[jira] [Created] (MESOS-4994) Overloads for defering member function invocations match too tightly

["Benjamin Bannier (JIRA)" <ji...@apache.org>]

Benjamin Bannier created MESOS-4994:
---------------------------------------

             Summary: Overloads for defering member function invocations match too tightly
                 Key: MESOS-4994
                 URL: https://issues.apache.org/jira/browse/MESOS-4994
             Project: Mesos
          Issue Type: Bug
          Components: libprocess
            Reporter: Benjamin Bannier
            Priority: Minor


libprocess' {{defer}} provides the following overloads

{code}
Deferred<void()> defer(const Process<T>& process, void (T::*method)())
Deferred<void()> defer(const Process<T>* process, void (T::*method)())
Deferred<Future<R>()> defer(const PID<T>& pid, Future<R> (T::*method)())
Deferred<Future<R>()> defer(const Process<T>& process, Future<R> (T::*method)())
Deferred<void()> defer(const PID<T>& pid, void (T::*method)())
Deferred<Future<R>()> defer(const PID<T>& pid, R (T::*method)())
Deferred<Future<R>()> defer(const Process<T>& process, R (T::*method)())
Deferred<Future<R>()> defer(const Process<T>* process, R (T::*method)())
Deferred<Future<R>()> defer(const Process<T>* process, Future<R> (T::*method)())
_Deferred<Future<R>()> defer(const UPID& pid, F&& f)
{code}

Here all overloads but the last require that the first argument is (in a LSP
sense) either a {{Process<T>}} or a {{PID<T>}}, and only member functions of
{{T}} are supported. Consider the following setup,
{code}
struct Base : public Process<Base> {
  double b() { return 0; } // non-const due to MESOS-XXXX
};

struct Derived : public Base {
  double d() { return 0; } // non-const due to MESOS-XXXX
};
{code}

We can then {{defer}} for a {{Base}} like
{code}
defer(base, &Base::b);
{code}
which invokes an overload taking {{const Process<T>&}} and {{R (T::*method)()}}, but on the other hand use of {{Derived}} is more cumbersome,
{code}
defer(derived.self(), &Derived::d);
{code}
This effectively performs an explicit cast of the {{Derived}} to a {{PID<Derived>}} so the overload taking {{const PID<T>&}} and {{R (T::*method)()}} can be taken. The overload taking for the {{Base}} case cannot be taken here since while {{Derived}} is a {{Process<Base>}}, there exists no {{&Base::d}} (like one would have expected).

We should investigate ways to allow the same {{self}}-less {{defer}} syntax for both {{Base}} and {{Derived}}-like classes. This might be possible by decoupling the types {{T}} for the first and second arguments.





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