add wait-if for waiter

include/waiter.h

@@ -10,7 +10,7 @@ public:
     explicit waiter(char const * name);
     waiter(waiter&& rhs);
 
-    ~waiter();
+    virtual ~waiter();
 
     void swap(waiter& rhs);
     waiter& operator=(waiter rhs);
@@ -22,9 +22,14 @@ public:
     void close();
 
     bool wait();
+    bool wait_if_pred();
+
     bool notify();
     bool broadcast();
 
+protected:
+    virtual bool pred();
+
 private:
     class waiter_;
     waiter_* p_;

src/platform/waiter_linux.h

@@ -199,7 +199,9 @@ public:
 
     void broadcast(handle_t h) {
         if (h == invalid()) return;
-        sem_.post([this] { return counter_.load(std::memory_order_relaxed); });
+        sem_.post([this] {
+            return counter_.load(std::memory_order_relaxed);
+        });
     }
 };
 

src/waiter.cpp

@@ -67,6 +67,14 @@ bool waiter::wait() {
     return impl(p_)->w_.wait_if([] { return true; });
 }
 
+bool waiter::pred() {
+    return true;
+}
+
+bool waiter::wait_if_pred() {
+    return impl(p_)->w_.wait_if([this] { return pred(); });
+}
+
 bool waiter::notify() {
     return impl(p_)->w_.notify();
 }