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diff --git a/automotive/vehicle/aidl/impl/vhal/include/PendingRequestPool.h b/automotive/vehicle/aidl/impl/vhal/include/PendingRequestPool.h
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+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef android_hardware_automotive_vehicle_aidl_impl_vhal_include_PendingRequestPool_H_
+#define android_hardware_automotive_vehicle_aidl_impl_vhal_include_PendingRequestPool_H_
+
+#include <android-base/result.h>
+#include <android-base/thread_annotations.h>
+
+#include <atomic>
+#include <list>
+#include <mutex>
+#include <thread>
+#include <unordered_map>
+#include <unordered_set>
+
+namespace android {
+namespace hardware {
+namespace automotive {
+namespace vehicle {
+
+// A thread-safe pending request pool that tracks whether each request has timed-out.
+class PendingRequestPool final {
+  public:
+    using TimeoutCallbackFunc = std::function<void(const std::unordered_set<int64_t>&)>;
+
+    explicit PendingRequestPool(int64_t timeoutInSec);
+
+    ~PendingRequestPool();
+
+    // Adds a list of requests to the request pool.
+    // The clientId is the key for all the requests. It could be a number or an address to a data
+    // structure that represents a client. The caller must maintain this data structure.
+    // All the request IDs must be unique for one client, if any of the requestIds is duplicate with
+    // any pending request IDs for the client, this function returns error and no requests would be
+    // added. Otherwise, they would be added to the request pool.
+    // The callback would be called if requests are not finished within {@code mTimeoutInNano}
+    // seconds.
+    android::base::Result<void> addRequests(const void* clientId,
+                                            const std::unordered_set<int64_t>& requestIds,
+                                            std::shared_ptr<TimeoutCallbackFunc> callback);
+
+    // Checks whether the request is currently pending.
+    bool isRequestPending(const void* clientId, int64_t requestId) const;
+
+    // Tries to mark the requests as finished and remove them from the pool if the request is
+    // currently pending. Returns the list of request that is pending and has been finished
+    // successfully. This function would try to finish any valid requestIds even though some of the
+    // requestIds are not valid.
+    std::unordered_set<int64_t> tryFinishRequests(const void* clientId,
+                                                  const std::unordered_set<int64_t>& requestIds);
+
+    // Returns how many pending requests in the pool, for testing purpose.
+    size_t countPendingRequests(const void* clientId) const;
+
+  private:
+    // The maximum number of pending requests allowed per client. If exceeds this number, adding
+    // more requests would fail. This is to prevent spamming from client.
+    static constexpr size_t MAX_PENDING_REQUEST_PER_CLIENT = 10000;
+
+    struct PendingRequest {
+        std::unordered_set<int64_t> requestIds;
+        int64_t timeoutTimestamp;
+        std::shared_ptr<TimeoutCallbackFunc> callback;
+    };
+
+    int64_t mTimeoutInNano;
+    mutable std::mutex mLock;
+    std::unordered_map<const void*, std::list<PendingRequest>> mPendingRequestsByClient
+            GUARDED_BY(mLock);
+    std::thread mThread;
+    std::atomic<bool> mThreadStop = false;
+    std::condition_variable mCv;
+    std::mutex mCvLock;
+
+    bool isRequestPendingLocked(const void* clientId, int64_t requestId) const REQUIRES(mLock);
+
+    // Checks whether the requests in the pool has timed-out, run periodically in a separate thread.
+    void checkTimeout();
+};
+
+}  // namespace vehicle
+}  // namespace automotive
+}  // namespace hardware
+}  // namespace android
+
+#endif  // android_hardware_automotive_vehicle_aidl_impl_vhal_include_PendingRequestPool_H_