cc: Audit and static_assert the types in PaintOpBuffer can be moved.

cc/paint/paint_op_buffer.h

Index: cc/paint/paint_op_buffer.h
diff --git a/cc/paint/paint_op_buffer.h b/cc/paint/paint_op_buffer.h
index db9eaadcf22c8c8ffb89e4005e94ebb5f23d9718..51846d136b9ea856a6e432a6f501b47d2d805895 100644
--- a/cc/paint/paint_op_buffer.h
+++ b/cc/paint/paint_op_buffer.h
@@ -72,6 +72,91 @@ enum class PaintOpType : uint8_t {
   LastPaintOpType = Translate,
 };
 
+// It is a requirement for every field in a PaintOp that we can memcpy it to
+// another place in memory and free the only one (without calling any
+// constructors or destructors). Specializations of this template are for
+// types that we know to be safe for memcpy+free, with static asserts if
+// possible, and explanations if not.
+template <typename T>
+struct SafePaintOpData;

[vmpstr, 2017/06/01 22:00:55]

Don't need the forward declare anymore

+
+template <typename T>
+struct SafePaintOpData {
+  using type = T;
+
+  // TODO(danakj): This could use std::is_trivially_move_constructible once that
+  // is available on all our toolchains.
+  static_assert(base::is_trivially_copyable<type>::value,
+                "This type is not trivially copyable, which means it may not be"
+                " okay to change its memory address via memcpy+free. A template"
+                " specialization should be provided for this type that verifies"
+                " or explains why it is valid to memcpy+free this type.");
+};
+
+// sk_sp<T> pointers behave if their memory address is changed because there
+// is never a pointer back to the sk_sp<T> itself from inside the T. This is
+// true because multiple sk_sp<> may point to the same T and they are
+// transient compared to the lifetime of any T. Additionally, the move
+// constructor doesn't change any state that needs to be reflected in the new
+// object.
+template <typename U>
+struct SafePaintOpData<sk_sp<U>> {
+  using type = sk_sp<U>;
+};
+// scoped_refptr<> is able to be moved in memory for the same reasons as
+// sk_sp<>.
+template <typename U>
+struct SafePaintOpData<scoped_refptr<U>> {
+  using type = scoped_refptr<U>;
+};
+
+// PaintFlags is a collection of trivial data types and sk_sp<> ref pointers.
+// The trivial data types satisfy std::is_trivially_copyable, and sk_sp<> is
+// valid to have its memory address changed for reasons decribed on
+// SafePaintOpData<sk_sp>.
+template <>
+struct SafePaintOpData<PaintFlags> {
+  using type = PaintFlags;
+};
+
+// PaintImage is a set of trivial types and an sk_sp<> which is able to be
+// moved in memory as described on SafePaintOpData<sk_sp>. PaintImage does not
+// hold any pointers back to itself directly or indirectly.
+template <>
+struct SafePaintOpData<PaintImage> {
+  using type = PaintImage;
+};
+
+// ThreadsafePath is an SkPath which contains trivial types, sk_sp<>, and
+// SkAtomic<uint8_t>. sk_sp<> is valid as described on SafePaintOpData<sk_sp>.
+// SkAtomic<uint8_t> stores a trivial type internally, and takes its address
+// but does not store it.
+template <>
+struct SafePaintOpData<ThreadsafePath> {
+  using type = ThreadsafePath;
+};
+
+// WARNING: It is a requirement for every field in a PaintOp that we can memcpy
+// it to another place in memory and free the only one (without calling any
+// constructors or destructors). We think of this as moving the memory address
+// of the data type. This is required because PaintOpBuffer stores PaintOps in
+// a memory buffer that is realloced when it grows, changing the address of
+// everything inside the buffer.
+// std::is_trivially_copyable<> would guarantee this, but is a superset of the
+// guarantees that we need. For instance sk_sp<> works correctly when we
+// memcpy+free it, but it has a non-trivial move constructor and destructor,
+// which would make it fail a check for std::is_trivially_copyable<>.
+// std::is_trivially_move_constructible<> also provides enough as we are moving
+// the objects from one address to another conceptually, but also is not
+// satisfied by sk_sp<> since it modifies the moved-from type.
+//
+// THEREFORE: For each field in a PaintOp, declare it as a
+// SafePaintOpData<T>::type, which provides us with an explanation of safety for
+// each type.
+//
+// BEWARE: std::vector does *not* work if its memory address is changed, as
+// implementations can (and do) sometimes have a pointer back to the vector from
+// its internals, so running its move-assignment operator is required.
 struct CC_PAINT_EXPORT PaintOp {

[vmpstr, 2017/06/01 22:00:55]

maybe we can add something like

template <typename T>
using Member = SafePaintOpData<T>::type;

to PaintOp and then everything can just be 

Member<bool> flag; type of things? AFAIK that would still trigger the static
assert?

