[Hacky prototype] Create a shared-memory high-performance reporting service.

services/blamer/shared_memory_heap.h

Index: services/blamer/shared_memory_heap.h
diff --git a/services/blamer/shared_memory_heap.h b/services/blamer/shared_memory_heap.h
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+// Copyright 2017 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.
+
+// Defines a shared memory heap. One of these is created by each process
+// using the blame service. This heap takes care of registering all of its slabs
+// with the central blamer service, allowing it to iterate over all blame
+// information across all processes.
+
+#ifndef SERVICES_BLAMER_SHARED_MEMORY_HEAP_H_
+#define SERVICES_BLAMER_SHARED_MEMORY_HEAP_H_
+
+#include "base/metrics/persistent_memory_allocator.h"
+#include "mojo/public/cpp/system/buffer.h"
+#include "services/blamer/heap_object_types.h"
+#include "services/blamer/public/interfaces/shared_memory_heap_registry.mojom.h"
+
+namespace blamer {
+
+// This class is thread safe. Most of the operations will actually be performed
+// without using any locks, however occasionally a lock will need to be
+// acquired in support of some operations.
+class SharedMemoryHeap {
+ public:
+  static constexpr size_t kSlabSizeBits = 22;
+  static constexpr size_t kSlabSize = 1 << kSlabSizeBits;
+
+  // Represents a pointer in a process-portable way. This is what would be sent
+  // over IPC to communicate pointers between processes. Knowing the ID of the
+  // process owning the memory and the |raw_value| of the PortablePointer is
+  // sufficient to extract an actual pointer to the object in the process
+  // hosting the service.
+  struct PortablePointer {
+    union {
+      uint32_t raw_value;
+      struct {
+        unsigned int slab_id : 32 - kSlabSizeBits;
+        unsigned int slab_offset : kSlabSizeBits;
+      };
+    };
+  };
+  static_assert(sizeof(PortablePointer) == 4,
+                "unexpected PortablePointer size");
+
+  // Encapsulates pointers as they are doled out by this shared memory
+  // allocator. Dereferencing has a non-trivial cost so the result should
+  // ideally be cached by clients that need to do this often.
+  template<typename ObjectType>
+  struct TypedLocalPointer {
+    TypedLocalPointer(void* slab, PortablePointer pointer)
+        : slab(slab), pointer(pointer) {
+    }
+
+    ObjectType* Get() const {
+      auto* allocator = reinterpret_cast<base::PersistentMemoryAllocator*>(
+          slab);
+      base::PersistentMemoryAllocator::Reference ref = pointer.slab_offset;
+      return allocator->GetAsObject<ObjectType>(ref);
+    }
+    ObjectType* operator->() const { return Get(); }
+    ObjectType& operator*() const { return *Get(); }
+
+    void* slab;
+    PortablePointer pointer;
+  };
+
+  SharedMemoryHeap(mojom::SharedMemoryHeapRegistryPtr heap_registry);
+  ~SharedMemoryHeap();
+
+  // Allocates an object on the shared memory heap. Uses the actual object size.
+  template<typename ObjectType>
+  TypedLocalPointer<ObjectType> Allocate() {
+    return MakePointer<ObjectType>(Allocate(
+        ObjectType::kPersistentTypeId, sizeof(ObjectType)));
+  }
+
+  // Allocates an object on the shared memory heap. Uses the request object
+  // size.
+  template<typename ObjectType>
+  TypedLocalPointer<ObjectType> Allocate(size_t object_size) {
+    return MakePointer<ObjectType>(Allocate(
+        ObjectType::kPersistentTypeId, object_size));
+  }
+
+  // Frees an object. Once freed the object will no longer be visible to
+  // remote processes.
+  template<typename ObjectType>
+  void Free(const TypedLocalPointer<ObjectType>& pointer) {
+    Free(ObjectType::kPersistentTypeId, MakePointer(pointer));
+  }
+
+ private:
+  struct LocalPointer {
+    void* slab;
+    PortablePointer pointer;
+  };
+
+  struct Slab {
+    Slab();
+    Slab(Slab&& other);
+    ~Slab();
+
+    mojo::ScopedSharedBufferHandle buffer;
+    mojo::ScopedSharedBufferMapping mapping;
+    std::unique_ptr<base::PersistentMemoryAllocator> allocator;
+
+   private:
+    DISALLOW_COPY_AND_ASSIGN(Slab);
+  };
+
+  // Returns the current slab, creating one if none exists.
+  base::PersistentMemoryAllocator* GetCurrentSlab();
+
+  // Creates a new slab, and updates |current_slab_|. Returns a pointer to the
+  // newly created slab.
+  base::PersistentMemoryAllocator* CreateNewSlab(
+      base::PersistentMemoryAllocator* current_slab);
+
+  // Creates a TypedLocalPointer from an untyped LocalPointer.
+  template<typename ObjectType>
+  TypedLocalPointer<ObjectType> MakePointer(const LocalPointer& pointer) {
+    return TypedLocalPointer<ObjectType>(pointer.slab, pointer.pointer);
+  }
+
+  // Creates an untyped LocalPointer from a TypedLocalPointer.
+  template<typename ObjectType>
+  LocalPointer MakePointer(const TypedLocalPointer<ObjectType>& pointer) {
+    LocalPointer lp = {};
+    lp.slab = pointer.slab;
+    lp.slab = pointer.pointer = pointer.pointer;
+    return lp;
+  }
+
+  // Allocates an object from the current slab. Creates a new slab if the
+  // current slab fails to perform the allocation.
+  LocalPointer Allocate(HeapObjectType object_type, size_t object_size);
+
+  // Frees the provided object.
+  void Free(HeapObjectType object_type, const LocalPointer& pointer);
+
+  // Pointer to the heap registry to be notified of new slabs.
+  mojom::SharedMemoryHeapRegistryPtr heap_registry_;
+
+  // The current persistent memory allocator. This is updated in a lock-free
+  // manner. The common use case is to grab the current allocator and make an
+  // allocation, which is very fast.
+  std::atomic<base::PersistentMemoryAllocator*> current_slab_;
+
+  // |current_slab_| is used as a lock for modifying this safely.
+  std::vector<Slab> slabs_;
+};
+
+}  // namespace blamer
+
+#endif  // SERVICES_BLAMER_SHARED_MEMORY_HEAP_H_