Never call unknown code while holding a lock (e.g., a callback)

Reason

If you don't know what a piece of code does, you are risking deadlock.

Example

void do_this(Foo* p)
{
    lock_guard<mutex> lck {my_mutex};
    // ... do something ...
    p->act(my_data);
    // ...
}

If you don't know what Foo::act does (maybe it is a virtual function invoking a derived class member of a class not yet written), it might call do_this (recursively) and cause a deadlock on my_mutex. Maybe it will lock on a different mutex and not return in a reasonable time, causing delays to any code calling do_this.

Example

A common example of the "calling unknown code" problem is a call to a function that tries to gain locked access to the same object. Such problem can often be solved by using a recursive_mutex. For example:

recursive_mutex my_mutex;

template<typename Action>
void do_something(Action f)
{
    unique_lock<recursive_mutex> lck {my_mutex};
    // ... do something ...
    f(this);    // f will do something to *this
    // ...
}

If, as it is likely, f() invokes operations on *this, we must make sure that the object's invariant holds before the call.

Enforcement

• Flag calling a virtual function with a non-recursive mutex held
• Flag calling a callback with a non-recursive mutex held