Decoder: prevent modification of inline tables (#702)

Fixes #701

internal/tracker/seen.go

@@ -145,6 +145,11 @@ func (s *SeenTracker) checkTable(node *ast.Node) error {
 
 		if idx < 0 {
 			idx = s.create(parentIdx, k, tableKind, false)
+		} else {
+			entry := s.entries[idx]
+			if entry.kind == valueKind {
+				return fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
+			}
 		}
 		parentIdx = idx
 	}
@@ -186,7 +191,13 @@ func (s *SeenTracker) checkArrayTable(node *ast.Node) error {
 
 		if idx < 0 {
 			idx = s.create(parentIdx, k, tableKind, false)
+		} else {
+			entry := s.entries[idx]
+			if entry.kind == valueKind {
+				return fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), entry.kind)
+			}
 		}
+
 		parentIdx = idx
 	}
 
@@ -281,6 +292,14 @@ func (s *SeenTracker) checkInlineTable(parentIdx int, node *ast.Node) error {
 			return err
 		}
 	}
+
+	// As inline tables are self-contained, the tracker does not
+	// need to retain the details of what they contain. The
+	// keyValue element that creates the inline table is kept to
+	// mark the presence of the inline table and prevent
+	// redefinition of its keys: check* functions cannot walk into
+	// a value.
+	s.clear(parentIdx)
 	return nil
 }
 

unmarshaler_test.go

@@ -2270,7 +2270,34 @@ Program = "hugo"
 	require.Equal(t, expected, p)
 }
 
-//nolint:funlen
+func TestIssue701(t *testing.T) {
+	// Expected behavior:
+	// Return an error since a cannot be modified. From the TOML spec:
+	//
+	// > Inline tables are fully self-contained and define all
+	// keys and sub-tables within them. Keys and sub-tables cannot
+	// be added outside the braces.
+
+	docs := []string{
+		`
+a={}
+[a.b]
+z=0
+`,
+		`
+a={}
+[[a.b]]
+z=0
+`,
+	}
+
+	for _, doc := range docs {
+		var v interface{}
+		err := toml.Unmarshal([]byte(doc), &v)
+		assert.Error(t, err)
+	}
+}
+
 func TestUnmarshalDecodeErrors(t *testing.T) {
 	examples := []struct {
 		desc string