1 files changed, 187 insertions, 0 deletions
diff --git a/ipc/glue/SideVariant.h b/ipc/glue/SideVariant.h
new file mode 100644
index 0000000000..3082feebde
--- /dev/null
+++ b/ipc/glue/SideVariant.h
@@ -0,0 +1,187 @@
+/* -*- 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 mozilla_ipc_SidedVariant_h
+#define mozilla_ipc_SidedVariant_h
+
+#include <variant>
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ipc/ProtocolUtils.h"
+#include "ipc/IPCMessageUtils.h"
+
+namespace mozilla {
+namespace ipc {
+
+/**
+ * Helper type used by IPDL structs and unions to hold actor pointers with a
+ * dynamic side.
+ *
+ * When sent over IPC, ParentSide will be used for send/recv on parent actors,
+ * and ChildSide will be used for send/recv on child actors.
+ */
+template <typename ParentSide, typename ChildSide>
+struct SideVariant {
+ public:
+  SideVariant() = default;
+  template <typename U,
+            std::enable_if_t<std::is_convertible_v<U&&, ParentSide>, int> = 0>
+  MOZ_IMPLICIT SideVariant(U&& aParent) : mParent(std::forward<U>(aParent)) {}
+  template <typename U,
+            std::enable_if_t<std::is_convertible_v<U&&, ChildSide>, int> = 0>
+  MOZ_IMPLICIT SideVariant(U&& aChild) : mChild(std::forward<U>(aChild)) {}
+  MOZ_IMPLICIT SideVariant(std::nullptr_t) {}
+
+  MOZ_IMPLICIT SideVariant& operator=(ParentSide aParent) {
+    mParent = aParent;
+    mChild = nullptr;
+    return *this;
+  }
+  MOZ_IMPLICIT SideVariant& operator=(ChildSide aChild) {
+    mChild = aChild;
+    mParent = nullptr;
+    return *this;
+  }
+  MOZ_IMPLICIT SideVariant& operator=(std::nullptr_t) {
+    mChild = nullptr;
+    mParent = nullptr;
+    return *this;
+  }
+
+  MOZ_IMPLICIT operator bool() const { return mParent || mChild; }
+
+  bool IsNull() const { return !operator bool(); }
+  bool IsParent() const { return mParent; }
+  bool IsChild() const { return mChild; }
+
+  ParentSide AsParent() const {
+    MOZ_ASSERT(IsNull() || IsParent());
+    return mParent;
+  }
+  ChildSide AsChild() const {
+    MOZ_ASSERT(IsNull() || IsChild());
+    return mChild;
+  }
+
+ private:
+  // As the values are both pointers, this is the same size as a variant would
+  // be, but has less risk of type confusion, and supports an overall `nullptr`
+  // value which is neither parent nor child.
+  ParentSide mParent = nullptr;
+  ChildSide mChild = nullptr;
+};
+
+}  // namespace ipc
+
+// NotNull specialization to expose AsChild and AsParent on the NotNull itself
+// avoiding unnecessary unwrapping.
+template <typename ParentSide, typename ChildSide>
+class NotNull<mozilla::ipc::SideVariant<ParentSide, ChildSide>> {
+  template <typename U>
+  friend constexpr NotNull<U> WrapNotNull(U aBasePtr);
+  template <typename U>
+  friend constexpr NotNull<U> WrapNotNullUnchecked(U aBasePtr);
+  template <typename U>
+  friend class NotNull;
+
+  using BasePtr = mozilla::ipc::SideVariant<ParentSide, ChildSide>;
+
+  BasePtr mBasePtr;
+
+  // This constructor is only used by WrapNotNull() and MakeNotNull<U>().
+  template <typename U>
+  constexpr explicit NotNull(U aBasePtr) : mBasePtr(aBasePtr) {}
+
+ public:
+  // Disallow default construction.
+  NotNull() = delete;
+
+  // Construct/assign from another NotNull with a compatible base pointer type.
+  template <typename U, typename = std::enable_if_t<
+                            std::is_convertible_v<const U&, BasePtr>>>
+  constexpr MOZ_IMPLICIT NotNull(const NotNull<U>& aOther)
+      : mBasePtr(aOther.get()) {
+    static_assert(sizeof(BasePtr) == sizeof(NotNull<BasePtr>),
+                  "NotNull must have zero space overhead.");
+    static_assert(offsetof(NotNull<BasePtr>, mBasePtr) == 0,
+                  "mBasePtr must have zero offset.");
+  }
+
+  template <typename U,
+            typename = std::enable_if_t<std::is_convertible_v<U&&, BasePtr>>>
+  constexpr MOZ_IMPLICIT NotNull(MovingNotNull<U>&& aOther)
+      : mBasePtr(NotNull{std::move(aOther)}) {}
+
+  // Disallow null checks, which are unnecessary for this type.
+  explicit operator bool() const = delete;
+
+  // Explicit conversion to a base pointer. Use only to resolve ambiguity or to
+  // get a castable pointer.
+  constexpr const BasePtr& get() const { return mBasePtr; }
+
+  // Implicit conversion to a base pointer. Preferable to get().
+  constexpr operator const BasePtr&() const { return get(); }
+
+  bool IsParent() const { return get().IsParent(); }
+  bool IsChild() const { return get().IsChild(); }
+
+  NotNull<ParentSide> AsParent() const { return WrapNotNull(get().AsParent()); }
+  NotNull<ChildSide> AsChild() const { return WrapNotNull(get().AsChild()); }
+};
+
+}  // namespace mozilla
+
+namespace IPC {
+
+template <typename ParentSide, typename ChildSide>
+struct ParamTraits<mozilla::ipc::SideVariant<ParentSide, ChildSide>> {
+  typedef mozilla::ipc::SideVariant<ParentSide, ChildSide> paramType;
+
+  static void Write(IPC::MessageWriter* aWriter, const paramType& aParam) {
+    if (!aWriter->GetActor()) {
+      aWriter->FatalError("actor required to serialize this type");
+      return;
+    }
+
+    if (aWriter->GetActor()->GetSide() == mozilla::ipc::ParentSide) {
+      if (aParam && !aParam.IsParent()) {
+        aWriter->FatalError("invalid side");
+        return;
+      }
+      WriteParam(aWriter, aParam.AsParent());
+    } else {
+      if (aParam && !aParam.IsChild()) {
+        aWriter->FatalError("invalid side");
+        return;
+      }
+      WriteParam(aWriter, aParam.AsChild());
+    }
+  }
+
+  static ReadResult<paramType> Read(IPC::MessageReader* aReader) {
+    if (!aReader->GetActor()) {
+      aReader->FatalError("actor required to deserialize this type");
+      return {};
+    }
+
+    if (aReader->GetActor()->GetSide() == mozilla::ipc::ParentSide) {
+      auto parentSide = ReadParam<ParentSide>(aReader);
+      if (!parentSide) {
+        return {};
+      }
+      return std::move(*parentSide);
+    }
+    auto childSide = ReadParam<ChildSide>(aReader);
+    if (!childSide) {
+      return {};
+    }
+    return std::move(*childSide);
+  }
+};
+
+}  // namespace IPC
+
+#endif  // mozilla_ipc_SidedVariant_h