(this is a nit, I'm just trying to improve the ergonomics of using this a bit,
but maybe this is more explicit. not sure)

   uint32_t type : 8;
   uint32_t skip : 24;
@@ -126,9 +211,13 @@ struct CC_PAINT_EXPORT PaintOpWithFlags : PaintOp {
   // provide a modified PaintFlags. The RasterWithFlags() method is static with
   // a const PaintOpWithFlags* parameter so that it can be used as a function
   // pointer.
-  PaintFlags flags;
+  SafePaintOpData<PaintFlags>::type flags;
 };
 
+// A PaintOp with an additional block of memory with variable size. The
+// additional data held in the PaintOpWithData must be able to be moved in
+// memory without consequences. As such it must not (directly or indirectly)
+// contain any pointers back to itself or to the PaintOp.
 struct CC_PAINT_EXPORT PaintOpWithData : PaintOpWithFlags {
   // Having data is just a helper for ops that have a varying amount of data and
   // want a way to store that inline.  This is for ops that pass in a
@@ -139,7 +228,7 @@ struct CC_PAINT_EXPORT PaintOpWithData : PaintOpWithFlags {
 
   // Get data out by calling paint_op_data.  This can't be part of the class
   // because it needs to know the size of the derived type.
-  size_t bytes;
+  SafePaintOpData<size_t>::type bytes;
 
  protected:
   // For some derived object T, return the internally stored data.
@@ -170,8 +259,16 @@ struct CC_PAINT_EXPORT PaintOpWithArrayBase : PaintOpWithFlags {
       : PaintOpWithFlags(flags) {}
 };
 
+// A PaintOp with an additional block of memory with variable size that is
+// an array of a known type. The types held in the array must be able to be
+// moved in memory without consequences. As such it must not (directly or
+// indirectly) contain any pointers back to itself or to the PaintOp. We use
+// std::is_trivially_copyable() to verify this (it is more strict but satisfies
+// our needs).
 template <typename M>
 struct CC_PAINT_EXPORT PaintOpWithArray : PaintOpWithArrayBase {
+  static_assert(base::is_trivially_copyable<M>::value, "");
+
   // Paint op that has a M[count] and a char[bytes].
   // Array data is stored first so that it can be aligned with T's alignment
   // with the arbitrary unaligned char data after it.
@@ -180,8 +277,8 @@ struct CC_PAINT_EXPORT PaintOpWithArray : PaintOpWithArrayBase {
   PaintOpWithArray(const PaintFlags& flags, size_t bytes, size_t count)
       : PaintOpWithArrayBase(flags), bytes(bytes), count(count) {}
 
-  size_t bytes;
-  size_t count;
+  SafePaintOpData<size_t>::type bytes;
+  SafePaintOpData<size_t>::type count;
 
  protected:
   template <typename T>
@@ -241,9 +338,9 @@ struct CC_PAINT_EXPORT AnnotateOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  PaintCanvas::AnnotationType annotation_type;
-  SkRect rect;
-  sk_sp<SkData> data;
+  SafePaintOpData<PaintCanvas::AnnotationType>::type annotation_type;
+  SafePaintOpData<SkRect>::type rect;
+  SafePaintOpData<sk_sp<SkData>>::type data;
 };
 
 struct CC_PAINT_EXPORT ClipPathOp final : PaintOp {
@@ -255,9 +352,9 @@ struct CC_PAINT_EXPORT ClipPathOp final : PaintOp {
                      const SkMatrix& original_ctm);
   int CountSlowPaths() const;
 
-  ThreadsafePath path;
-  SkClipOp op;
-  bool antialias;
+  SafePaintOpData<ThreadsafePath>::type path;
+  SafePaintOpData<SkClipOp>::type op;
+  SafePaintOpData<bool>::type antialias;
 };
 
 struct CC_PAINT_EXPORT ClipRectOp final : PaintOp {
@@ -268,9 +365,9 @@ struct CC_PAINT_EXPORT ClipRectOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkRect rect;
-  SkClipOp op;
-  bool antialias;
+  SafePaintOpData<SkRect>::type rect;
+  SafePaintOpData<SkClipOp>::type op;
+  SafePaintOpData<bool>::type antialias;
 };
 
 struct CC_PAINT_EXPORT ClipRRectOp final : PaintOp {
@@ -281,9 +378,9 @@ struct CC_PAINT_EXPORT ClipRRectOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkRRect rrect;
-  SkClipOp op;
-  bool antialias;
+  SafePaintOpData<SkRRect>::type rrect;
+  SafePaintOpData<SkClipOp>::type op;
+  SafePaintOpData<bool>::type antialias;
 };
 
