https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/snprintf-snprintf-snprintf-l-snwprintf-snwprintf-l?view=vs-2017
"Beginning with the UCRT in Visual Studio 2015 and Windows 10, snprintf is no
longer identical to _snprintf. The snprintf function behavior is now C99
standard compliant."

In other words, VS 2015 et ff. snprintf() now promises to nul-terminate the
buffer even in the overflow case, which is what snprintf_hack::snprintf() was
for.

This removal was motivated by ambiguous-call errors generated by VS 2017 for
library snprintf() vs. snprintf_hack::snprintf().

Before this change, you had to literally pass LLSD::emptyArray() to get no-op
behavior.

LLStoreListener is an adapter initialized with a reference to an LLEventPump
on which to listen, a reference to a variable into which to store received
data, and an optional llsd::drill() path to extract desired data from each
event received on the subject LLEventPump.

In effect, LLStoreListener is like a miniature LLEventAPI whose only operation
is to store to its destination variable.

We include both const and non-const overloads. The latter returns LLSD&, so
you can assign to the located element.

In fact we already implemented the non-const logic in a less public form as
storeToLLSDPath() in lleventcoro.cpp. Reimplement the latter to use the new
llsd::drill() function.

Remove call from LLAppViewer::cleanup().

Instead, make each LLSingleton<T>::deleteSingleton() call cleanupSingleton()
just before destroying the instance. Since deleteSingleton() is not a
destructor, it's fine to call cleanupSingleton() from there; and since
deleteAll() calls deleteSingleton() on every remaining instance, the former
cleanupAll() functionality has been subsumed into deleteAll().

Since cleanupSingleton() is now called at exactly one point in the instance's
lifetime, we no longer need a bool indicating whether it has been called.

The previous protocol of calling cleanupAll() before deleteAll() implemented a
two-phase cleanup strategy for the application. That is no longer needed.
Moreover, the cleanupAll() / deleteAll() sequence created a time window during
which individual LLSingleton<T> instances weren't usable (to the extent that
their cleanupSingleton() methods released essential resources) but still
existed -- so a getInstance() call would return the crippled instance rather
than recreating it.

Remove cleanupAll() calls from tests; adjust to new order of expected side
effects: instead of A::cleanupSingleton(), B::cleanupSingleton(), ~A(), ~B(),
now we get A::cleanupSingleton(), ~A(), B::cleanupSingleton(), ~B().

Use function-static LLMutex instances instead of module-static instances,
since some log calls are evidently issued before we get around to initializing
llerror.cpp module-static variables.

instead of deleteSingleton().

Specifically, clear static SingletonData and remove the instance from the
MasterList in the destructor.

Empirically, some consumers are manually deleting LLSingleton instances,
instead of calling deleteSingleton(). If deleteSingleton() handles cleanup
rather than the destructor, we're left with dangling pointers in the Master
List.

We don't also call cleanupSingleton() from the destructor because only
deleteSingleton() promises to call cleanupSingleton(). Hopefully whoever is
directly deleting an LLSingleton subclass instance isn't relying on
cleanupSingleton().

When calling LLParamSingleton::initParamSingleton() on a secondary thread, use
LLMainThreadTask::dispatch() to construct the instance on the main thread --
as with LLSingleton::getInstance().

So does LLLockedSingleton<T>::construct().

Given the viewer's mutually-dependent LLSingletons, given that different
threads might simultaneously request different LLSingletons from such a chain
of circular dependencies, the key to avoiding deadlock is to serialize all
LLSingleton construction on one thread: the main thread. Add comments to
LLSingleton::getInstance() explaining the problem and the solution.

Recast LLSingleton's static SingletonData to use LockStatic. Instead of using
Locker, and simply trusting that every reference to sData is within the
dynamic scope of a Locker instance, LockStatic enforces that: you can only
access SingletonData members via LockStatic.

Reorganize the switch in getInstance() to group the CONSTRUCTING error, the
INITIALIZING/INITIALIZED success case, and the DELETED/UNINITIALIZED
construction case.

When [re]constructing an instance, on the main thread, retain the lock and
call constructSingleton() (and capture_dependency()) directly.

