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diff --git a/layout/base/FrameProperties.h b/layout/base/FrameProperties.h
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+++ b/layout/base/FrameProperties.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef FRAMEPROPERTIES_H_
+#define FRAMEPROPERTIES_H_
+
+#include "mozilla/DebugOnly.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Unused.h"
+#include "nsTArray.h"
+#include "nsThreadUtils.h"
+
+class nsIFrame;
+
+namespace mozilla {
+
+struct FramePropertyDescriptorUntyped {
+  /**
+   * mDestructor will be called if it's non-null.
+   */
+  typedef void UntypedDestructor(void* aPropertyValue);
+  UntypedDestructor* mDestructor;
+  /**
+   * mDestructorWithFrame will be called if it's non-null and mDestructor
+   * is null. WARNING: The frame passed to mDestructorWithFrame may
+   * be a dangling frame pointer, if this is being called during
+   * presshell teardown. Do not use it except to compare against
+   * other frame pointers. No frame will have been allocated with
+   * the same address yet.
+   */
+  typedef void UntypedDestructorWithFrame(const nsIFrame* aFrame,
+                                          void* aPropertyValue);
+  UntypedDestructorWithFrame* mDestructorWithFrame;
+  /**
+   * mDestructor and mDestructorWithFrame may both be null, in which case
+   * no value destruction is a no-op.
+   */
+
+ protected:
+  /**
+   * At most one destructor should be passed in. In general, you should
+   * just use the static function FramePropertyDescriptor::New* below
+   * instead of using this constructor directly.
+   */
+  constexpr FramePropertyDescriptorUntyped(
+      UntypedDestructor* aDtor, UntypedDestructorWithFrame* aDtorWithFrame)
+      : mDestructor(aDtor), mDestructorWithFrame(aDtorWithFrame) {}
+};
+
+/**
+ * A pointer to a FramePropertyDescriptor serves as a unique property ID.
+ * The FramePropertyDescriptor stores metadata about the property.
+ * Currently the only metadata is a destructor function. The destructor
+ * function is called on property values when they are overwritten or
+ * deleted.
+ *
+ * To use this class, declare a global (i.e., file, class or function-scope
+ * static member) FramePropertyDescriptor and pass its address as
+ * aProperty in the FrameProperties methods.
+ */
+template <typename T>
+struct FramePropertyDescriptor : public FramePropertyDescriptorUntyped {
+  typedef void Destructor(T* aPropertyValue);
+  typedef void DestructorWithFrame(const nsIFrame* aaFrame, T* aPropertyValue);
+
+  template <Destructor Dtor>
+  static constexpr const FramePropertyDescriptor<T> NewWithDestructor() {
+    return {Destruct<Dtor>, nullptr};
+  }
+
+  template <DestructorWithFrame Dtor>
+  static constexpr const FramePropertyDescriptor<T>
+  NewWithDestructorWithFrame() {
+    return {nullptr, DestructWithFrame<Dtor>};
+  }
+
+  static constexpr const FramePropertyDescriptor<T> NewWithoutDestructor() {
+    return {nullptr, nullptr};
+  }
+
+ private:
+  constexpr FramePropertyDescriptor(UntypedDestructor* aDtor,
+                                    UntypedDestructorWithFrame* aDtorWithFrame)
+      : FramePropertyDescriptorUntyped(aDtor, aDtorWithFrame) {}
+
+  template <Destructor Dtor>
+  static void Destruct(void* aPropertyValue) {
+    Dtor(static_cast<T*>(aPropertyValue));
+  }
+
+  template <DestructorWithFrame Dtor>
+  static void DestructWithFrame(const nsIFrame* aFrame, void* aPropertyValue) {
+    Dtor(aFrame, static_cast<T*>(aPropertyValue));
+  }
+};
+
+// SmallValueHolder<T> is a placeholder intended to be used as template
+// argument of FramePropertyDescriptor for types which can fit into the
+// size of a pointer directly. This class should never be defined, so
+// that we won't use it for unexpected purpose by mistake.
