compiler: check for pkg nilness, fix #3202

Unfortunately, when import cycle happens somewhere deep in the import chain we
dont't get an error from packages.Load(). But it leaves some imports
uninitialized, so at least we can check for them.

Signed-off-by: Roman Khimov <roman@nspcc.ru>

pkg/compiler/analysis.go

@@ -282,7 +282,14 @@ func (c *codegen) visitPkg(pkg *packages.Package, seen map[string]bool) {
 		return
 	}
 	for _, imp := range pkg.Types.Imports() {
-		c.visitPkg(pkg.Imports[imp.Path()], seen)
+		var subpkg = pkg.Imports[imp.Path()]
+		if subpkg == nil {
+			if c.prog.Err == nil {
+				c.prog.Err = fmt.Errorf("failed to load %q package from %q, import cycle?", imp.Path(), pkg.PkgPath)
+			}
+			return
+		}
+		c.visitPkg(subpkg, seen)
 	}
 	seen[pkg.PkgPath] = true
 	c.packages = append(c.packages, pkg.PkgPath)

pkg/compiler/codegen.go

@@ -2209,6 +2209,9 @@ func (c *codegen) newLambda(u uint16, lit *ast.FuncLit) {
 func (c *codegen) compile(info *buildInfo, pkg *packages.Package) error {
 	c.mainPkg = pkg
 	c.analyzePkgOrder()
+	if c.prog.Err != nil {
+		return c.prog.Err
+	}
 	c.fillDocumentInfo()
 	funUsage := c.analyzeFuncAndGlobalVarUsage()
 	if c.prog.Err != nil {

pkg/compiler/import_test.go

@@ -2,7 +2,11 @@ package compiler_test
 
 import (
 	"math/big"
+	"strings"
 	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/compiler"
+	"github.com/stretchr/testify/require"
 )
 
 func TestImportFunction(t *testing.T) {
@@ -61,3 +65,27 @@ func TestImportNameSameAsOwn(t *testing.T) {
 	}`
 	eval(t, src, big.NewInt(3))
 }
+
+func TestImportCycleDirect(t *testing.T) {
+	src := `
+		package some
+		import "github.com/nspcc-dev/neo-go/pkg/compiler/testdata/importcycle/pkg2"
+		func Main() int {
+			return pkg2.A
+		}
+	`
+	_, _, err := compiler.CompileWithOptions("some.go", strings.NewReader(src), nil)
+	require.Error(t, err)
+}
+
+func TestImportCycleIndirect(t *testing.T) {
+	src := `
+		package some
+		import "github.com/nspcc-dev/neo-go/pkg/compiler/testdata/importcycle/pkg1"
+		func Main() int {
+			return pkg1.A
+		}
+	`
+	_, _, err := compiler.CompileWithOptions("some.go", strings.NewReader(src), nil)
+	require.Error(t, err)
+}

pkg/compiler/testdata/importcycle/pkg1/pkg1.go

@@ -0,0 +1,9 @@
+package pkg1
+
+import "github.com/nspcc-dev/neo-go/pkg/compiler/testdata/importcycle/pkg2"
+
+var A int
+
+func init() {
+	pkg2.A = 1
+}

pkg/compiler/testdata/importcycle/pkg2/pkg2.go

@@ -0,0 +1,11 @@
+package pkg2
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/compiler/testdata/importcycle/pkg3"
+)
+
+var A int
+
+func init() {
+	pkg3.A = 2
+}

pkg/compiler/testdata/importcycle/pkg3/pkg3.go

@@ -0,0 +1,11 @@
+package pkg3
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/compiler/testdata/importcycle/pkg2"
+)
+
+var A int
+
+func init() {
+	pkg2.A = 1
+}