Add dap::RWMutex. A read / write mutex.

Also known as a shared mutex.
`std::shared_mutex` was introduced in C++17, but cppdap is C++11.

To be used to fix #35.

CMakeLists.txt

@@ -194,6 +194,7 @@ if(CPPDAP_BUILD_TESTS)
         ${CPPDAP_SRC_DIR}/json_serializer_test.cpp
         ${CPPDAP_SRC_DIR}/network_test.cpp
         ${CPPDAP_SRC_DIR}/optional_test.cpp
+        ${CPPDAP_SRC_DIR}/rwmutex_test.cpp
         ${CPPDAP_SRC_DIR}/session_test.cpp
         ${CPPDAP_SRC_DIR}/socket_test.cpp
         ${CPPDAP_SRC_DIR}/typeinfo_test.cpp

src/rwmutex.h

@@ -0,0 +1,172 @@
+// Copyright 2020 Google LLC
+//
+// 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
+//
+//     https://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 dap_rwmutex_h
+#define dap_rwmutex_h
+
+#include <condition_variable>
+#include <mutex>
+
+namespace dap {
+
+////////////////////////////////////////////////////////////////////////////////
+// RWMutex
+////////////////////////////////////////////////////////////////////////////////
+
+// A RWMutex is a reader/writer mutual exclusion lock.
+// The lock can be held by an arbitrary number of readers or a single writer.
+// Also known as a shared mutex.
+class RWMutex {
+ public:
+  inline RWMutex() = default;
+
+  // lockReader() locks the mutex for reading.
+  // Multiple read locks can be held while there are no writer locks.
+  inline void lockReader();
+
+  // unlockReader() unlocks the mutex for reading.
+  inline void unlockReader();
+
+  // lockWriter() locks the mutex for writing.
+  // If the lock is already locked for reading or writing, lockWriter blocks
+  // until the lock is available.
+  inline void lockWriter();
+
+  // unlockWriter() unlocks the mutex for writing.
+  inline void unlockWriter();
+
+ private:
+  RWMutex(const RWMutex&) = delete;
+  RWMutex& operator=(const RWMutex&) = delete;
+
+  int readLocks = 0;
+  int pendingWriteLocks = 0;
+  std::mutex mutex;
+  std::condition_variable cv;
+};
+
+void RWMutex::lockReader() {
+  std::unique_lock<std::mutex> lock(mutex);
+  readLocks++;
+}
+
+void RWMutex::unlockReader() {
+  std::unique_lock<std::mutex> lock(mutex);
+  readLocks--;
+  if (readLocks == 0 && pendingWriteLocks > 0) {
+    cv.notify_one();
+  }
+}
+
+void RWMutex::lockWriter() {
+  std::unique_lock<std::mutex> lock(mutex);
+  if (readLocks > 0) {
+    pendingWriteLocks++;
+    cv.wait(lock, [&] { return readLocks == 0; });
+    pendingWriteLocks--;
+  }
+  lock.release();  // Keep lock held
+}
+
+void RWMutex::unlockWriter() {
+  if (pendingWriteLocks > 0) {
+    cv.notify_one();
+  }
+  mutex.unlock();
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// RLock
+////////////////////////////////////////////////////////////////////////////////
+
+// RLock is a RAII read lock helper for a RWMutex.
+class RLock {
+ public:
+  inline RLock(RWMutex& mutex);
+  inline ~RLock();
+
+  inline RLock(RLock&&);
+  inline RLock& operator=(RLock&&);
+
+ private:
+  RLock(const RLock&) = delete;
+  RLock& operator=(const RLock&) = delete;
+
+  RWMutex* m;
+};
+
+RLock::RLock(RWMutex& mutex) : m(&mutex) {
+  m->lockReader();
+}
+
+RLock::~RLock() {
+  if (m != nullptr) {
+    m->unlockReader();
+  }
+}
+
+RLock::RLock(RLock&& other) {
+  m = other.m;
+  other.m = nullptr;
+}
+
+RLock& RLock::operator=(RLock&& other) {
+  m = other.m;
+  other.m = nullptr;
+  return *this;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// WLock
+////////////////////////////////////////////////////////////////////////////////
+
+// WLock is a RAII write lock helper for a RWMutex.
+class WLock {
+ public:
+  inline WLock(RWMutex& mutex);
+  inline ~WLock();
+
+  inline WLock(WLock&&);
+  inline WLock& operator=(WLock&&);
+
+ private:
+  WLock(const WLock&) = delete;
+  WLock& operator=(const WLock&) = delete;
+
+  RWMutex* m;
+};
+
+WLock::WLock(RWMutex& mutex) : m(&mutex) {
+  m->lockWriter();
+}
+
+WLock::~WLock() {
+  if (m != nullptr) {
+    m->unlockWriter();
+  }
+}
+
+WLock::WLock(WLock&& other) {
+  m = other.m;
+  other.m = nullptr;
+}
+
+WLock& WLock::operator=(WLock&& other) {
+  m = other.m;
+  other.m = nullptr;
+  return *this;
+}
+}  // namespace dap
+
+#endif