+template <typename T>
+class SmallValueHolder;
+
+namespace detail {
+
+template <typename T>
+struct FramePropertyTypeHelper {
+  typedef T* Type;
+};
+template <typename T>
+struct FramePropertyTypeHelper<SmallValueHolder<T>> {
+  typedef T Type;
+};
+
+}  // namespace detail
+
+/**
+ * The FrameProperties class is optimized for storing 0 or 1 properties on
+ * a given frame. Storing very large numbers of properties on a single
+ * frame will not be efficient.
+ *
+ * Property values are passed as void* but do not actually have to be
+ * valid pointers. You can use NS_INT32_TO_PTR/NS_PTR_TO_INT32 to
+ * store int32_t values. Null/zero values can be stored and retrieved.
+ * Of course, the destructor function (if any) must handle such values
+ * correctly.
+ */
+class FrameProperties {
+ public:
+  template <typename T>
+  using Descriptor = const FramePropertyDescriptor<T>*;
+  using UntypedDescriptor = const FramePropertyDescriptorUntyped*;
+
+  template <typename T>
+  using PropertyType = typename detail::FramePropertyTypeHelper<T>::Type;
+
+  explicit FrameProperties() = default;
+
+  ~FrameProperties() {
+    MOZ_ASSERT(mProperties.Length() == 0, "forgot to delete properties");
+  }
+
+  /**
+   * Return true if we have no properties, otherwise return false.
+   */
+  bool IsEmpty() const { return mProperties.IsEmpty(); }
+
+  /**
+   * Set a property value. This requires a linear search through
+   * the properties of the frame. Any existing value for the property
+   * is destroyed.
+   */
+  template <typename T>
+  void Set(Descriptor<T> aProperty, PropertyType<T> aValue,
+           const nsIFrame* aFrame) {
+    void* ptr = ReinterpretHelper<T>::ToPointer(aValue);
+    SetInternal(aProperty, ptr, aFrame);
+  }
+
+  /**
+   * Add a property value; the descriptor MUST NOT already be present.
+   */
+  template <typename T>
+  void Add(Descriptor<T> aProperty, PropertyType<T> aValue) {
+    MOZ_ASSERT(!Has(aProperty), "duplicate frame property");
+    void* ptr = ReinterpretHelper<T>::ToPointer(aValue);
+    AddInternal(aProperty, ptr);
+  }
+
+  /**
+   * @return true if @aProperty is set. This requires a linear search through
+   * the properties of the frame.
+   *
+   * In most cases, this shouldn't be used outside of assertions, because if
+   * you're doing a lookup anyway it would be far more efficient to call Get()
+   * or Take() and check the aFoundResult outparam to find out whether the
+   * property is set. Legitimate non-assertion uses include:
+   *
+   *   - Checking if a frame property is set in cases where that's all we want
+   *     to know (i.e., we don't intend to read the actual value or remove the
+   *     property).
+   *
+   *   - Calling Has() before Set() in cases where we don't want to overwrite
+   *     an existing value for the frame property.
+   */
+  template <typename T>
+  bool Has(Descriptor<T> aProperty) const {
+    return mProperties.Contains(aProperty, PropertyComparator());
+  }
+
+  /**
+   * Get a property value. This requires a linear search through
+   * the properties of the frame. If the frame has no such property,
+   * returns zero-filled result, which means null for pointers and
+   * zero for integers and floating point types.
+   * @param aFoundResult if non-null, receives a value 'true' iff
+   * the frame has a value for the property. This lets callers
+   * disambiguate a null result, which can mean 'no such property' or
+   * 'property value is null'.
+   */
+  template <typename T>
+  PropertyType<T> Get(Descriptor<T> aProperty,
+                      bool* aFoundResult = nullptr) const {
+    void* ptr = GetInternal(aProperty, aFoundResult);
+    return ReinterpretHelper<T>::FromPointer(ptr);
+  }
+
+  /**
+   * Remove a property value, and return it without destroying it.
