core: add 'latch' synchronizer

The synchronizer allows any thread to block on it until it is
unlocked. It is unlocked once count_down() has been called given
number of times.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>

include/osv/latch.hh

+#ifndef _OSV_LATCH_HH
+#define _OSV_LATCH_HH
+
+#include <condition_variable>
+#include <mutex>
+
+class latch
+{
+private:
+    int _count;
+    std::mutex _mutex;
+    std::condition_variable _condvar;
+public:
+    latch(int count)
+        : _count(count)
+    {
+    }
+
+    void count_down()
+    {
+        std::lock_guard<std::mutex> l(_mutex);
+        if (--_count == 0) {
+            _condvar.notify_all();
+        }
+    }
+
+    void await()
+    {
+        std::unique_lock<std::mutex> l(_mutex);
+        while (_count > 0) {
+            _condvar.wait(l);
+        }
+    }
+};
+
+#endif