compiler: restrict return values count for manifest methods

Exported functions from main package shouldn't have more than one return
value.

pkg/compiler/analysis.go

@@ -13,8 +13,13 @@ import (
 	"golang.org/x/tools/go/packages"
 )
 
-// ErrMissingExportedParamName is returned when exported contract method has unnamed parameter.
+// Various exported functions usage errors.
-var ErrMissingExportedParamName = errors.New("exported method is not allowed to have unnamed parameter")
+var (
+	// ErrMissingExportedParamName is returned when exported contract method has unnamed parameter.
+	ErrMissingExportedParamName = errors.New("exported method is not allowed to have unnamed parameter")
+	// ErrInvalidExportedRetCount is returned when exported contract method has invalid return values count.
+	ErrInvalidExportedRetCount = errors.New("exported method is not allowed to have more than one return value")
+)
 
 var (
 	// Go language builtin functions.
@@ -289,7 +294,7 @@ func (c *codegen) analyzeFuncUsage() funcUsage {
 				if isMain && n.Name.IsExported() || isInitFunc(n) || isDeployFunc(n) {
 					diff[name] = true
 				}
-				// exported functions are not allowed to have unnamed parameters
+				// exported functions are not allowed to have unnamed parameters  or multiple return values
 				if isMain && n.Name.IsExported() && n.Recv == nil {
 					if n.Type.Params.List != nil {
 						for i, param := range n.Type.Params.List {
@@ -305,6 +310,9 @@ func (c *codegen) analyzeFuncUsage() funcUsage {
 							}
 						}
 					}
+					if retCnt := n.Type.Results.NumFields(); retCnt > 1 {
+						c.prog.Err = fmt.Errorf("%w: %s/%d return values", ErrInvalidExportedRetCount, n.Name, retCnt)
+					}
 				}
 				nodeCache[name] = declPair{n, c.importMap, pkgPath}
 				return false // will be processed in the next stage

