content/renderer/accessibility/ax_tree_snapshotter_impl.cc

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

#include "content/renderer/accessibility/ax_tree_snapshotter_impl.h"

#include "base/metrics/histogram_functions.h"
#include "content/renderer/render_frame_impl.h"
#include "third_party/blink/public/web/web_ax_context.h"
#include "third_party/blink/public/web/web_ax_object.h"
#include "third_party/blink/public/web/web_document.h"
#include "third_party/blink/public/web/web_frame.h"
#include "ui/accessibility/ax_error_types.h"
#include "ui/accessibility/ax_tree_serializer.h"
#include "ui/accessibility/ax_tree_update.h"

using blink::WebAXContext;
using blink::WebAXObject;
using blink::WebDocument;

namespace content {

constexpr char kAXTreeSnapshotterErrorHistogramName[] =
    "Accessibility.AXTreeSnapshotter.Snapshot.Error";

// These values are persisted to logs. Entries should not be renumbered and
// numeric values should never be reused.
enum class AXTreeSnapshotErrorReason {
  kGenericSerializationError = 0,
  kNoWebFrame = 1,
  kNoActiveDocument = 2,
  kNoExistingAXObjectCache = 3,
  kSerializeMaxNodesReached = 4,
  kSerializeTimeoutReached = 5,
  kSerializeMaxNodesAndTimeoutReached = 6,
  kMaxValue = kSerializeMaxNodesAndTimeoutReached,
};

AXTreeSnapshotterImpl::AXTreeSnapshotterImpl(RenderFrameImpl* render_frame,
                                             ui::AXMode ax_mode)
    : render_frame_(render_frame) {
  // Do not generate inline textboxes, which are expensive to create and just
  // present extra noise to snapshot consumers.
  ax_mode.set_mode(ui::AXMode::kInlineTextBoxes, false);

  DCHECK(render_frame->GetWebFrame());
  blink::WebDocument document_ = render_frame->GetWebFrame()->GetDocument();
  context_ = std::make_unique<WebAXContext>(document_, ax_mode);
}

AXTreeSnapshotterImpl::~AXTreeSnapshotterImpl() = default;

void AXTreeSnapshotterImpl::Snapshot(size_t max_node_count,
                                     base::TimeDelta timeout,
                                     ui::AXTreeUpdate* response) {
  base::UmaHistogramBoolean("Accessibility.AXTreeSnapshotter.Snapshot.Request",
                            true);

  if (!render_frame_->GetWebFrame()) {
    base::UmaHistogramEnumeration(kAXTreeSnapshotterErrorHistogramName,
                                  AXTreeSnapshotErrorReason::kNoWebFrame);
    return;
  }

  if (!context_->HasActiveDocument()) {
    base::UmaHistogramEnumeration(kAXTreeSnapshotterErrorHistogramName,
                                  AXTreeSnapshotErrorReason::kNoActiveDocument);
    return;
  }

  if (!context_->HasAXObjectCache()) {
    base::UmaHistogramEnumeration(
        kAXTreeSnapshotterErrorHistogramName,
        AXTreeSnapshotErrorReason::kNoExistingAXObjectCache);
    // TODO(chrishtr): not clear why this can happen.
    NOTREACHED_NORETURN();
  }

  std::set<ui::AXSerializationErrorFlag> out_error;
  if (context_->SerializeEntireTree(max_node_count, timeout, response,
                                    &out_error)) {
    std::set<ui::AXSerializationErrorFlag>::iterator max_nodes_iter =
        out_error.find(ui::AXSerializationErrorFlag::kMaxNodesReached);
    std::set<ui::AXSerializationErrorFlag>::iterator timeout_iter =
        out_error.find(ui::AXSerializationErrorFlag::kTimeoutReached);

    if (max_nodes_iter != out_error.end() && timeout_iter != out_error.end()) {
      base::UmaHistogramEnumeration(
          kAXTreeSnapshotterErrorHistogramName,
          AXTreeSnapshotErrorReason::kSerializeMaxNodesAndTimeoutReached);
    } else if (max_nodes_iter != out_error.end()) {
      base::UmaHistogramEnumeration(
          kAXTreeSnapshotterErrorHistogramName,
          AXTreeSnapshotErrorReason::kSerializeMaxNodesReached);
    } else if (timeout_iter != out_error.end()) {
      base::UmaHistogramEnumeration(
          kAXTreeSnapshotterErrorHistogramName,
          AXTreeSnapshotErrorReason::kSerializeTimeoutReached);
    }

    return;
  }

  base::UmaHistogramEnumeration(
      kAXTreeSnapshotterErrorHistogramName,
      AXTreeSnapshotErrorReason::kGenericSerializationError);
  // It failed again. Clear the response object because it might have errors.
  *response = ui::AXTreeUpdate();
  LOG(WARNING) << "Unable to serialize accessibility tree.";

  // As a confidence check, node_id_to_clear and event_from should be
  // uninitialized if this is a full tree snapshot. They'd only be set to
  // something if this was indeed a partial update to the tree (which we don't
  // want).
  DCHECK_EQ(0, response->node_id_to_clear);
  DCHECK_EQ(ax::mojom::EventFrom::kNone, response->event_from);
  DCHECK_EQ(ax::mojom::Action::kNone, response->event_from_action);
}

}  // namespace content