chromium/src/third_party/blink/renderer/platform/graphics/identifiability_paint_op_digest.h

// Copyright 2020 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef THIRD_PARTY_BLINK_RENDERER_PLATFORM_GRAPHICS_IDENTIFIABILITY_PAINT_OP_DIGEST_H_
#define THIRD_PARTY_BLINK_RENDERER_PLATFORM_GRAPHICS_IDENTIFIABILITY_PAINT_OP_DIGEST_H_

#include <memory>

#include "cc/paint/draw_image.h"
#include "cc/paint/image_provider.h"
#include "cc/paint/paint_cache.h"
#include "third_party/blink/public/common/privacy_budget/identifiable_token_builder.h"
#include "third_party/blink/renderer/platform/geometry/int_size.h"
#include "third_party/blink/renderer/platform/platform_export.h"
#include "third_party/skia/include/utils/SkNoDrawCanvas.h"

namespace blink {

// Manages digest calculation of operations performed on an HTML canvas using
// the serialization of PaintOps generated by those operations.
class PLATFORM_EXPORT IdentifiabilityPaintOpDigest {
 public:
  // Constructs based on the size of the CanvasResourceProvider.
  explicit IdentifiabilityPaintOpDigest(IntSize size);
  // For-testing constructor that allows setting a custom |max_digest_ops_|.
  IdentifiabilityPaintOpDigest(IntSize size, int max_digest_ops);
  ~IdentifiabilityPaintOpDigest();

  // When passed as |num_ops_to_visit| to MaybeUpdateDigest(), every
  // non-skipped PaintOp in the buffer contributes to digest calculation.
  static constexpr size_t kInfiniteOps = -1;

  // Maybe update the digest, if the user is participating in the study, and we
  // haven't exceeded the operation count.
  //
  // Only processes |num_ops_to_visit| PaintOps, which includes the first
  // |prefix_skip_count| PaintOps that are skipped. The prefix and suffixes that
  // aren't processed are internal rendering details, and don't correspond to
  // operations performed on the canvas context.
  void MaybeUpdateDigest(const sk_sp<const cc::PaintRecord>& paint_record,
                         size_t num_ops_to_visit);

  // Sets the number of operations to skip in the next PaintRecord passed to
  // MaybeUpdateDigest().
  void SetPrefixSkipCount(size_t prefix_skip_count) {
    prefix_skip_count_ = prefix_skip_count;
  }

  // The IdentifiabilityToken (digest), based on the PaintOps observed.
  IdentifiableToken GetToken() const { return builder_.GetToken(); }

  bool encountered_partially_digested_image() const {
    return encountered_partially_digested_image_;
  }

  bool encountered_skipped_ops() const { return encountered_skipped_ops_; }

 private:
  class IdentifiabilityImageProvider : public cc::ImageProvider {
   public:
    explicit IdentifiabilityImageProvider(IdentifiabilityPaintOpDigest* outer)
        : outer_(outer) {}

    ScopedResult GetRasterContent(const cc::DrawImage& draw_image) override;

   private:
    IdentifiabilityPaintOpDigest* const outer_;
  };

  // The maximum number of ops to digest during the lifetime of this
  // IdentifiabilityPaintOpDigest object.
  const int max_digest_ops_;

  // Potentially updated every MaybeUpdateDigest() call.
  IdentifiableTokenBuilder builder_;

  // The number of PaintOps that have contributed to the current digest -- used
  // to stop updating the digest after a threshold number of operations to avoid
  // hurting performance.
  int total_ops_digested_ = 0;

  // How many PaintOps to skip, as set by SetPrefixSkipCount().
  size_t prefix_skip_count_ = 0;

  // Resources needed for PaintOp serialization.

  // Size of the corresponding CanvasResourceProvider.
  IntSize size_;

  // Fake identifiability image provider; can be used to compute image digests.
  IdentifiabilityImageProvider image_provider_{this};

  // Real paint cache with Put() disabled.
  cc::ClientPaintCache paint_cache_;

  // Fake canvas needed for null checks.
  SkNoDrawCanvas nodraw_canvas_;

  // Used for PaintOp::Serialize() -- several options are not needed, since we
  // just need to compute a digest.
  cc::PaintOp::SerializeOptions serialize_options_;

  // If true, at least one DrawImage[Rect]Op was partially digested -- the whole
  // image wasn't digested for performance reasons.
  bool encountered_partially_digested_image_ = false;

  // If true, at least one op was skipped completely, for performance reasons.
  //
  // DrawTextBlobOps don't flip this flag to true, as they are handled
  // separately in the 2D canavs context object (to capture font fallback
  // fingerprinting). Also, prefix and suffix ops skipped don't flip this flag
  // to true.
  bool encountered_skipped_ops_ = false;
};

}  // namespace blink

#endif  // THIRD_PARTY_BLINK_RENDERER_PLATFORM_GRAPHICS_IDENTIFIABILITY_PAINT_OP_DIGEST_H_