fixed theoretical issue when deferredGet onSize() methods destructs the underlying channel object

include/channelimpl.h

@@ -598,7 +598,7 @@ public:
         Monitor monitor(this);
         
         // copy the callback (so that it will not be destructed during
-        // the "reportSuccess" call
+        // the "reportSuccess" call, if the channel is destructed during the call)
         auto cb = _oldestCallback;
 
         // call the callback

src/deferredget.cpp

@@ -27,10 +27,10 @@ const std::shared_ptr<Deferred> &DeferredGet::reportSuccess(uint32_t messageCoun
     // we grab a self pointer to ensure that the deferred object stays alive
     auto self = shared_from_this();
 
-    // report the size (technically, the channel object could be destructed now, but we ignore that case)
-    if (_sizeCallback) _sizeCallback(messageCount);
-    
-    // we now know the name, so we can install the message callback on the channel
+    // we now know the name, so we can install the message callback on the channel, the self
+    // pointer is also captured, which ensures that 'this' is not destructed, all members stay
+    // accessible, and that the onFinalize() function will only be called after the message
+    // is reported (onFinalize() is called from the destructor of this DeferredGet object)
     _channel->install("", [self, this](const Message &message, uint64_t deliveryTag, bool redelivered) {
 
         // install a monitor to deal with the case that the channel is removed
@@ -43,6 +43,10 @@ const std::shared_ptr<Deferred> &DeferredGet::reportSuccess(uint32_t messageCoun
         if (monitor.valid()) _channel->uninstall("");
     });
 
+    // report the size (note that this is the size _minus_ the message that is retrieved
+    // (and for which the callback will be called later), so it could be zero)
+    if (_sizeCallback) _sizeCallback(messageCount);
+    
     // return next object
     return _next;
 }