Add a DiscardableMemoryShmemeAllocator.

mojo/services/html_viewer/discardable_memory_allocator.cc

+// Copyright 2015 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.
+
+#include "mojo/services/html_viewer/discardable_memory_allocator.h"
+
+#include "base/memory/discardable_memory.h"
+#include "base/memory/weak_ptr.h"
+#include "base/stl_util.h"
+
+namespace html_viewer {
+
+// Represents an actual memory chunk. This is an object owned by
+// DiscardableMemoryAllocator. DiscardableMemoryChunkImpl are passed to the
+// rest of the program, and access this memory through a weak ptr.
+class DiscardableMemoryAllocator::OwnedMemoryChunk {
+ public:
+  explicit OwnedMemoryChunk(size_t size, DiscardableMemoryAllocator* allocator)

[jamesr, 2015/03/19 22:09:56]

nit: no explicit for 2-arg ctors

(yes it has a meaning with c++11 uniform initialization syntax, but we don't
allow that in chromium and i think we probably never will)

+      : is_locked_(true),
+        size_(size),
+        memory_(new uint8_t[size]),
+        allocator_(allocator),
+        weak_factory_(this) {}
+  ~OwnedMemoryChunk() {}
+
+  void Lock() {
+    DCHECK(!is_locked_);
+    is_locked_ = true;
+    allocator_->NotifyLocked(unlocked_position_);
+  }
+
+  void Unlock() {
+    DCHECK(is_locked_);
+    is_locked_ = false;
+    unlocked_position_ = allocator_->NotifyUnlocked(this);
+  }
+
+  bool locked() const { return is_locked_; }

[jamesr, 2015/03/19 22:09:56]

call it is_locked() to match the member (and i think it's a nicer name)

+  size_t size() const { return size_; }
+  void* Memory() const {
+    DCHECK(is_locked_);
+    return memory_.get();
+  }
+
+  base::WeakPtr<OwnedMemoryChunk> GetWeakPtr() {
+    return weak_factory_.GetWeakPtr();
+  }
+
+ private:
+  bool is_locked_;
+  size_t size_;
+  scoped_ptr<uint8_t[]> memory_;
+  DiscardableMemoryAllocator* allocator_;
+
+  std::list<OwnedMemoryChunk*>::iterator unlocked_position_;
+
+  base::WeakPtrFactory<OwnedMemoryChunk> weak_factory_;
+
+  DISALLOW_IMPLICIT_CONSTRUCTORS(OwnedMemoryChunk);
+};
+
+namespace {
+
+// Interface to the rest of the program. These objects are owned outside of the
+// allocator and wrap a weak ptr.
+class DiscardableMemoryChunkImpl : public base::DiscardableMemory {
+ public:
+  explicit DiscardableMemoryChunkImpl(
+      base::WeakPtr<DiscardableMemoryAllocator::OwnedMemoryChunk> chunk)
+      : memory_chunk_(chunk) {}
+  ~DiscardableMemoryChunkImpl() override {
+    // Either the memory chunk is invalid (because the backing has gone away),
+    // or the memory chunk is unlocked (because leaving the chunk locked once
+    // we deallocate means the chunk will never get cleaned up).
+    DCHECK(!memory_chunk_ || !memory_chunk_->locked());
+  }
+
+  // Overridden from DiscardableMemoryChunk:
+  bool Lock() override {
+    if (!memory_chunk_)
+      return false;
+
+    memory_chunk_->Lock();
+    return true;
+  }
+
+  void Unlock() override {
+    DCHECK(memory_chunk_);
+    memory_chunk_->Unlock();
+  }
+
+  void* Memory() const override {
+    if (memory_chunk_)
+      return memory_chunk_->Memory();
+    return nullptr;
+  }
+
+ private:
+  base::WeakPtr<DiscardableMemoryAllocator::OwnedMemoryChunk> memory_chunk_;
+
+  DISALLOW_IMPLICIT_CONSTRUCTORS(DiscardableMemoryChunkImpl);
+};
+
+}  // namespace
+
+DiscardableMemoryAllocator::DiscardableMemoryAllocator(
+    size_t desired_max_memory)
+    : desired_max_memory_(desired_max_memory),
+      total_live_memory_(0u),
+      locked_chunks_(0) {
+}
+
+DiscardableMemoryAllocator::~DiscardableMemoryAllocator() {
+  DCHECK_EQ(0, locked_chunks_);
+  STLDeleteElements(&live_unlocked_chunks_);
+}
+
+scoped_ptr<base::DiscardableMemory>
+DiscardableMemoryAllocator::AllocateLockedDiscardableMemory(size_t size) {
+  OwnedMemoryChunk* chunk = new OwnedMemoryChunk(size, this);
+  total_live_memory_ += size;
+  locked_chunks_++;
+
+  // Go through the list of unlocked live chunks starting from the least
+  // recently used, freeing as many as we can until we get our size under the
+  // desired maximum.
+  auto it = live_unlocked_chunks_.begin();
+  while (total_live_memory_ > desired_max_memory_ &&
+         it != live_unlocked_chunks_.end()) {
+    total_live_memory_ -= (*it)->size();
+    delete *it;
+    it = live_unlocked_chunks_.erase(it);
+  }
+
+  return make_scoped_ptr(new DiscardableMemoryChunkImpl(chunk->GetWeakPtr()));
+}
+
+std::list<DiscardableMemoryAllocator::OwnedMemoryChunk*>::iterator
+DiscardableMemoryAllocator::NotifyUnlocked(
+    DiscardableMemoryAllocator::OwnedMemoryChunk* chunk) {
+  locked_chunks_--;
+  return live_unlocked_chunks_.insert(live_unlocked_chunks_.end(), chunk);
+}
+
+void DiscardableMemoryAllocator::NotifyLocked(
+    std::list<OwnedMemoryChunk*>::iterator it) {
+  locked_chunks_++;
+  live_unlocked_chunks_.erase(it);
+}
+
+}  // namespace html_viewer