1 /* 2 * Copyright 2021 Google LLC 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #ifndef FCP_BASE_REENTRANCY_GUARD_H_ 18 #define FCP_BASE_REENTRANCY_GUARD_H_ 19 20 #include <atomic> 21 22 #include "fcp/base/monitoring.h" 23 24 namespace fcp { 25 26 /** 27 * ReentrancyGuard class is used to enforce strictly sequential calling pattern. 28 * Usage pattern: 29 * 30 * Status Method(...) { 31 * ReentrancyGuard guard; 32 * FCP_RETURN_IF_ERROR(guard.Check(&in_use_)); 33 * 34 * // The rest of the method body... 35 * } 36 * 37 * in_use_ above is std::atomic<bool> value stored in the object which methods 38 * are enforced. 39 */ 40 class ReentrancyGuard { 41 public: Check(std::atomic<bool> * in_use)42 Status Check(std::atomic<bool>* in_use) { 43 FCP_CHECK(in_use != nullptr); 44 bool expected_value = false; 45 if (!in_use->compare_exchange_strong(expected_value, true)) { 46 return FCP_STATUS(FAILED_PRECONDITION) 47 << "Concurrent method calls detected"; 48 } 49 50 in_use_ = in_use; 51 return FCP_STATUS(OK); 52 } 53 ~ReentrancyGuard()54 ~ReentrancyGuard() { 55 if (in_use_ != nullptr) { 56 in_use_->store(false); 57 } 58 } 59 60 private: 61 // Pointer to atomic boolean value which is owned by the object which methods 62 // are guarded against reentrancy. This value is set to true when inside 63 // a method call; otherwise false. 64 // Note: std::atomic is used here rather than Mutex is emphasise non-blocking 65 // nature of the implementation. The purpose in_use_ is only to check against 66 // reentrancy rather than synchronization. 67 std::atomic<bool>* in_use_ = nullptr; 68 }; 69 70 } // namespace fcp 71 72 #endif // FCP_BASE_REENTRANCY_GUARD_H_ 73