On a secondary thread, unlock LockStatic and use LLMainThreadTask::dispatch()
to call getInstance() on the main thread. Since we might end up enqueuing
multiple such tasks, it's important to let getInstance() notice when the
instance has already been constructed and simply return the existing pointer.

Add loginfos() method, sibling to logerrs(), logwarns() and logdebugs().
Produce loginfos() messages when dispatching to the main thread, when actually
running on the main thread and when resuming the suspended requesting thread.

Make deleteSingleton() manage all associated state, instead of delegating some
of that work to ~LLSingleton(). Now, within LockStatic, extract the instance
pointer and set state to DELETED; that lets subsequent code, which retains the
only remaining pointer to the instance, remove the master-list entry, call the
subclass cleanupSingleton() and destructor without needing to hold the lock.

In fact, entirely remove ~LLSingleton().

Import LLSingletonBase::cleanup_() method to wrap the call to subclass
cleanupSingleton() in try/catch.

Remove cleanupAll() calls from llsingleton_test.cpp, and reorder the success
cases to reflect the fact that T::cleanupSingleton() is called immediately
before ~T() for each distinct LLSingleton subclass T.

When getInstance() on a secondary thread dispatches to the main thread, it
necessarily unlocks its LockStatic lock. But an LLSingleton dependency chain
strongly depends on the function stack on which getInstance() is invoked --
the task dispatched to the main thread doesn't know the dependencies tracked
on the requesting thread stack. So, once the main thread delivers the instance
pointer, the requesting thread captures its own dependencies for that
instance.

Back in the requesting thread, obtaining the current EInitState to pass to
capture_dependencies() would have required relocking LockStatic. Instead, I've
convinced myself that (a) capture_dependencies() only wanted to know
EInitState to produce an error for CONSTRUCTING, and (b) in CONSTRUCTING
state, we never get as far as capture_dependencies() because getInstance()
produces an error first.

Eliminate the EInitState parameter from all capture_dependencies() methods.
Remove the LLSingletonBase::capture_dependency() stanza that tested
EInitState. Make the capture_dependencies() variants that accepted LockStatic
instead accept LLSingletonBase*. That lets getInstance(), in the
LLMainThreadTask case, pass the newly-returned instance pointer.

For symmetry, make pop_initializing() accept LLSingletonBase* as well, instead
of accepting LockStatic and extracting mInstance.

The Windows implementation of demangle() assumed that a "mangled" class name
produced by typeid(class).name() always starts with the prefix "class ",
checked for that and removed it. If the mangled name didn't start with that
prefix, it would emit a debug message and return the full name.

When the class in question is actually a struct, the prefix is "struct "
instead. But when demangle() was being called before logging had been fully
initialized, the debug message remarking that it didn't start with "class "
crashed.

Look for either "class " or "struct " prefix. Remove whichever is found and
return the rest of the name. If neither is found, only log if logging is
available.

Monty's code review reveals that conflating dispatch() with [un]lock
functionality is inconsistent and unnecessary.

If already running on the main thread, LLMaintThreadTask simply runs the work
inline. Otherwise it queues it for the main thread using LLEventTimer, using
std::future to retrieve the result.

LLThread::currentID() used to return a U32, a distinct unsigned value
incremented by explicitly constructing LLThread or by calling LLThread::
registerThreadID() early in a thread launched by other means. The latter
imposed an unobvious requirement on new code based on std::thread. Using
std::thread::id instead delegates to the compiler/library the problem of
distinguishing threads launched by any means.

Change lots of explicit U32 declarations. Introduce LLThread::id_t typedef to
avoid having to run around fixing uses again if we later revisit this decision.

LLMutex, which stores an LLThread::id_t, wants a distinguished value meaning
NO_THREAD, and had an enum with that name. But as std::thread::id promises
that the default-constructed value is distinct from every valid value,
NO_THREAD becomes unnecessary and goes away.

Because LLMutex now stores LLThread::id_t instead of U32, make llmutex.h
#include "llthread.h" instead of the other way around. This makes LLMutex an
incomplete type within llthread.h, so move LLThread::lockData() and
unlockData() to the .cpp file. Similarly, remove llrefcount.h's #include
"llmutex.h" to break circularity; instead forward-declare LLMutex.

It turns out that a number of source files assumed that #include "llthread.h"
would get the definition for LLMutex. Sprinkle #include "llmutex.h" as needed.