 struct CC_PAINT_EXPORT ConcatOp final : PaintOp {
@@ -293,7 +390,7 @@ struct CC_PAINT_EXPORT ConcatOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  ThreadsafeMatrix matrix;
+  SafePaintOpData<ThreadsafeMatrix>::type matrix;
 };
 
 struct CC_PAINT_EXPORT DrawArcOp final : PaintOpWithFlags {
@@ -320,10 +417,10 @@ struct CC_PAINT_EXPORT DrawArcOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkRect oval;
-  SkScalar start_angle;
-  SkScalar sweep_angle;
-  bool use_center;
+  SafePaintOpData<SkRect>::type oval;
+  SafePaintOpData<SkScalar>::type start_angle;
+  SafePaintOpData<SkScalar>::type sweep_angle;
+  SafePaintOpData<bool>::type use_center;
 };
 
 struct CC_PAINT_EXPORT DrawCircleOp final : PaintOpWithFlags {
@@ -345,9 +442,9 @@ struct CC_PAINT_EXPORT DrawCircleOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkScalar cx;
-  SkScalar cy;
-  SkScalar radius;
+  SafePaintOpData<SkScalar>::type cx;
+  SafePaintOpData<SkScalar>::type cy;
+  SafePaintOpData<SkScalar>::type radius;
 };
 
 struct CC_PAINT_EXPORT DrawColorOp final : PaintOp {
@@ -358,8 +455,8 @@ struct CC_PAINT_EXPORT DrawColorOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkColor color;
-  SkBlendMode mode;
+  SafePaintOpData<SkColor>::type color;
+  SafePaintOpData<SkBlendMode>::type mode;
 };
 
 struct CC_PAINT_EXPORT DrawDisplayItemListOp final : PaintOp {
@@ -379,7 +476,7 @@ struct CC_PAINT_EXPORT DrawDisplayItemListOp final : PaintOp {
   bool HasDiscardableImages() const;
   int CountSlowPaths() const;
 
-  scoped_refptr<DisplayItemList> list;
+  SafePaintOpData<scoped_refptr<DisplayItemList>>::type list;
 };
 
 struct CC_PAINT_EXPORT DrawDRRectOp final : PaintOpWithFlags {
@@ -400,8 +497,8 @@ struct CC_PAINT_EXPORT DrawDRRectOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkRRect outer;
-  SkRRect inner;
+  SafePaintOpData<SkRRect>::type outer;
+  SafePaintOpData<SkRRect>::type inner;
 };
 
 struct CC_PAINT_EXPORT DrawImageOp final : PaintOpWithFlags {
@@ -424,9 +521,9 @@ struct CC_PAINT_EXPORT DrawImageOp final : PaintOpWithFlags {
                               const SkMatrix& original_ctm);
   bool HasDiscardableImages() const;
 
-  PaintImage image;
-  SkScalar left;
-  SkScalar top;
+  SafePaintOpData<PaintImage>::type image;
+  SafePaintOpData<SkScalar>::type left;
+  SafePaintOpData<SkScalar>::type top;
 };
 
 struct CC_PAINT_EXPORT DrawImageRectOp final : PaintOpWithFlags {
@@ -450,10 +547,10 @@ struct CC_PAINT_EXPORT DrawImageRectOp final : PaintOpWithFlags {
                               const SkMatrix& original_ctm);
   bool HasDiscardableImages() const;
 
-  PaintImage image;
-  SkRect src;
-  SkRect dst;
-  PaintCanvas::SrcRectConstraint constraint;
+  SafePaintOpData<PaintImage>::type image;
+  SafePaintOpData<SkRect>::type src;
+  SafePaintOpData<SkRect>::type dst;
+  SafePaintOpData<PaintCanvas::SrcRectConstraint>::type constraint;
 };
 
 struct CC_PAINT_EXPORT DrawIRectOp final : PaintOpWithFlags {
@@ -472,7 +569,7 @@ struct CC_PAINT_EXPORT DrawIRectOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkIRect rect;
+  SafePaintOpData<SkIRect>::type rect;
 };
 
 struct CC_PAINT_EXPORT DrawLineOp final : PaintOpWithFlags {
@@ -497,10 +594,10 @@ struct CC_PAINT_EXPORT DrawLineOp final : PaintOpWithFlags {
 
   int CountSlowPaths() const;
 
-  SkScalar x0;
-  SkScalar y0;
-  SkScalar x1;
-  SkScalar y1;
+  SafePaintOpData<SkScalar>::type x0;
+  SafePaintOpData<SkScalar>::type y0;
+  SafePaintOpData<SkScalar>::type x1;
+  SafePaintOpData<SkScalar>::type y1;
 };
 
 struct CC_PAINT_EXPORT DrawOvalOp final : PaintOpWithFlags {
@@ -519,7 +616,7 @@ struct CC_PAINT_EXPORT DrawOvalOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkRect oval;
+  SafePaintOpData<SkRect>::type oval;
 };
 
 struct CC_PAINT_EXPORT DrawPathOp final : PaintOpWithFlags {
@@ -539,7 +636,7 @@ struct CC_PAINT_EXPORT DrawPathOp final : PaintOpWithFlags {
                               const SkMatrix& original_ctm);
   int CountSlowPaths() const;
 
