1 files changed, 53 insertions, 4 deletions
diff --git a/pkg/state/decode_unsafe.go b/pkg/state/decode_unsafe.go
index d048f61a1..f1208e2a2 100644
--- a/pkg/state/decode_unsafe.go
+++ b/pkg/state/decode_unsafe.go
@@ -15,13 +15,62 @@
 package state
 
 import (
+	"fmt"
 	"reflect"
+	"runtime"
 	"unsafe"
 )
 
-// unsafePointerTo is logically equivalent to reflect.Value.Addr, but works on
-// values representing unexported fields. This bypasses visibility, but not
-// type safety.
-func unsafePointerTo(obj reflect.Value) reflect.Value {
+// reflectValueRWAddr is equivalent to obj.Addr(), except that the returned
+// reflect.Value is usable in assignments even if obj was obtained by the use
+// of unexported struct fields.
+//
+// Preconditions: obj.CanAddr().
+func reflectValueRWAddr(obj reflect.Value) reflect.Value {
 	return reflect.NewAt(obj.Type(), unsafe.Pointer(obj.UnsafeAddr()))
 }
+
+// reflectValueRWSlice3 is equivalent to arr.Slice3(i, j, k), except that the
+// returned reflect.Value is usable in assignments even if obj was obtained by
+// the use of unexported struct fields.
+//
+// Preconditions:
+// * arr.Kind() == reflect.Array.
+// * i, j, k >= 0.
+// * i <= j <= k <= arr.Len().
+func reflectValueRWSlice3(arr reflect.Value, i, j, k int) reflect.Value {
+	if arr.Kind() != reflect.Array {
+		panic(fmt.Sprintf("arr has kind %v, wanted %v", arr.Kind(), reflect.Array))
+	}
+	if i < 0 || j < 0 || k < 0 {
+		panic(fmt.Sprintf("negative subscripts (%d, %d, %d)", i, j, k))
+	}
+	if i > j {
+		panic(fmt.Sprintf("subscript i (%d) > j (%d)", i, j))
+	}
+	if j > k {
+		panic(fmt.Sprintf("subscript j (%d) > k (%d)", j, k))
+	}
+	if k > arr.Len() {
+		panic(fmt.Sprintf("subscript k (%d) > array length (%d)", k, arr.Len()))
+	}
+
+	sliceTyp := reflect.SliceOf(arr.Type().Elem())
+	if i == arr.Len() {
+		// By precondition, i == j == k == arr.Len().
+		return reflect.MakeSlice(sliceTyp, 0, 0)
+	}
+	slh := reflect.SliceHeader{
+		// reflect.Value.CanAddr() == false for arrays, so we need to get the
+		// address from the first element of the array.
+		Data: arr.Index(i).UnsafeAddr(),
+		Len:  j - i,
+		Cap:  k - i,
+	}
+	slobj := reflect.NewAt(sliceTyp, unsafe.Pointer(&slh)).Elem()
+	// Before slobj is constructed, arr holds the only pointer-typed pointer to
+	// the array since reflect.SliceHeader.Data is a uintptr, so arr must be
+	// kept alive.
+	runtime.KeepAlive(arr)
+	return slobj
+}