Re-init ServiceRegistry on every Initialize() call

[ausbin]

Without this, the sequence of calls

xacc::Initialize()
xacc::Finalize()
xacc::Initialize()

will cause CppMicroservices to throw the following:

terminate called after throwing an instance of 'std::runtime_error'
  what():  The bundle context is no longer valid

Testing

I ran ctest and did some end-to-end testing with QIR-EE.

[danclaudino]

Thanks @ausbin. This seems such an edge case. I'm curious when one would ever use/need the following.

xacc::Initialize()
xacc::Finalize()
xacc::Initialize()

[sethrj]

release gives up the pointer for someone else to hold on to (in this case, it's leaking memory). I think you want reset or = {} or = nullptr.

[ausbin]

Thanks, will fix.

I also will look into why the tests are segfaulting. I wonder if something is accessing the service registry after finalize()

[ausbin]

Good point @danclaudino. Is that sequence of calls even supported? I assumed so, but if you are only supposed to call xacc::Initialize() and xacc::Finalize() once per process (understandable), then we should fix QIR-EE instead. Because right now, XaccQuantum in QIR-EE calls xacc::Initialize() and xacc::Finalize() each time it is constructed and destructed (respectively)

If this is meant to be supported, I will investigate the Python segfault further (I looked a bit and some Python code is indeed touching the ServiceRegistry before xacc::Initialize()). Otherwise, I will close this PR.

[sethrj]

This seems such an edge case. I'm curious when one would ever use/need the following.

xacc::Initialize()
xacc::Finalize()
xacc::Initialize()

@danclaudino Is the expectation that you only execute one quantum circuit for every use of XACC? Or that you're supposed to assume that you're the only library in a larger toolchain that uses XACC (and thus only your library will ever call init/finalize)? If Finalize doesn't finalize, why bother having it at all? Maybe it would be better to just "initialize" the global variables once and then have an if (initialized) return; in the initialize function to prevent subsequent errors.

The usual reason to have multiple initialize/finalize is to reset states for reproducibility, unit testing, etc.