1 files changed, 145 insertions, 0 deletions
diff --git a/src/rocksdb/cache/cache_helpers.h b/src/rocksdb/cache/cache_helpers.h
new file mode 100644
index 000000000..7ea2365b8
--- /dev/null
+++ b/src/rocksdb/cache/cache_helpers.h
@@ -0,0 +1,145 @@
+//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+
+#pragma once
+
+#include <cassert>
+
+#include "rocksdb/cache.h"
+#include "rocksdb/rocksdb_namespace.h"
+
+namespace ROCKSDB_NAMESPACE {
+
+// Returns the cached value given a cache handle.
+template <typename T>
+T* GetFromCacheHandle(Cache* cache, Cache::Handle* handle) {
+  assert(cache);
+  assert(handle);
+
+  return static_cast<T*>(cache->Value(handle));
+}
+
+// Simple generic deleter for Cache (to be used with Cache::Insert).
+template <typename T>
+void DeleteCacheEntry(const Slice& /* key */, void* value) {
+  delete static_cast<T*>(value);
+}
+
+// Turns a T* into a Slice so it can be used as a key with Cache.
+template <typename T>
+Slice GetSlice(const T* t) {
+  return Slice(reinterpret_cast<const char*>(t), sizeof(T));
+}
+
+// Generic resource management object for cache handles that releases the handle
+// when destroyed. Has unique ownership of the handle, so copying it is not
+// allowed, while moving it transfers ownership.
+template <typename T>
+class CacheHandleGuard {
+ public:
+  CacheHandleGuard() = default;
+
+  CacheHandleGuard(Cache* cache, Cache::Handle* handle)
+      : cache_(cache),
+        handle_(handle),
+        value_(GetFromCacheHandle<T>(cache, handle)) {
+    assert(cache_ && handle_ && value_);
+  }
+
+  CacheHandleGuard(const CacheHandleGuard&) = delete;
+  CacheHandleGuard& operator=(const CacheHandleGuard&) = delete;
+
+  CacheHandleGuard(CacheHandleGuard&& rhs) noexcept
+      : cache_(rhs.cache_), handle_(rhs.handle_), value_(rhs.value_) {
+    assert((!cache_ && !handle_ && !value_) || (cache_ && handle_ && value_));
+
+    rhs.ResetFields();
+  }
+
+  CacheHandleGuard& operator=(CacheHandleGuard&& rhs) noexcept {
+    if (this == &rhs) {
+      return *this;
+    }
+
+    ReleaseHandle();
+
+    cache_ = rhs.cache_;
+    handle_ = rhs.handle_;
+    value_ = rhs.value_;
+
+    assert((!cache_ && !handle_ && !value_) || (cache_ && handle_ && value_));
+
+    rhs.ResetFields();
+
+    return *this;
+  }
+
+  ~CacheHandleGuard() { ReleaseHandle(); }
+
+  bool IsEmpty() const { return !handle_; }
+
+  Cache* GetCache() const { return cache_; }
+  Cache::Handle* GetCacheHandle() const { return handle_; }
+  T* GetValue() const { return value_; }
+
+  void TransferTo(Cleanable* cleanable) {
+    if (cleanable) {
+      if (handle_ != nullptr) {
+        assert(cache_);
+        cleanable->RegisterCleanup(&ReleaseCacheHandle, cache_, handle_);
+      }
+    }
+    ResetFields();
+  }
+
+  void Reset() {
+    ReleaseHandle();
+    ResetFields();
+  }
+
+ private:
+  void ReleaseHandle() {
+    if (IsEmpty()) {
+      return;
+    }
+
+    assert(cache_);
+    cache_->Release(handle_);
+  }
+
+  void ResetFields() {
+    cache_ = nullptr;
+    handle_ = nullptr;
+    value_ = nullptr;
+  }
+
+  static void ReleaseCacheHandle(void* arg1, void* arg2) {
+    Cache* const cache = static_cast<Cache*>(arg1);
+    assert(cache);
+
+    Cache::Handle* const cache_handle = static_cast<Cache::Handle*>(arg2);
+    assert(cache_handle);
+
+    cache->Release(cache_handle);
+  }
+
+ private:
+  Cache* cache_ = nullptr;
+  Cache::Handle* handle_ = nullptr;
+  T* value_ = nullptr;
+};
+
+// Build an aliasing shared_ptr that keeps `handle` in cache while there
+// are references, but the pointer is to the value for that cache entry,
+// which must be of type T. This is copyable, unlike CacheHandleGuard, but
+// does not provide access to caching details.
+template <typename T>
+std::shared_ptr<T> MakeSharedCacheHandleGuard(Cache* cache,
+                                              Cache::Handle* handle) {
+  auto wrapper = std::make_shared<CacheHandleGuard<T>>(cache, handle);
+  return std::shared_ptr<T>(wrapper, static_cast<T*>(cache->Value(handle)));
+}
+
+}  // namespace ROCKSDB_NAMESPACE