In the SAFE_SSL code in llcorehttp/httpcommon.cpp, there's an ssl_thread_id()
callback that returns an unsigned long to the SSL library. When LLThread::
currentID() was U32, we could simply return that. But std::thread::id is very
deliberately opaque, and can't be reinterpret_cast to unsigned long.
Fortunately it can be hashed because std::hash is specialized with that type.

Add a namespaced free function in .cpp file to report LL_ERRS as needed.

Per code review, use a more indicative namespace name.

The pattern of requiring a lock to permit *any* access to a static instance of
something seems generally useful. Break out lockstatic.h; recast
LLInstanceTracker to use it.

Moving LockStatic to an external template class instead of a nested class in
LLInstanceTrackerBase leaves LLInstanceTrackerBase pretty empty. Get rid of it.

And *that* means we can move the definition of the StaticData used by each
LLInstanceTracker specialization into the class itself, rather than having to
define it beforehand in namespace LLInstanceTrackerStuff.

The CONSTRUCTED state was only briefly set between constructSingleton() and
finishInitializing(). But as no consumer code is executed between setting
CONSTRUCTED and setting INITIALIZING, it was impossible to reach the switch
statement in either getInstance() method in state CONSTRUCTED. So there was no
point in state CONSTRUCTED. Remove it.

With CONSTRUCTED gone, we only ever call finishInitializing() right after
constructSingleton(). Merge finishInitializing() into constructSingleton().

No need to capture a separate list of completed LLEventTimer instances to
delete after the primary loop, since at this point we're looping over a
snapshot and can directly delete each completed timer.

Remove warnings about LLSingleton not being thread-safe because, at this point,
we have devoted considerable effort to trying to make it thread-safe.

Add LLSingleton<T>::Locker, a nested class which both provides a function-
static mutex and a scoped lock that uses it. Instantiating Locker, which has a
nullary constructor, replaces the somewhat cumbersome idiom of declaring a
std::unique_lock<std::recursive_mutex> lk(getMutex);

This eliminates (or rather, absorbs) the typedefs and getMutex() method from
LLParamSingleton. Replace explicit std::unique_lock declarations in
LLParamSingleton methods with Locker declarations.

Remove LLSingleton<T>::SingletonInitializer nested struct. Instead of
getInstance() relying on function-static initialization to protect (only)
constructSingleton() calls, explicitly use a Locker instance to cover its
whole scope, and make the UNINITIALIZED case call constructSingleton().
Rearrange cases so that after constructSingleton(), control falls through to
the CONSTRUCTED case and the finishInitializing() call.

Use a Locker instance in other public-facing methods too: instanceExists(),
wasDeleted(), ~LLSingleton(). Make destructor protected so it can only be called
via deleteSingleton() (but must be accessible to subclasses for overrides).

Remove LLSingletonBase::get_master() and get_initializing(), which permitted
directly manipulating the master list and the initializing stack without any
locking mechanism. Replace with get_initializing_size().

Similarly, replace LLSingleton_manage_master::get_initializing() with
get_initializing_size(). Use in constructSingleton() in place of
get_initializing().size().

Remove LLSingletonBase::capture_dependency()'s list_t parameter, which
accepted the list returned by get_initializing(). Encapsulate that retrieval
within the scope of the new lock in capture_dependency().

Add LLSingleton_manage_master::capture_dependency(LLSingletonBase*, EInitState)
to forward (or not) a call to LLSingletonBase::capture_dependency(). Nullary
LLSingleton<T>::capture_dependency() calls new LLSingleton_manage_master method.

Equip LLSingletonBase::MasterList with a mutex of its own, separate from the
one donated by the LLSingleton machinery, to serialize use of MasterList data
members. Introduce MasterList::Lock nested class to lock the MasterList mutex
while providing a reference to the MasterList instance. Introduce subclasses
LockedMaster, which provides a reference to the actual mMaster master list
while holding the MasterList lock; and LockedInitializing, which does the same
for the initializing list. Make mMaster and get_initializing_() private so
that consuming code can *only* access those lists via LockedInitializing and
LockedMaster.

Make MasterList::cleanup_initializing_() private, with a LockedInitializing
public forwarding method. This avoids another call to MasterList::instance(),
and also mandates that the lock is currently held during every call.

