Import: module 'CountingSemaphore' from libpEpAdapter

src/CountingSemaphore.hh

@@ -0,0 +1,69 @@
+// This file is under GNU General Public License 3.0
+// see LICENSE.txt
+
+#ifndef LIBPEPADAPTER_COUNTING_SEMAPHORE_HH
+#define LIBPEPADAPTER_COUNTING_SEMAPHORE_HH
+
+#include <mutex>
+#include <condition_variable>
+#include <atomic>
+
+namespace pEp {
+    class CountingSemaphore {
+        std::mutex mtx;
+        std::condition_variable cv;
+
+        // To synchronize threads, ALWAYS use <atomic> types
+        std::atomic_uint _count;
+
+    public:
+        CountingSemaphore(unsigned count = 0) : _count(count) {}
+
+        // Change the stored count.
+        CountingSemaphore& operator=(unsigned count)
+        {
+            std::unique_lock<std::mutex> lock(mtx);
+            _count.store (count);
+            if (count != 0)
+                cv.notify_one ();
+            return *this;
+        }
+
+        // Return the current stored count.
+        unsigned load()
+        {
+            return _count.load ();
+        }
+
+        // Atomically decrement the stored count, blocking in case the count
+        // is currently zero.  When the method terminates the counter is
+        // guaranteed to have been decremented by one unit.
+        //
+        // This is Dijkstra's P operation, used to atomically acquire a resource.
+        void p()
+        {
+            std::unique_lock<std::mutex> lock(mtx);
+            // FIXME: is the loop even needed?  Any received notification will
+            // wake up only one thread, which will see the count as non-zero...
+            // I guess an if conditional would work just as well as a while
+            // loop, but I will follow the canonical pattern just in case some
+            // new bizarre operation other than V is added later.
+            while (_count.load() == 0)
+                cv.wait(lock);
+            _count --;
+        }
+
+        // Atomically increment the stored count.  This may wake up one thread
+        // which is currently executing the p method.
+        //
+        // This is Dijkstra's V operation, used to atomically release a resource.
+        void v()
+        {
+            std::unique_lock<std::mutex> lock(mtx);
+            _count ++;
+            cv.notify_one();
+        }
+    };
+} // namespace pEp
+
+#endif // LIBPEPADAPTER_COUNTING_SEMAPHORE_HH