maint: update deps

deps/zstd/contrib/pzstd/utils/ResourcePool.h

@@ -0,0 +1,96 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under both the BSD-style license (found in the
+ * LICENSE file in the root directory of this source tree) and the GPLv2 (found
+ * in the COPYING file in the root directory of this source tree).
+ */
+#pragma once
+
+#include <cassert>
+#include <functional>
+#include <memory>
+#include <mutex>
+#include <vector>
+
+namespace pzstd {
+
+/**
+ * An unbounded pool of resources.
+ * A `ResourcePool<T>` requires a factory function that takes allocates `T*` and
+ * a free function that frees a `T*`.
+ * Calling `ResourcePool::get()` will give you a new `ResourcePool::UniquePtr`
+ * to a `T`, and when it goes out of scope the resource will be returned to the
+ * pool.
+ * The `ResourcePool<T>` *must* survive longer than any resources it hands out.
+ * Remember that `ResourcePool<T>` hands out mutable `T`s, so make sure to clean
+ * up the resource before or after every use.
+ */
+template <typename T>
+class ResourcePool {
+ public:
+  class Deleter;
+  using Factory = std::function<T*()>;
+  using Free = std::function<void(T*)>;
+  using UniquePtr = std::unique_ptr<T, Deleter>;
+
+ private:
+  std::mutex mutex_;
+  Factory factory_;
+  Free free_;
+  std::vector<T*> resources_;
+  unsigned inUse_;
+
+ public:
+  /**
+   * Creates a `ResourcePool`.
+   *
+   * @param factory  The function to use to create new resources.
+   * @param free     The function to use to free resources created by `factory`.
+   */
+  ResourcePool(Factory factory, Free free)
+      : factory_(std::move(factory)), free_(std::move(free)), inUse_(0) {}
+
+  /**
+   * @returns  A unique pointer to a resource.  The resource is null iff
+   *           there are no available resources and `factory()` returns null.
+   */
+  UniquePtr get() {
+    std::lock_guard<std::mutex> lock(mutex_);
+    if (!resources_.empty()) {
+      UniquePtr resource{resources_.back(), Deleter{*this}};
+      resources_.pop_back();
+      ++inUse_;
+      return resource;
+    }
+    UniquePtr resource{factory_(), Deleter{*this}};
+    ++inUse_;
+    return resource;
+  }
+
+  ~ResourcePool() noexcept {
+    assert(inUse_ == 0);
+    for (const auto resource : resources_) {
+      free_(resource);
+    }
+  }
+
+  class Deleter {
+    ResourcePool *pool_;
+  public:
+    explicit Deleter(ResourcePool &pool) : pool_(&pool) {}
+
+    void operator() (T *resource) {
+      std::lock_guard<std::mutex> lock(pool_->mutex_);
+      // Make sure we don't put null resources into the pool
+      if (resource) {
+        pool_->resources_.push_back(resource);
+      }
+      assert(pool_->inUse_ > 0);
+      --pool_->inUse_;
+    }
+  };
+};
+
+}