fix critical proposer selection bug #4259

beacon-chain/core/helpers/validators.go

@@ -213,6 +213,7 @@ func ComputeProposerIndex(state *pb.BeaconState, indices []uint64, seed [32]byte
 		if err != nil {
 			return 0, err
 		}
+		candidateIndex = indices[candidateIndex]
 		b := append(seed[:], bytesutil.Bytes8(i/32)...)
 		randomByte := hashutil.Hash(b)[i%32]
 		effectiveBal := state.Validators[candidateIndex].EffectiveBalance

beacon-chain/core/helpers/validators_test.go

@@ -405,3 +405,63 @@ func TestActiveValidatorIndices(t *testing.T) {
 		})
 	}
 }
+
+func TestComputeProposerIndex(t *testing.T) {
+	seed := bytesutil.ToBytes32([]byte("seed"))
+	type args struct {
+		state   *pb.BeaconState
+		indices []uint64
+		seed    [32]byte
+	}
+	tests := []struct {
+		name    string
+		args    args
+		want    uint64
+		wantErr bool
+	}{
+		{
+			name: "all_active_indices",
+			args: args{
+				state: &pb.BeaconState{
+					Validators: []*ethpb.Validator{
+						&ethpb.Validator{EffectiveBalance: params.BeaconConfig().MaxEffectiveBalance},
+						&ethpb.Validator{EffectiveBalance: params.BeaconConfig().MaxEffectiveBalance},
+						&ethpb.Validator{EffectiveBalance: params.BeaconConfig().MaxEffectiveBalance},
+						&ethpb.Validator{EffectiveBalance: params.BeaconConfig().MaxEffectiveBalance},
+					},
+				},
+				indices: []uint64{0,1,2,3,4},
+				seed: seed,
+			},
+			want: 2,
+		},
+		{ // Regression test for https://github.com/prysmaticlabs/prysm/issues/4259.
+			name: "1_active_index",
+			args: args{
+				state: &pb.BeaconState{
+					Validators: []*ethpb.Validator{
+						&ethpb.Validator{EffectiveBalance: params.BeaconConfig().MaxEffectiveBalance},
+						&ethpb.Validator{EffectiveBalance: params.BeaconConfig().MaxEffectiveBalance},
+						&ethpb.Validator{EffectiveBalance: params.BeaconConfig().MaxEffectiveBalance},
+						&ethpb.Validator{EffectiveBalance: params.BeaconConfig().MaxEffectiveBalance},
+					},
+				},
+				indices: []uint64{3},
+				seed: seed,
+			},
+			want: 3,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := ComputeProposerIndex(tt.args.state, tt.args.indices, tt.args.seed)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ComputeProposerIndex() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if got != tt.want {
+				t.Errorf("ComputeProposerIndex() got = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}