Similarly, move LLSingletonBase::log_initializing() to a LockedInitializing
log() method.
(transplanted from dca0f16266c7bddedb51ae7d7dca468ba87060d5)

The previous implementation went to some effort to crash if anyone attempted
to create or destroy an LLInstanceTracker subclass instance during traversal.
That restriction is manageable within a single thread, but becomes unworkable
if it's possible that a given subclass might be used on more than one thread.

Remove LLInstanceTracker::instance_iter, beginInstances(), endInstances(),
also key_iter, beginKeys() and endKeys(). Instead, introduce key_snapshot()
and instance_snapshot(), the only means of iterating over LLInstanceTracker
instances. (These are intended to resemble functions, but in fact the current
implementation simply presents the classes.) Iterating over a captured
snapshot defends against container modifications during traversal. The term
'snapshot' reminds the coder that a new instance created during traversal will
not be considered. To defend against instance deletion during traversal, a
snapshot stores std::weak_ptrs which it lazily dereferences, skipping on the
fly any that have expired.

Dereferencing instance_snapshot::iterator gets you a reference rather than a
pointer. Because some use cases want to delete all existing instances, add an
instance_snapshot::deleteAll() method that extracts the pointer. Those cases
used to require explicitly copying instance pointers into a separate
container; instance_snapshot() now takes care of that. It remains the caller's
responsibility to ensure that all instances of that LLInstanceTracker subclass
were allocated on the heap.

Replace unkeyed static LLInstanceTracker::getInstance(T*) -- which returned
nullptr if that instance had been destroyed -- with new getWeak() method
returning std::weak_ptr<T>. Caller must detect expiration of that weak_ptr.

Adjust tests accordingly.

Use of std::weak_ptr to detect expired instances requires engaging
std::shared_ptr in the constructor. We now store shared_ptrs in the static
containers (std::map for keyed, std::set for unkeyed).

Make LLInstanceTrackerBase a template parameterized on the type of the static
data it manages. For that reason, hoist static data class declarations out of
the class definitions to an LLInstanceTrackerStuff namespace.

Remove the static atomic sIterationNestDepth and its methods incrementDepth(),
decrementDepth() and getDepth(), since they were used only to forbid creation
and destruction during traversal.

Add a std::mutex to static data. Introduce an internal LockStatic class that
locks the mutex while providing a pointer to static data, making that the only
way to access the static data.

The LLINSTANCETRACKER_DTOR_NOEXCEPT macro goes away because we no longer
expect ~LLInstanceTracker() to throw an exception in test programs.
That affects LLTrace::StatBase as well as LLInstanceTracker itself.

Adapt consumers to the new LLInstanceTracker API.

VS 2013 thought we were storing an initialization-list.

An exception in the LLSingleton subclass constructor, or in its
initSingleton() method, could leave the LLSingleton machinery in a bad state:
the failing instance would remain in the MasterList, also on the stack of
initializing LLSingletons. Catch exceptions in either and perform relevant
cleanup.

This problem is highlighted by test programs, in which LL_ERRS throws an
exception rather than crashing the whole process.

In the relevant catch clauses, clean up the initializing stack BEFORE logging.
Otherwise we get tangled up recording bogus dependencies.

Move capture_dependency() out of finishInitializing(): it must be called by
every valid getInstance() call, both from LLSingleton and LLParamSingleton.

Introduce new CONSTRUCTED EInitState value to distinguish "have called the
constructor but not yet the initSingleton() method" from "currently within
initSingleton() method." This is transient, but we execute the 'switch' on
state within that moment. One could argue that the previous enum used
INITIALIZING for current CONSTRUCTED, and INITIALIZED meant INITIALIZING too,
but this is clearer.

Introduce template LLSingletonBase::classname() helper methods to clarify
verbose demangle(typeid(stuff).name()) calls.

Similarly, introduce LLSingleton::pop_initializing() shorthand method.

Add try/catch clauses to constructSingleton() (to catch exceptions in the
subclass constructor) and finishInitializing() (to catch exceptions in the
subclass initSingleton() method). Each of these catch clauses rethrows the
exception -- they're for cleanup, not for ultimate handling.

