[libcxx-commits] [libcxx] [libc++] Refactor tests for std::condition_variable (PR #91530)

[via libcxx-commits]

================
@@ -20,82 +19,142 @@
 //              Predicate pred);
 
 #include <condition_variable>
+#include <atomic>
+#include <cassert>
+#include <chrono>
 #include <mutex>
 #include <thread>
-#include <chrono>
-#include <cassert>
 
 #include "make_test_thread.h"
 #include "test_macros.h"
 
-class Pred
-{
-    int& i_;
-public:
-    explicit Pred(int& i) : i_(i) {}
-
-    bool operator()() {return i_ != 0;}
-};
-
-std::condition_variable cv;
-std::mutex mut;
-
-int test1 = 0;
-int test2 = 0;
-
-int runs = 0;
-
-void f()
-{
-    typedef std::chrono::system_clock Clock;
-    typedef std::chrono::milliseconds milliseconds;
-    std::unique_lock<std::mutex> lk(mut);
-    assert(test2 == 0);
-    test1 = 1;
-    cv.notify_one();
-    Clock::time_point t0 = Clock::now();
-    bool r = cv.wait_for(lk, milliseconds(250), Pred(test2));
-    ((void)r); // Prevent unused warning
-    Clock::time_point t1 = Clock::now();
-    if (runs == 0)
-    {
-        assert(t1 - t0 < milliseconds(250));
-        assert(test2 != 0);
-    }
-    else
-    {
-        assert(t1 - t0 - milliseconds(250) < milliseconds(50));
-        assert(test2 == 0);
-    }
-    ++runs;
+template <class Function>
+std::chrono::microseconds measure(Function f) {
+  std::chrono::high_resolution_clock::time_point start = std::chrono::high_resolution_clock::now();
+  f();
+  std::chrono::high_resolution_clock::time_point end = std::chrono::high_resolution_clock::now();
+  return std::chrono::duration_cast<std::chrono::microseconds>(end - start);
 }
 
-int main(int, char**)
-{
-    {
-        std::unique_lock<std::mutex>lk(mut);
-        std::thread t = support::make_test_thread(f);
-        assert(test1 == 0);
-        while (test1 == 0)
-            cv.wait(lk);
-        assert(test1 != 0);
-        test2 = 1;
-        lk.unlock();
-        cv.notify_one();
-        t.join();
-    }
-    test1 = 0;
-    test2 = 0;
-    {
-        std::unique_lock<std::mutex>lk(mut);
-        std::thread t = support::make_test_thread(f);
-        assert(test1 == 0);
-        while (test1 == 0)
-            cv.wait(lk);
-        assert(test1 != 0);
-        lk.unlock();
-        t.join();
-    }
+int main(int, char**) {
+  // Test unblocking via a call to notify_one() in another thread.
+  //
+  // To test this, we set a very long timeout in wait_for() and we try to minimize
+  // the likelihood that we got awoken by a spurious wakeup by updating the
+  // likely_spurious flag only immediately before we perform the notification.
+  {
+    std::atomic<bool> ready           = false;
+    std::atomic<bool> likely_spurious = true;
+    auto timeout                      = std::chrono::seconds(3600);
+    std::condition_variable cv;
+    std::mutex mutex;
+
+    std::thread t1 = support::make_test_thread([&] {
+      std::unique_lock<std::mutex> lock(mutex);
+      auto elapsed = measure([&] {
+        ready       = true;
+        bool result = cv.wait_for(lock, timeout, [&] { return !likely_spurious; });
+        assert(result); // return value should be true since we didn't time out
+      });
+      assert(elapsed < timeout);
+    });
+
+    std::thread t2 = support::make_test_thread([&] {
+      while (!ready) {
+        // spin
+      }
+
+      // Acquire the same mutex as t1. This ensures that the condition variable has started
+      // waiting (and hence released that mutex). We don't actually need to hold the lock, we
+      // simply use it as a signal that the condition variable has started waiting.
+      std::unique_lock<std::mutex> lock(mutex);
+      lock.unlock();
+
+      likely_spurious = false;
+      cv.notify_one();
+    });
+
+    t2.join();
+    t1.join();
+  }
+
+  // Test unblocking via a timeout.
+  //
+  // To test this, we create a thread that waits on a condition variable with a certain
+  // timeout, and we never awaken it. The "stop waiting" predicate always returns false,
+  // which means that we can't get out of the wait via a spurious wakeup.
+  {
+    auto timeout = std::chrono::milliseconds(250);
+    std::condition_variable cv;
+    std::mutex mutex;
+
+    std::thread t1 = support::make_test_thread([&] {
+      std::unique_lock<std::mutex> lock(mutex);
+      auto elapsed = measure([&] {
+        bool result = cv.wait_for(lock, timeout, [] { return false; }); // never stop waiting (until timeout)
+        assert(!result); // return value should be false since the predicate returns false after the timeout
+      });
+      assert(elapsed >= timeout);
+    });
+
+    t1.join();
+  }
+
+  // Test unblocking via a spurious wakeup.
+  //
+  // To test this, we set a fairly long timeout in wait_for() and we basically never
+  // wake up the condition variable. This way, we are hoping to get out of the wait
+  // via a spurious wakeup.
+  //
+  // However, since spurious wakeups are not required to even happen, this test is
+  // only trying to trigger that code path, but not actually asserting that it is
+  // taken. In particular, we do need to eventually ensure we get out of the wait
+  // by standard means, so we actually wake up the thread at the end.
+  {
+    std::atomic<bool> ready  = false;
+    std::atomic<bool> awoken = false;
+    auto timeout             = std::chrono::seconds(3600);
+    std::condition_variable cv;
+    std::mutex mutex;
+
+    std::thread t1 = support::make_test_thread([&] {
+      std::unique_lock<std::mutex> lock(mutex);
+      auto elapsed = measure([&] {
+        ready       = true;
+        bool result = cv.wait_for(lock, timeout, [&] { return true; });
+        awoken      = true;
+        assert(result); // return value should be true since we didn't time out
+      });
+      assert(elapsed < timeout); // can technically fail if t2 never executes and we timeout, but very unlikely
+    });
+
+    std::thread t2 = support::make_test_thread([&] {
+      while (!ready) {
+        // spin
+      }
+
+      // Acquire the same mutex as t1. This ensures that the condition variable has started
+      // waiting (and hence released that mutex). We don't actually need to hold the lock, we
+      // simply use it as a signal that the condition variable has started waiting.
+      std::unique_lock<std::mutex> lock(mutex);
+      lock.unlock();
+
+      // Give some time for t1 to be awoken spuriously so that code path is used.
+      std::this_thread::sleep_for(std::chrono::seconds(1));
+
+      // We would want to assert that the thread has been awoken after this time,
+      // however nothing guarantees us that it ever gets spuriously awoken, so
+      // we can't really check anything. This is still left here as documentation.
+      assert(awoken || !awoken);
----------------
huixie90 wrote:

Although unlikely but technically this assert can still fail if the awoken is changed between two atomic loads. Maybe just store the load locally?, or just keep the lock until the end of this line. 

https://github.com/llvm/llvm-project/pull/91530