Implement ScopeLock and use in libPS/libPS

New ScopeLock Class

The ScopeLock class provides a RAII interface for obtaining a lock on a resource. It works with objects that implement the new ILockable interface. The object to be locked for the lifetime of the ScopeLock instance is a raw pointer. This allows the usage of ScopeLock inside the target ILockable object itself.

An example of usage is:

bool doStuff(std::shared_ptr<ILockable> dev)
{
    ScopeLock lock(dev); // contructor will call dev.get() to get the raw pointer
    if (!dev->action1())
        return false;

    if (!dev->action2())
        return false;

    return true;
}

This addresses issue #65.

New ILockable Interface

The ILockable interface generalizes the interface for requesting exclusive access to an object via the lock/unlock functions. The interface documentation describes how the resource access lock should behave, but does not implement it. The rules are the same as for the current ICom classes.

Calling lock gives user exclusive access to the device represented by the class across multiple processes. Calling unlock releases that access.

Multiple calls to lock should increment a counter, which is then decremented by a call to unlock. Only once the counter has reached 0 should the access be released.

The ICom class now inherits from ILockable.

Transition libCom and libPS to ScopeLock

All classes in libCom and libPS now use ScopeLock for obtaining exclusive access to a communication object corresponding to a power supply.

Add spam_powersupply Example

The spam_powersupply "example" can be used for testing the multi-process safety of a power supply implementation. It works by setting a random number to the output voltage level to a channel and the reading that value back out continuously. If the read value does not match the set value, then an error is printed.

The idea is to run several instances of spam_powersupply at the same time, each one addressing a different channel of a multi-channel power supply. Thus is the reading of channel A interferes with reading of channel B, then the read value of channel B will be wrong.