[boost] futures, deadlocking and mutual recursion.

From: Achilleas Margaritis (axilmar_at_[hidden])
Date: 2007-12-01 19:43:00

Isn't there a deadlock issue with futures? suppose I have the following 
code:

class A {
     int data;

public:
     int getData() const {
         return data;
     }

     void someAction(B *b) {
         future<int> i = bind(b, &B::someAction, this);
         data = i.wait() + 1;
     }
};

class B {
     int data;

public:
     int someAction(A *a) {
         future<int> j = bind(a, &A::getData);
         return j.wait() + 1;
     }
};

int main() {
      A *a = new A;
      B *b = new B;
      a->someAction(b);
}

The above contains a deadlock:

class A builds a future 'i' in function 'someAction' over class B, then 
it waits for B's method 'someAction' to wait.

class B, when its method 'someAction' is invoked, builds a future 'j' 
over method 'getData' of A...then it tries to evaluate 'j'.

At this point, the instance of A is blocked waiting for B to finish its 
processing, and B waits for A to finish its processing, thus deadlocking 
the program.

My question is: is this a real problem, and if so, how is it handled by 
boost::futures?

Another way to introduce the deadlock is by mutual recursion using futures.