libutil: allow graceful dropping of Pool::Handle

not needed yet, but returning a resource from the exception handling
path that has ownership of a handle is currently not well-supported.
we could also add a default constructor to Handle, but then we would
also need to change the pool reference to a pointer. eventually that
should be done since now resources can be swapped between pools with
clever moves, but since that's not a problem yet we won't do it now.

Change-Id: I26eb06581f7be34569e9e67a33da736128d167af

src/libutil/pool.hh

@@ -108,6 +108,19 @@ public:
 
         Handle(Pool & pool, std::shared_ptr<R> r) : pool(pool), r(r) { }
 
+        void drop(bool stillValid)
+        {
+            {
+                auto state_(pool.state.lock());
+                if (stillValid)
+                    state_->idle.emplace_back(std::move(r));
+                assert(state_->inUse);
+                state_->inUse--;
+            }
+            pool.wakeup.notify_one();
+            r = nullptr;
+        }
+
     public:
         Handle(Handle && h) : pool(h.pool), r(h.r) { h.r.reset(); }
 
@@ -115,15 +128,13 @@ public:
 
         ~Handle()
         {
-            if (!r) return;
-            {
-                auto state_(pool.state.lock());
-                if (!std::uncaught_exceptions())
-                    state_->idle.push_back(ref<R>(r));
-                assert(state_->inUse);
-                state_->inUse--;
+            if (r)
+                drop(std::uncaught_exceptions() == 0);
         }
-            pool.wakeup.notify_one();
+
+        void release()
+        {
+            drop(true);
         }
 
         R * operator -> () { return &*r; }