Fix S2583/2589 FP: Return inside lock and using causes FP after the block

[pavel-mikula-sonarsource]

We have FP when flow exists with return from within the explicit (try/finally) or implicit (foreach, lock, using) TryRegion/FinallyRegion.

Flow if(a)

The return true visits finally block with flag=true state, and then goes to exit block.

Flow if(b)

flag is set to false, normal exit of implicit try/finally visits the finally block with flag=false state,and then goes to exit block.

Flow with b==false

flag remains true and normal exist of implicit try/finally visits the finally block. And because this state has already been visited, it's thrown away.

ToDo

Take closer look on why it doesn't reproduce on the try/finally scenario, that's part of the reproducer. It could involve LVA, where the a is or is not disposed properly.

Possible solution

To fix this, we need to consider what are the finally blocks that should be visited after as a part of the unique key.

We need to consider FinallyPoint of the node, including all FinallyPoint.Previous nodes, as the path could be different in nested scenarios.

public bool Method(bool a, bool b)
{
    bool flag = true;
    lock (Lock)
    {
        if (a)
        {
            return true;
        }
        if (b)
        {
            flag = false;
        }
    }
    if (flag)           // Noncompliant FP
    {
        return true;    // Secondary FP
    }
    else
    {
        return false;
    }
}