reflect: fix size of StructOf ending in zero-sized field

Update #9401.
Fixes #18016.

Change-Id: Icc24dd10dab1ad8e5cf295e0727d437afa5025c0
Reviewed-on: https://go-review.googlesource.com/33475
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/reflect/all_test.go

@@ -4059,6 +4059,38 @@ func TestStructOf(t *testing.T) {
 		}
 	}
 
+	// Check size and alignment with a trailing zero-sized field.
+	st = StructOf([]StructField{
+		{
+			Name: "F1",
+			Type: TypeOf(byte(0)),
+		},
+		{
+			Name: "F2",
+			Type: TypeOf([0]*byte{}),
+		},
+	})
+	stt = TypeOf(struct {
+		G1 byte
+		G2 [0]*byte
+	}{})
+	if st.Size() != stt.Size() {
+		t.Errorf("constructed zero-padded struct size = %v, want %v", st.Size(), stt.Size())
+	}
+	if st.Align() != stt.Align() {
+		t.Errorf("constructed zero-padded struct align = %v, want %v", st.Align(), stt.Align())
+	}
+	if st.FieldAlign() != stt.FieldAlign() {
+		t.Errorf("constructed zero-padded struct field align = %v, want %v", st.FieldAlign(), stt.FieldAlign())
+	}
+	for i := 0; i < st.NumField(); i++ {
+		o1 := st.Field(i).Offset
+		o2 := stt.Field(i).Offset
+		if o1 != o2 {
+			t.Errorf("constructed zero-padded struct field %v offset = %v, want %v", i, o1, o2)
+		}
+	}
+
 	// check duplicate names
 	shouldPanic(func() {
 		StructOf([]StructField{

src/reflect/type.go

@@ -2400,6 +2400,7 @@ func StructOf(fields []StructField) Type {
 		hasGCProg = false // records whether a struct-field type has a GCProg
 	)
 
+	lastzero := uintptr(0)
 	repr = append(repr, "struct {"...)
 	for i, field := range fields {
 		if field.Type == nil {
@@ -2570,9 +2571,22 @@ func StructOf(fields []StructField) Type {
 		}
 		size = f.offset + ft.size
 
+		if ft.size == 0 {
+			lastzero = size
+		}
+
 		fs[i] = f
 	}
 
+	if size > 0 && lastzero == size {
+		// This is a non-zero sized struct that ends in a
+		// zero-sized field. We add an extra byte of padding,
+		// to ensure that taking the address of the final
+		// zero-sized field can't manufacture a poitner to the
+		// next object in the heap. See issue 9401.
+		size++
+	}
+
 	var typ *structType
 	var ut *uncommonType
 	var typPin interface {