

core/v3/signature_build_test.go

@@ -66,3 +66,25 @@ func TestBuildRegularSignatureCollectsAllSignersDespiteSubdigestLeaf(t *testing.
 		require.Equal(t, first, data, "signature encoding must be deterministic")
 	}
 }
+
+// A subdigest leaf reports max signersWeight even for an empty signer set, so
+// signing-power validation must still fail when no signature is collected at all.
+func TestBuildSignatureValidationRejectsEmptySignerSetDespiteSubdigestLeaf(t *testing.T) {
+	config := &v3.WalletConfig{
+		Threshold_: 2,
+		Tree: v3.WalletConfigTreeNodes(
+			v3.WalletConfigTreeSubdigestLeaf{Subdigest: common.BigToHash(big.NewInt(1))},
+			&v3.WalletConfigTreeAddressLeaf{Weight: 1, Address: common.HexToAddress("0x1111111111111111111111111111111111111111")},
+		),
+	}
+
+	signingFunc := func(ctx context.Context, signer core.Signer, _ []core.SignerSignature) (core.SignerSignatureType, []byte, error) {
+		return 0, nil, nil
+	}
+
+	_, err := config.BuildRegularSignature(context.Background(), signingFunc, true)
+	require.ErrorContains(t, err, "not enough signers")
+
+	_, err = config.BuildNoChainIDSignature(context.Background(), signingFunc, true)
+	require.ErrorContains(t, err, "not enough signers")
+}

core/v3/v3.go

@@ -1999,7 +1999,9 @@ func (c *WalletConfig) BuildSubdigestSignature(noChainID bool) (core.Signature[*
 }
 
 func (c *WalletConfig) BuildRegularSignature(ctx context.Context, sign core.SigningFunction, validateSigningPower bool, checkpointerData ...[]byte) (core.Signature[*WalletConfig], error) {
+	var isValid bool
 	configSigners := c.Signers()
+	threshold := new(big.Int).SetUint64(uint64(c.Threshold_))
 
 	signCtx, signCancel := context.WithCancel(ctx)
 	defer signCancel()
@@ -2023,14 +2025,17 @@ func (c *WalletConfig) BuildRegularSignature(ctx context.Context, sign core.Sign
 			// threshold. signersWeight counts payload-blind subdigest leaves as satisfied,
 			// and cancelling on that estimate nondeterministically drops signatures that
 			// recovery of a non-matching payload still needs.
-			weight := c.Tree.collectedSignersWeight(signedSigners)
-			if weight.Cmp(new(big.Int).SetUint64(uint64(c.Threshold_))) >= 0 {
+			if c.Tree.collectedSignersWeight(signedSigners).Cmp(threshold) >= 0 {
 				signCancel()
 			}
+
+			if c.Tree.signersWeight(signedSigners).Cmp(threshold) >= 0 {
+				isValid = true
+			}
 		}
 	}
 
-	if validateSigningPower && c.Tree.signersWeight(signedSigners).Cmp(new(big.Int).SetUint64(uint64(c.Threshold_))) < 0 {
+	if !isValid && validateSigningPower {
 		return nil, fmt.Errorf("not enough signers to build regular signature")
 	}
 
@@ -2050,7 +2055,9 @@ func (c *WalletConfig) BuildRegularSignature(ctx context.Context, sign core.Sign
 }
 
 func (c *WalletConfig) BuildNoChainIDSignature(ctx context.Context, sign core.SigningFunction, validateSigningPower bool, checkpointerData ...[]byte) (core.Signature[*WalletConfig], error) {
+	var isValid bool
 	configSigners := c.Signers()
+	threshold := new(big.Int).SetUint64(uint64(c.Threshold_))
 
 	signCtx, signCancel := context.WithCancel(ctx)
 	defer signCancel()
@@ -2074,14 +2081,17 @@ func (c *WalletConfig) BuildNoChainIDSignature(ctx context.Context, sign core.Si
 			// threshold. signersWeight counts payload-blind subdigest leaves as satisfied,
 			// and cancelling on that estimate nondeterministically drops signatures that
 			// recovery of a non-matching payload still needs.
-			weight := c.Tree.collectedSignersWeight(signedSigners)
-			if weight.Cmp(new(big.Int).SetUint64(uint64(c.Threshold_))) >= 0 {
+			if c.Tree.collectedSignersWeight(signedSigners).Cmp(threshold) >= 0 {
 				signCancel()
 			}
+
+			if c.Tree.signersWeight(signedSigners).Cmp(threshold) >= 0 {
+				isValid = true
+			}
 		}
 	}
 
-	if validateSigningPower && c.Tree.signersWeight(signedSigners).Cmp(new(big.Int).SetUint64(uint64(c.Threshold_))) < 0 {
+	if !isValid && validateSigningPower {
 		return nil, fmt.Errorf("not enough signers to build no chain ID signature")
 	}