+   *
+   * This requires a linear search through the properties of the frame.
+   * If the frame has no such property, returns zero-filled result, which means
+   * null for pointers and zero for integers and floating point types.
+   * @param aFoundResult if non-null, receives a value 'true' iff
+   * the frame had a value for the property. This lets callers
+   * disambiguate a null result, which can mean 'no such property' or
+   * 'property value is null'.
+   */
+  template <typename T>
+  PropertyType<T> Take(Descriptor<T> aProperty, bool* aFoundResult = nullptr) {
+    void* ptr = TakeInternal(aProperty, aFoundResult);
+    return ReinterpretHelper<T>::FromPointer(ptr);
+  }
+
+  /**
+   * Remove and destroy a property value. This requires a linear search through
+   * the properties of the frame. If the frame has no such property, nothing
+   * happens.
+   */
+  template <typename T>
+  void Remove(Descriptor<T> aProperty, const nsIFrame* aFrame) {
+    RemoveInternal(aProperty, aFrame);
+  }
+
+  /**
+   * Call @aFunction for each property or until @aFunction returns false.
+   */
+  template <class F>
+  void ForEach(F aFunction) const {
+#ifdef DEBUG
+    size_t len = mProperties.Length();
+#endif
+    for (const auto& prop : mProperties) {
+      bool shouldContinue = aFunction(prop.mProperty, prop.mValue);
+      MOZ_ASSERT(len == mProperties.Length(),
+                 "frame property list was modified by ForEach callback!");
+      if (!shouldContinue) {
+        return;
+      }
+    }
+  }
+
+  /**
+   * Remove and destroy all property values for the frame.
+   */
+  void RemoveAll(const nsIFrame* aFrame) {
+    nsTArray<PropertyValue> toDelete = std::move(mProperties);
+    for (auto& prop : toDelete) {
+      prop.DestroyValueFor(aFrame);
+    }
+    MOZ_ASSERT(mProperties.IsEmpty(), "a property dtor added new properties");
+  }
+
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    // We currently report only the shallow size of the mProperties array.
+    // As for the PropertyValue entries: we don't need to measure the mProperty
+    // field of because it always points to static memory, and we can't measure
+    // mValue because the type is opaque.
+    // XXX Can we do better, e.g. with a method on the descriptor?
+    return mProperties.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+
+ private:
+  // Prevent copying of FrameProperties; we should always return/pass around
+  // references to it, not copies!
+  FrameProperties(const FrameProperties&) = delete;
+  FrameProperties& operator=(const FrameProperties&) = delete;
+
+  inline void SetInternal(UntypedDescriptor aProperty, void* aValue,
+                          const nsIFrame* aFrame);
+
+  inline void AddInternal(UntypedDescriptor aProperty, void* aValue);
+
+  inline void* GetInternal(UntypedDescriptor aProperty,
+                           bool* aFoundResult) const;
+
+  inline void* TakeInternal(UntypedDescriptor aProperty, bool* aFoundResult);
+
+  inline void RemoveInternal(UntypedDescriptor aProperty,
+                             const nsIFrame* aFrame);
+
+  template <typename T>
+  struct ReinterpretHelper {
+    static_assert(sizeof(PropertyType<T>) <= sizeof(void*),
+                  "size of the value must never be larger than a pointer");
+
+    static void* ToPointer(PropertyType<T> aValue) {
+      void* ptr = nullptr;
+      memcpy(&ptr, &aValue, sizeof(aValue));
+      return ptr;
+    }
+
+    static PropertyType<T> FromPointer(void* aPtr) {
+      PropertyType<T> value;
+      memcpy(&value, &aPtr, sizeof(value));
+      return value;
+    }
+  };
+
+  template <typename T>
+  struct ReinterpretHelper<T*> {
+    static void* ToPointer(T* aValue) { return static_cast<void*>(aValue); }
+
+    static T* FromPointer(void* aPtr) { return static_cast<T*>(aPtr); }
+  };
+
+  /**
+   * Stores a property descriptor/value pair.
