LLSingleton explicitly supports circular dependencies: initialization
performed during an LLSingleton subclass's initSingleton() method may
recursively call that same subclass's getInstance() method. On the other hand,
circularity from a subclass constructor cannot be permitted, else
getInstance() would have to return a partially-constructed object.
Our dependency tracking circularity check initially forbade both. Loosen it to
permit references from within initSingleton().
Part of LLError's logging infrastructure is implemented with an LLSingleton.
Therefore, attempts to log from within LLSingleton machinery could potentially
go south if LLError's LLSingleton is not yet initialized.
Introduce LLError::is_available() in llerrorcontrol.h and llerror.cpp.
Make LLSingletonBase::logwarns() and logerrs() consult LLError::is_available()
before attempting to use LL_WARNS or LL_ERRS, respectively.
Moreover, make all LLSingleton internal logging use logwarns() and logerrs()
instead of directly engaging LL_ERRS or LL_WARNS.
Introduce LLSingleton::cleanupSingleton() canonical method as the place to put
any subclass cleanup logic that might take nontrivial realtime or throw an
exception. Neither is appropriate in a destructor.
Track all extant LLSingleton subclass instances on a master list, which
permits adding LLSingletonBase::cleanupAll() and deleteAll() methods.
Also notice when any LLSingleton subclass constructor (or initSingleton()
method) calls instance() or getInstance() for another LLSingleton, and capture
that other LLSingleton instance as a dependency of the first. This permits
cleanupAll() and deleteAll() to perform a dependency sort on the master list,
thus cleaning up (or deleting) leaf LLSingletons AFTER the LLSingletons that
depend on them.
Make C++ runtime's final static destructor call LLSingletonBase::deleteAll()
instead of deleting individual LLSingleton instances in arbitrary order.
Eliminate "llerror.h" from llsingleton.h, a longstanding TODO.
Nat Goodspeed