BUG: Improve thead safety of ObjectFactoryBase::Initialize

[blowekamp]

Block concurent threads until initializaiton is completed.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Added license to new files (if any)
• Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5
• Added ITK examples for all new major features (if any)

Refer to the ITK Software Guide for
further development details if necessary.

[N-Dekker]

Looks like you're doing double-checked locking, which is considered an anti-pattern nowadays! If one thread sets m_PimplGlobals->m_Initialized = true while at the same time, another thread tries to evaluate m_PimplGlobals->m_Initialized outside the locked region, we have undefined behavior.

[blowekamp]

What is the updated pattern? To ensure it's only initialized once and block other threads until it it initialed?

[dzenanz]

std::call_once?

[blowekamp]

My bigger concern now is if using std::mutex during static initialization is OK aka ( defined behavior ).

evaluate m_PimplGlobals->m_Initialized outside the locked region, we have undefined behavior.

Not sure how undefined the undefined read behavior would be, maybe just not 100% synced between threads. It's will either be true or false, either will be OK.

[N-Dekker]

std::call_once?

Thanks, @dzenanz. I just gave it a try, using std::call_once. Please check! (Still draft!)

• WIP: Use call_once for dynamic loading ObjectFactoryBase::Initialize() #4227

[N-Dekker]

Please consider removing the ObjectFactoryBase::InitializeFactoryList() call, as in:

• STYLE: Remove private ObjectFactoryBase member InitializeFactoryList() #4225

[N-Dekker]

Would it be possible to move this code (calling ObjectFactoryBase::RegisterInternal() and ObjectFactoryBase::LoadDynamicFactories()) to the default-constructor of PimplGlobals?

ITK/Modules/Core/Common/src/itkObjectFactoryBase.cxx

Lines 244 to 247 in c3a8eab

	ObjectFactoryBase::RegisterInternal();
	#if defined(ITK_DYNAMIC_LOADING) && !defined(ITK_WRAPPING)
	ObjectFactoryBase::LoadDynamicFactories();
	#endif

[blowekamp]

4227 looks like a better approach.