add call_with_lock(). Moved from Json Adapter.

call_with_lock.cc

@@ -0,0 +1,8 @@
+// this file is under GNU GPL 3.0, see LICENSE.txt
+
+#include "call_with_lock.hh"
+
+namespace pEp
+{
+	std::mutex call_with_lock_mutex;
+}

call_with_lock.hh

@@ -0,0 +1,25 @@
+// this file is under GNU GPL 3.0, see LICENSE.txt
+
+#ifndef PEP_CALL_WITH_LOCK_HH
+#define PEP_CALL_WITH_LOCK_HH
+
+#include <mutex>
+
+namespace pEp
+{
+	extern std::mutex call_with_lock_mutex;
+
+	// TODO: use && and std::forward<> to avoid copying of the arguments.
+	// It is not relevant, yet, because at the moment we use this function template only
+	// for init() and release() which have cheap-to-copy pointer parameters only
+	template<class R, class... Args>
+	R call_with_lock( R(*fn)(Args...), Args... args)
+	{
+		std::lock_guard<std::mutex> L(call_with_lock_mutex);
+		return fn(args...);
+	}
+
+
+}
+
+#endif // PEP_CALL_WITH_LOCK_HH