-  ThreadsafePath path;
+  SafePaintOpData<ThreadsafePath>::type path;
 };
 
 struct CC_PAINT_EXPORT DrawPosTextOp final : PaintOpWithArray<SkPoint> {
@@ -576,7 +673,7 @@ struct CC_PAINT_EXPORT DrawRecordOp final : PaintOp {
   bool HasDiscardableImages() const;
   int CountSlowPaths() const;
 
-  sk_sp<const PaintRecord> record;
+  SafePaintOpData<sk_sp<const PaintRecord>>::type record;
 };
 
 struct CC_PAINT_EXPORT DrawRectOp final : PaintOpWithFlags {
@@ -595,7 +692,7 @@ struct CC_PAINT_EXPORT DrawRectOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkRect rect;
+  SafePaintOpData<SkRect>::type rect;
 };
 
 struct CC_PAINT_EXPORT DrawRRectOp final : PaintOpWithFlags {
@@ -614,7 +711,7 @@ struct CC_PAINT_EXPORT DrawRRectOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkRRect rrect;
+  SafePaintOpData<SkRRect>::type rrect;
 };
 
 struct CC_PAINT_EXPORT DrawTextOp final : PaintOpWithData {
@@ -636,8 +733,8 @@ struct CC_PAINT_EXPORT DrawTextOp final : PaintOpWithData {
   void* GetData() { return GetDataForThis(this); }
   const void* GetData() const { return GetDataForThis(this); }
 
-  SkScalar x;
-  SkScalar y;
+  SafePaintOpData<SkScalar>::type x;
+  SafePaintOpData<SkScalar>::type y;
 };
 
 struct CC_PAINT_EXPORT DrawTextBlobOp final : PaintOpWithFlags {
@@ -659,9 +756,9 @@ struct CC_PAINT_EXPORT DrawTextBlobOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  sk_sp<SkTextBlob> blob;
-  SkScalar x;
-  SkScalar y;
+  SafePaintOpData<sk_sp<SkTextBlob>>::type blob;
+  SafePaintOpData<SkScalar>::type x;
+  SafePaintOpData<SkScalar>::type y;
 };
 
 struct CC_PAINT_EXPORT NoopOp final : PaintOp {
@@ -685,7 +782,7 @@ struct CC_PAINT_EXPORT RotateOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkScalar degrees;
+  SafePaintOpData<SkScalar>::type degrees;
 };
 
 struct CC_PAINT_EXPORT SaveOp final : PaintOp {
@@ -711,7 +808,7 @@ struct CC_PAINT_EXPORT SaveLayerOp final : PaintOpWithFlags {
                               SkCanvas* canvas,
                               const SkMatrix& original_ctm);
 
-  SkRect bounds;
+  SafePaintOpData<SkRect>::type bounds;
 };
 
 struct CC_PAINT_EXPORT SaveLayerAlphaOp final : PaintOp {
@@ -722,8 +819,8 @@ struct CC_PAINT_EXPORT SaveLayerAlphaOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkRect bounds;
-  uint8_t alpha;
+  SafePaintOpData<SkRect>::type bounds;
+  SafePaintOpData<uint8_t>::type alpha;
 };
 
 struct CC_PAINT_EXPORT ScaleOp final : PaintOp {
@@ -733,8 +830,8 @@ struct CC_PAINT_EXPORT ScaleOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkScalar sx;
-  SkScalar sy;
+  SafePaintOpData<SkScalar>::type sx;
+  SafePaintOpData<SkScalar>::type sy;
 };
 
 struct CC_PAINT_EXPORT SetMatrixOp final : PaintOp {
@@ -750,7 +847,7 @@ struct CC_PAINT_EXPORT SetMatrixOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  ThreadsafeMatrix matrix;
+  SafePaintOpData<ThreadsafeMatrix>::type matrix;
 };
 
 struct CC_PAINT_EXPORT TranslateOp final : PaintOp {
@@ -760,8 +857,8 @@ struct CC_PAINT_EXPORT TranslateOp final : PaintOp {
                      SkCanvas* canvas,
                      const SkMatrix& original_ctm);
 
-  SkScalar dx;
-  SkScalar dy;
+  SafePaintOpData<SkScalar>::type dx;
+  SafePaintOpData<SkScalar>::type dy;
 };
 
 using LargestPaintOp = DrawDRRectOp;