pkg/compiler/compiler_test.go

@@ -2,6 +2,7 @@ package compiler_test
 
 import (
 	"fmt"
+	"math/big"
 	"os"
 	"path/filepath"
 	"strings"
@@ -427,3 +428,147 @@ func TestUnnamedParameterCheck(t *testing.T) {
 		require.NoError(t, err) // it's OK for exported method to have unnamed params as it won't be included into manifest
 	})
 }
+
+func TestReturnValuesCountCheck(t *testing.T) {
+	t.Run("void", func(t *testing.T) {
+		t.Run("exported", func(t *testing.T) {
+			t.Run("func", func(t *testing.T) {
+				src := `package testcase
+					var a int
+					func Main() {
+						a = 5
+					}`
+				_, _, err := compiler.CompileWithOptions("test.go", strings.NewReader(src), nil)
+				require.NoError(t, err)
+			})
+			t.Run("method", func(t *testing.T) {
+				src := `package testcase
+					type A int
+					var a int
+					func (rcv A) Main() {
+						a = 5
+					}`
+				_, _, err := compiler.CompileWithOptions("test.go", strings.NewReader(src), nil)
+				require.NoError(t, err)
+			})
+		})
+		t.Run("unexported", func(t *testing.T) {
+			src := `package testcase
+					var a int
+					func main() {
+						a = 5
+					}`
+			_, _, err := compiler.CompileWithOptions("test.go", strings.NewReader(src), nil)
+			require.NoError(t, err)
+		})
+	})
+	t.Run("single return", func(t *testing.T) {
+		t.Run("exported", func(t *testing.T) {
+			t.Run("func", func(t *testing.T) {
+				src := `package testcase
+					var a int
+					func Main() int {
+						a = 5
+						return a
+					}`
+				eval(t, src, big.NewInt(5))
+			})
+			t.Run("method", func(t *testing.T) {
+				src := `package testcase
+					type A int
+					var a int
+					func (rcv A) Main() int {
+						a = 5
+						return a
+					}`
+				_, _, err := compiler.CompileWithOptions("test.go", strings.NewReader(src), nil)
+				require.NoError(t, err)
+			})
+		})
+		t.Run("unexported", func(t *testing.T) {
+			src := `package testcase
+					var a int
+					func main() int {
+						a = 5
+						return a
+					}`
+			_, _, err := compiler.CompileWithOptions("test.go", strings.NewReader(src), nil)
+			require.NoError(t, err)
+		})
+	})
+	t.Run("multiple unnamed return vals", func(t *testing.T) {
+		t.Run("exported", func(t *testing.T) {
+			t.Run("func", func(t *testing.T) {
+				src := `package testcase
+					var a int
+					func Main() (int, int) {
+						a = 5
+						return a, a
+					}`
+				_, _, err := compiler.CompileWithOptions("test.go", strings.NewReader(src), nil)
+				require.Error(t, err)
+				require.ErrorIs(t, err, compiler.ErrInvalidExportedRetCount)
+			})
+			t.Run("method", func(t *testing.T) {
+				src := `package testcase
+					type A int
+					var a int
+					func (rcv A) Main() (int, int) {
+						a = 5
+						return a, a
+					}`
+				_, _, err := compiler.CompileWithOptions("test.go", strings.NewReader(src), nil)
+				require.NoError(t, err) // OK for method to have multiple return values as it won't be included into manifest
+			})
+		})
+		t.Run("unexported", func(t *testing.T) {
+			src := `package testcase
+					var a int
+					func main() (int, int) {
+						a = 5
+						return a, a
+					}`
+			_, _, err := compiler.CompileWithOptions("test.go", strings.NewReader(src), nil)
+			require.NoError(t, err) // OK for unexported function to have multiple return values as it won't be included into manifest
+		})
+	})
+	t.Run("multiple named return vals", func(t *testing.T) {
+		t.Run("exported", func(t *testing.T) {
+			t.Run("func", func(t *testing.T) {
+				src := `package testcase
+					var a int
+					func Main() (a int, b int) {
+						a = 5
+						b = 2
+						return
+					}`
+				_, _, err := compiler.CompileWithOptions("test.go", strings.NewReader(src), nil)
+				require.Error(t, err)
+				require.ErrorIs(t, err, compiler.ErrInvalidExportedRetCount)
+			})
+			t.Run("method", func(t *testing.T) {
+				src := `package testcase
+					type A int
+					var a int
+					func (rcv A) Main() (a int, b int) {
+						a = 5
+						b = 2
+						return
+					}`
+				_, _, err := compiler.CompileWithOptions("test.go", strings.NewReader(src), nil)
+				require.NoError(t, err) // OK for method to have multiple return values as it won't be included into manifest
+			})
+		})
+		t.Run("unexported", func(t *testing.T) {
+			src := `package testcase
+					var a int
+					func main() (a int, b int) {
+						a = 5
+						b = 2
+						return
+					}`
+			_, _, err := compiler.CompileWithOptions("test.go", strings.NewReader(src), nil)
+			require.NoError(t, err) // OK for unexported function to have multiple return values as it won't be included into manifest
+		})
+	})
+}

pkg/compiler/return_test.go

@@ -4,9 +4,6 @@ import (
 	"fmt"
 	"math/big"
 	"testing"
-
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
 )
 
 func TestReturnInt64(t *testing.T) {
@@ -99,7 +96,11 @@ func TestSingleReturn(t *testing.T) {
 
 func TestNamedReturn(t *testing.T) {
 	src := `package foo
-	func Main() (a int, b int) {
+	func Main() int {
+		a, b := f()
+		return a + b
+	}
+	func f() (a int, b int) {
 		a = 1
 		b = 2
 		c := 3
@@ -107,21 +108,16 @@ func TestNamedReturn(t *testing.T) {
 		return %s
 	}`
 
-	runCase := func(ret string, result ...interface{}) func(t *testing.T) {
+	runCase := func(ret string, result *big.Int) func(t *testing.T) {
 		return func(t *testing.T) {
 			src := fmt.Sprintf(src, ret)
-			v := vmAndCompile(t, src)
+			eval(t, src, result)
-			require.NoError(t, v.Run())
-			require.Equal(t, len(result), v.Estack().Len())
-			for i := range result {
-				assert.EqualValues(t, result[i], v.Estack().Pop().Value())
-			}
 		}
 	}
 
-	t.Run("NormalReturn", runCase("a, b", big.NewInt(1), big.NewInt(2)))
+	t.Run("NormalReturn", runCase("a, b", big.NewInt(3)))
-	t.Run("EmptyReturn", runCase("", big.NewInt(1), big.NewInt(2)))
+	t.Run("EmptyReturn", runCase("", big.NewInt(3)))
-	t.Run("AnotherVariable", runCase("b, c", big.NewInt(2), big.NewInt(3)))
+	t.Run("AnotherVariable", runCase("b, c", big.NewInt(5)))
 }
 
 func TestTypeAssertReturn(t *testing.T) {