+   */
+  struct PropertyValue {
+    PropertyValue() : mProperty(nullptr), mValue(nullptr) {}
+    PropertyValue(UntypedDescriptor aProperty, void* aValue)
+        : mProperty(aProperty), mValue(aValue) {}
+
+    void DestroyValueFor(const nsIFrame* aFrame) {
+      if (mProperty->mDestructor) {
+        mProperty->mDestructor(mValue);
+      } else if (mProperty->mDestructorWithFrame) {
+        mProperty->mDestructorWithFrame(aFrame, mValue);
+      }
+    }
+
+    UntypedDescriptor mProperty;
+    void* mValue;
+  };
+
+  /**
+   * Used with an array of PropertyValues to allow lookups that compare
+   * only on the FramePropertyDescriptor.
+   */
+  class PropertyComparator {
+   public:
+    bool Equals(const PropertyValue& a, const PropertyValue& b) const {
+      return a.mProperty == b.mProperty;
+    }
+    bool Equals(UntypedDescriptor a, const PropertyValue& b) const {
+      return a == b.mProperty;
+    }
+    bool Equals(const PropertyValue& a, UntypedDescriptor b) const {
+      return a.mProperty == b;
+    }
+  };
+
+  nsTArray<PropertyValue> mProperties;
+};
+
+inline void* FrameProperties::GetInternal(UntypedDescriptor aProperty,
+                                          bool* aFoundResult) const {
+  MOZ_ASSERT(aProperty, "Null property?");
+
+  return mProperties.ApplyIf(
+      aProperty, 0, PropertyComparator(),
+      [&aFoundResult](const PropertyValue& aPV) -> void* {
+        if (aFoundResult) {
+          *aFoundResult = true;
+        }
+        return aPV.mValue;
+      },
+      [&aFoundResult]() -> void* {
+        if (aFoundResult) {
+          *aFoundResult = false;
+        }
+        return nullptr;
+      });
+}
+
+inline void FrameProperties::SetInternal(UntypedDescriptor aProperty,
+                                         void* aValue, const nsIFrame* aFrame) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aProperty, "Null property?");
+
+  mProperties.ApplyIf(
+      aProperty, 0, PropertyComparator(),
+      [&](PropertyValue& aPV) {
+        aPV.DestroyValueFor(aFrame);
+        aPV.mValue = aValue;
+      },
+      [&]() { mProperties.AppendElement(PropertyValue(aProperty, aValue)); });
+}
+
+inline void FrameProperties::AddInternal(UntypedDescriptor aProperty,
+                                         void* aValue) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aProperty, "Null property?");
+
+  mProperties.AppendElement(PropertyValue(aProperty, aValue));
+}
+
+inline void* FrameProperties::TakeInternal(UntypedDescriptor aProperty,
+                                           bool* aFoundResult) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aProperty, "Null property?");
+
+  auto index = mProperties.IndexOf(aProperty, 0, PropertyComparator());
+  if (index == nsTArray<PropertyValue>::NoIndex) {
+    if (aFoundResult) {
+      *aFoundResult = false;
+    }
+    return nullptr;
+  }
+
+  if (aFoundResult) {
+    *aFoundResult = true;
+  }
+
+  void* result = mProperties.Elements()[index].mValue;
+  mProperties.RemoveElementAt(index);
+
+  return result;
+}
+
+inline void FrameProperties::RemoveInternal(UntypedDescriptor aProperty,
+                                            const nsIFrame* aFrame) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aProperty, "Null property?");
+
+  auto index = mProperties.IndexOf(aProperty, 0, PropertyComparator());
+  if (index != nsTArray<PropertyValue>::NoIndex) {
+    mProperties.Elements()[index].DestroyValueFor(aFrame);
+    mProperties.RemoveElementAt(index);
+  }
+}
+
+}  // namespace mozilla
+
+#endif /* FRAMEPROPERTIES_H_ */