src/rwmutex_test.cpp

@@ -0,0 +1,113 @@
+// Copyright 2020 Google LLC
+//
+// 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
+//
+//     https://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.
+
+#include "rwmutex.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+#include <array>
+#include <thread>
+#include <vector>
+
+namespace {
+constexpr const size_t NumThreads = 8;
+}
+
+// Check that WLock behaves like regular mutex.
+TEST(RWMutex, WLock) {
+  dap::RWMutex rwmutex;
+  int counter = 0;
+
+  std::vector<std::thread> threads;
+  for (size_t i = 0; i < NumThreads; i++) {
+    threads.emplace_back([&] {
+      for (int j = 0; j < 1000; j++) {
+        dap::WLock lock(rwmutex);
+        counter++;
+        EXPECT_EQ(counter, 1);
+        counter--;
+      }
+    });
+  }
+
+  for (auto& thread : threads) {
+    thread.join();
+  }
+
+  EXPECT_EQ(counter, 0);
+}
+
+TEST(RWMutex, NoRLockWithWLock) {
+  dap::RWMutex rwmutex;
+
+  std::vector<std::thread> threads;
+  std::array<int, NumThreads> counters = {};
+
+  {  // With WLock held...
+    dap::WLock wlock(rwmutex);
+
+    for (size_t i = 0; i < counters.size(); i++) {
+      int* counter = &counters[i];
+      threads.emplace_back([&rwmutex, counter] {
+        dap::RLock lock(rwmutex);
+        for (int j = 0; j < 1000; j++) {
+          (*counter)++;
+        }
+      });
+    }
+
+    // RLocks should block
+    for (int counter : counters) {
+      EXPECT_EQ(counter, 0);
+    }
+  }
+
+  for (auto& thread : threads) {
+    thread.join();
+  }
+
+  for (int counter : counters) {
+    EXPECT_EQ(counter, 1000);
+  }
+}
+
+TEST(RWMutex, NoWLockWithRLock) {
+  dap::RWMutex rwmutex;
+
+  std::vector<std::thread> threads;
+  size_t counter = 0;
+
+  {  // With RLocks held...
+    dap::RLock rlockA(rwmutex);
+    dap::RLock rlockB(rwmutex);
+    dap::RLock rlockC(rwmutex);
+
+    for (size_t i = 0; i < NumThreads; i++) {
+      threads.emplace_back(std::thread([&] {
+        dap::WLock lock(rwmutex);
+        counter++;
+      }));
+    }
+
+    // ... WLocks should block
+    EXPECT_EQ(counter, 0U);
+  }
+
+  for (auto& thread : threads) {
+    thread.join();
+  }
+
+  EXPECT_EQ(counter, NumThreads);
+}