Introduce LLSingletonBase::reset_initializing(list_t::size_t). The idea is
that since we can't know whether the exception happened before or after the
push_initializing() call in LLSingletonBase's constructor, we can't just pop
the stack. Instead, constructSingleton() captures the stack size before
attempting to construct the new LLSingleton subclass. On exception, it calls
reset_initializing() to restore the stack to that size.

Naturally that requires a corresponding LLSingleton_manage_master method,
whose MasterList specialization is a no-op.

finishInitializing()'s exception handling is a bit simpler because it has a
constructed LLSingleton subclass instance in hand, therefore
push_initializing() has definitely been called, therefore it can call
pop_initializing().

Break out new static capture_dependency() method from finishInitializing()
because, in the previous LLSingleton::getInstance() implementation, the logic
now wrapped in capture_dependency() was reached even in the INITIALIZED case.
TODO: Add a new EInitState to differentiate "have been constructed, now
calling initSingleton()" from "fully initialized, normal case" -- in the
latter control path we should not be calling capture_dependency().

The LLSingleton_manage_master<LLSingletonBase::MasterList> specialization's
get_initializing() function (which called get_initializing_from()) was
potentially dangerous. get_initializing() is called by push_initializing(),
which (in the general case) is called by LLSingletonBase's constructor. If
somehow the MasterList's LLSingletonBase constructor ended up calling
get_initializing(), it would have called get_initializing_from(), passing an
LLSingletonBase which had not yet been constructed into the MasterList. In
particular, its mInitializing map would not yet have been initialized at all.

Since the MasterList must not, by design, depend on any other LLSingletons,
LLSingleton_manage_master<LLSingletonBase::MasterList>::get_initializing()
need not return a list from the official mInitializing map anyway. It can, and
should, and now does, return a static dummy list. That obviates
get_initializing_from(), which is removed.

That in turn means we no longer need to pass get_initializing() an
LLSingletonBase*. Remove that parameter.

LLParamSingleton contained a static member mutex. Unfortunately that wasn't
guaranteed to be initialized by the time its getInstance() was entered. Use a
function-local static instead.

from LLParamSingleton::initSingleton().

This was forbidden, but AndreyK points out cases in which LLParamSingleton::
initSingleton() should in fact be allowed to circle back to its own instance()
method. Use a recursive_mutex instead of plain mutex to permit that; remove
LL_ERRS preventing it.

Add LLParamSingleton::instance() method that calls
LLParamSingleton::getInstance(). Inheriting LLSingleton::instance() called
LLSingleton::getInstance() -- not at all what we want.

Add LLParamSingleton unit tests.

Simplify LLSingleton::SingletonLifetimeManager to SingletonInitializer: that
struct has not been responsible for deletion ever since LLSingletonBase
acquired dependency-ordered deleteAll().

Move SingletonData::mInitState changes from SingletonLifetimeManager to
constructSingleton() method. Similarly, constructSingleton() now sets
SingletonData::mInstance instead of making its caller store the pointer.

Add variadic arguments to LLSingleton::constructSingleton() so we can reuse it
for LLParamSingleton.

Add finishInitializing() method to encapsulate logic reused for
getInstance()'s INITIALIZING and DELETED cases.

Make LLParamSingleton a subclass of LLSingleton, just as LLLockedSingleton is
a subclass of LLParamSingleton. Make LLParamSingleton a friend of LLSingleton,
so it can access private members of LLSingleton without also granting access
to any DERIVED_CLASS subclass. This eliminates the need for protected
getInitState().

LLParamSingleton::initParamSingleton() reuses LLSingleton::constructSingleton()
and finishInitializing(). Its getInstance() method completely replaces
LLSingleton::getInstance(): in most EInitStates, LLParamSingleton::getInstance()
is an error.

Use a std::mutex to serialize calls to LLParamSingleton::initParamSingleton()
and getInstance(). While LLSingleton::getInstance() relies on the "initialized
exactly once" guarantee for block-scope static declarations, LLParamSingleton
cannot rely on the same mechanism.

LLLockedSingleton is now a very succinct subclass of LLParamSingleton -- they
have very similar functionality.

Giving the LLSINGLETON() macro variadic arguments eliminates the need for a
separate LLPARAMSINGLETON() macro, while continuing to support existing usage.

Use them in place of awkward try/catch test boilerplate.