Merge pull request #13594 from jordan-woyak/state-cleanups

State: Simplify interthread communication and general cleanups.

Source/UnitTests/Common/MutexTest.cpp

@@ -6,9 +6,11 @@
 #include <algorithm>
 #include <chrono>
 #include <mutex>
+#include <shared_mutex>
 #include <thread>
 
 #include "Common/Mutex.h"
+#include "Common/TransferableSharedMutex.h"
 
 template <typename MutexType>
 static void DoAtomicMutexTests(const char mutex_name[])
@@ -100,3 +102,123 @@ TEST(Mutex, AtomicMutex)
   DoAtomicMutexTests<Common::AtomicMutex>("AtomicMutex");
   DoAtomicMutexTests<Common::SpinMutex>("SpinMutex");
 }
+
+TEST(Mutex, TransferableSharedMutex)
+{
+  Common::TransferableSharedMutex work_mutex;
+
+  bool worker_done = false;
+
+  static constexpr auto SLEEP_TIME = std::chrono::microseconds{1};
+
+  // lock() on main thread, unlock() on worker thread.
+  std::thread thread{[&, lk = std::unique_lock{work_mutex}] {
+    std::this_thread::sleep_for(SLEEP_TIME);
+    worker_done = true;
+  }};
+
+  // lock() waits for the thread to unlock().
+  {
+    std::lock_guard lk{work_mutex};
+    EXPECT_TRUE(worker_done);
+  }
+
+  thread.join();
+
+  // Prevent below workers from incrementing `done_count`.
+  Common::TransferableSharedMutex done_mutex;
+  std::unique_lock done_lk{done_mutex};
+
+  // try_*() fails when holding an exclusive lock.
+  EXPECT_FALSE(done_mutex.try_lock());
+  EXPECT_FALSE(done_mutex.try_lock_shared());
+
+  static constexpr int THREAD_COUNT = 4;
+  static constexpr int REPEAT_COUNT = 100;
+  static constexpr int TOTAL_ITERATIONS = THREAD_COUNT * REPEAT_COUNT;
+
+  std::atomic<int> work_count = 0;
+  std::atomic<int> done_count = 0;
+  int additional_work_count = 0;
+
+  std::atomic<int> try_lock_fail_count = 0;
+  std::atomic<int> try_lock_shared_fail_count = 0;
+
+  std::vector<std::thread> threads(THREAD_COUNT);
+  for (auto& t : threads)
+  {
+    // lock_shared() multiple times on main thread.
+    t = std::thread{[&, work_lk = std::shared_lock{work_mutex}]() mutable {
+      std::this_thread::sleep_for(SLEEP_TIME);
+
+      // try_lock() fails after lock_shared().
+      EXPECT_FALSE(work_mutex.try_lock());
+
+      // Main thread already holds done_mutex.
+      EXPECT_FALSE(done_mutex.try_lock());
+      EXPECT_FALSE(done_mutex.try_lock_shared());
+
+      ++work_count;
+
+      // Signal work is done.
+      work_lk.unlock();
+
+      // lock_shared() blocks until main thread unlock()s.
+      {
+        std::shared_lock lk{done_mutex};
+        ++done_count;
+      }
+
+      // Contesting all of [try_]lock[_shared] doesn't explode.
+      for (int i = 0; i != REPEAT_COUNT; ++i)
+      {
+        while (!work_mutex.try_lock())
+        {
+          try_lock_fail_count.fetch_add(1, std::memory_order_relaxed);
+        }
+        work_mutex.unlock();
+
+        while (!work_mutex.try_lock_shared())
+        {
+          try_lock_shared_fail_count.fetch_add(1, std::memory_order_relaxed);
+        }
+        work_mutex.unlock_shared();
+        {
+          std::lock_guard lk{work_mutex};
+          ++additional_work_count;
+        }
+        std::shared_lock lk{work_mutex};
+      }
+    }};
+  }
+
+  // lock() waits for threads to unlock_shared().
+  {
+    std::lock_guard lk{work_mutex};
+    EXPECT_EQ(work_count.load(std::memory_order_relaxed), THREAD_COUNT);
+  }
+
+  std::this_thread::sleep_for(SLEEP_TIME);
+
+  // The threads are still blocking on done_mutex.
+  EXPECT_EQ(done_count, 0);
+
+  done_lk.unlock();
+  std::ranges::for_each(threads, &std::thread::join);
+
+  // The threads finished.
+  EXPECT_EQ(done_count, THREAD_COUNT);
+
+  EXPECT_EQ(additional_work_count, TOTAL_ITERATIONS);
+
+  GTEST_LOG_(INFO) << "try_lock() failure %: "
+                   << (try_lock_fail_count * 100.0 / (TOTAL_ITERATIONS + try_lock_fail_count));
+
+  GTEST_LOG_(INFO) << "try_lock_shared() failure %: "
+                   << (try_lock_shared_fail_count * 100.0 /
+                       (TOTAL_ITERATIONS + try_lock_shared_fail_count));
+
+  // Things are still sane after contesting in worker threads.
+  done_lk.lock();
+  std::lock_guard lk{work_mutex};
+}