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diff --git a/xpcom/base/RLBoxSandboxPool.h b/xpcom/base/RLBoxSandboxPool.h
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+++ b/xpcom/base/RLBoxSandboxPool.h
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+/* -*- Mode: C++; tab-width: 20; 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 SECURITY_RLBOX_SANDBOX_POOL_H_
+#define SECURITY_RLBOX_SANDBOX_POOL_H_
+
+#include "nsCOMPtr.h"
+#include "nsITimer.h"
+#include "nsTArray.h"
+#include "nsINamed.h"
+
+#include "mozilla/Mutex.h"
+#include "mozilla/rlbox/rlbox_types.hpp"
+
+namespace mozilla {
+
+class RLBoxSandboxDataBase;
+class RLBoxSandboxPoolData;
+
+// The RLBoxSandboxPool class is used to manage a pool of sandboxes that are
+// reused -- to save sandbox creation time and memory -- and automatically
+// destroyed when no longer in used. The sandbox pool is threadsafe and can be
+// used to share unused sandboxes across a thread pool.
+//
+// Each sandbox pool manages a particular kind of sandbox (e.g., expat
+// sandboxes, woff2 sandboxes, etc.); this is largely because different
+// sandboxes might have different callbacks and attacker assumptions. Hence,
+// RLBoxSandboxPool is intended to be subclassed for the different kinds of
+// sandbox pools. Each sandbox pool class needs to implement the
+// CreateSandboxData() method, which returns a pointer to a RLBoxSandboxDataBase
+// object.  RLBoxSandboxDataBase itself should be subclassed to implement
+// sandbox-specific details.
+class RLBoxSandboxPool : public nsITimerCallback, public nsINamed {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSITIMERCALLBACK
+  NS_DECL_NSINAMED
+
+  RLBoxSandboxPool(size_t aDelaySeconds = 10)
+      : mPool(),
+        mDelaySeconds(aDelaySeconds),
+        mMutex("RLBoxSandboxPool::mMutex"){};
+
+  void Push(UniquePtr<RLBoxSandboxDataBase> sbx);
+  // PopOrCreate returns a sandbox from the pool if the pool is not empty and
+  // tries to mint a new one otherwise. If creating a new sandbox fails, the
+  // function returns a nullptr. The parameter aMinSize is the minimum size of
+  // the sandbox memory.
+  UniquePtr<RLBoxSandboxPoolData> PopOrCreate(uint64_t aMinSize = 0);
+
+ protected:
+  // CreateSandboxData takes a parameter which is the size of the sandbox memory
+  virtual UniquePtr<RLBoxSandboxDataBase> CreateSandboxData(uint64_t aSize) = 0;
+  virtual ~RLBoxSandboxPool() = default;
+
+ private:
+  void StartTimer() MOZ_REQUIRES(mMutex);
+  void CancelTimer() MOZ_REQUIRES(mMutex);
+
+  nsTArray<UniquePtr<RLBoxSandboxDataBase>> mPool MOZ_GUARDED_BY(mMutex);
+  const size_t mDelaySeconds MOZ_GUARDED_BY(mMutex);
+  nsCOMPtr<nsITimer> mTimer MOZ_GUARDED_BY(mMutex);
+  mozilla::Mutex mMutex;
+};
+
+// The RLBoxSandboxDataBase class serves as the subclass for all sandbox data
+// classes, which keep track of the RLBox sandbox and any relevant sandbox data
+// (e.g., callbacks).
+class RLBoxSandboxDataBase {
+ public:
+  const uint64_t mSize;
+  explicit RLBoxSandboxDataBase(uint64_t aSize) : mSize(aSize) {}
+  virtual ~RLBoxSandboxDataBase() = default;
+};
+
+// This class is used wrap sandbox data objects (RLBoxSandboxDataBase) when they
+// are popped from sandbox pools. The wrapper destructor pushes the sandbox back
+// into the pool.
+class RLBoxSandboxPoolData {
+ public:
+  RLBoxSandboxPoolData(UniquePtr<RLBoxSandboxDataBase> aSbxData,
+                       RefPtr<RLBoxSandboxPool> aPool) {
+    mSbxData = std::move(aSbxData);
+    mPool = aPool;
+    MOZ_COUNT_CTOR(RLBoxSandboxPoolData);
+  }
+
+  RLBoxSandboxDataBase* SandboxData() const { return mSbxData.get(); };
+
+  ~RLBoxSandboxPoolData() {
+    mPool->Push(std::move(mSbxData));
+    MOZ_COUNT_DTOR(RLBoxSandboxPoolData);
+  };
+
+ private:
+  UniquePtr<RLBoxSandboxDataBase> mSbxData;
+  RefPtr<RLBoxSandboxPool> mPool;
+};
+
+}  // namespace mozilla
+
+#endif