Cleanup _Execute_once()

[StephanTLavavej]

• Revert XFG: made _Execute_once XFG-compatible. #1318, _Execute_once bug fix. #1356, and Move structs into unnamed namespaces #1398 (xonce.cpp only) while preserving Remove XP (and Server 2003) support from STL #1194's change to directly call InitOnceExecuteOnce().
  • XFG is being removed from the toolset (see internal VSO-1726733, MSVC-PR-462377). Some things like ASan still need to handle XFG hashes, but things like the STL no longer need unusual contortions. I checked with @amyw-msft and she said this should be okay to remove.
• Drop _Execute_once() usage in ppltasks.cpp, and as an associated change, simplify dllmain.cpp.
  • See below.
• Use call_once() instead of _Execute_once() in excptptr.cpp.
  • After Formatting thread::id, stacktrace_entry, and basic_stacktrace #3861, they're both declared by xcall_once.h, so we don't need to use the internal _Execute_once() (with a worse interface) anymore.
  • We're using namespace std; so we don't need qualification.
  • Because call_once() supports stateful lambdas, we can simply capture _Storage by reference.
  • We're in stl/src so we don't really need to worry about True Placement New being hijacked, but let's static_cast<void*> to preserve the original code and make it clear that we are indeed invoking True Placement New.
• Remove declarations from xcall_once.h.
  • They were guarded by _CRTBLD after Various cleanups #3935, so this doesn't affect users.
  • The definition of _Execute_once() exactly repeated the declaration, except for extern "C++" which is fine to drop because stl/src is always built classically.
• Update comment now that _Execute_once() is unused.
  • Also drop pointless "_Execute_once function" comment.

The code removal in ppltasks.cpp looks like I'm just thoughtlessly annihilating a bunch of weird logic, but I actually performed it carefully step-by-step. This code was _CRT_APP-only, for "async causality tracing". As far as I can tell (looking up TFS changesets 923210 on 2013-04-02 and 1175213 on 2014-02-07, as this code was introduced and moved around), this was implemented in the hopes of improving debugging, back when ppltasks was envisioned as a top-level feature and the new major model for concurrency, instead of the minor implementation detail of <future> that it eventually ended up as. The async causality logging mechanism never existed for Desktop, and was conditionally available for App (it depended on OS version). That is, it's not necessary for any of <future>'s work, and this was so undocumented, I strongly doubt that anyone aside from the original developer in 2013-2014 ever used it (I certainly haven't).

Attempting to use real std::call_once() here ran into problems, because App didn't have access to the necessary Windows API functions that power it instead of _Execute_once(). In theory that could have been solvable with enough MSBuild effort, but I think that we can simply remove this code, and I'm willing to take the risk and bear any potential servicing burden if I'm wrong.

This doesn't affect bincompat because it's internal to stl/src and There's Only One STL (stl/src is always consistent with itself). I also verified that the exports of binaries\x86chk\bin\i386\app\msvcp140d_app.dll are unchanged.

The steps that I followed started by considering what would happen if the causality APIs weren't detected as present, and then performing logical transformations after that.

• Step 1: Suppose we can never get the causality APIs.
  • In AsyncCausalityTracer, m_causalityAPIs and m_isSupported are private data members, initialized to nullptr and false. Let's pretend that GetActivationFactory() fails. In that case, the data members remain nullptr and false forever. Accordingly, we can hardcode the return values of get() and isCausalitySupported(). Also, release() becomes a no-op.
• Step 2: There are no causality static factories to cleanup.
  • Now that asyncCausalityTracer.release() is a no-op, __crtCleanupCausalityStaticFactories() does nothing. Note that it wasn't exported - it was used only for cross-TU interaction between ppltasks.cpp and dllmain.cpp in stl/src, so we can remove this function outright. Then, dllmain.cpp becomes a stub.
• Step 3: asyncCausalityTracer.isCausalitySupported() is always false now.
  • Accordingly, we can drop all of these branches. Note that this drops _M_scheduled = true;.
• Step 4: _M_scheduled is always false now.
  • In <ppltasks.h>, _TaskEventLogger's constructor assigned _M_scheduled = false;, and we previously removed the only way for it to be set to true. Accordingly, we can drop this branch.
• Step 5: Remove unused code.
  • From bottom to top:
  • const GUID PPLTaskCausalityPlatformID wasn't exported. It had internal linkage, so we can definitely drop it.
  • Nothing mentions the AsyncCausalityTracer type or the asyncCausalityTracer global variable anymore.
  • Finally, we don't need the using-directives here.
• Step 6: Unify Vulcan and Romulus.
  • The app implementation of _TaskEventLogger is now identical to the desktop implementation (after we drop the _IsContinuation parameter name). Unify them.
• Pre-merge with make extern "C" functions explicitly noexcept #4106 by marking extern "C" DllMain() as noexcept.
  • Also drop the commented-out parameter names to avoid wrapping; they aren't useful when the function does nothing.

[AlexGuteniev]

What is XFG and why is it removed?

[StephanTLavavej]

According to my limited understanding, it was a variant of the control flow guard feature, but was abandoned because it proved to be unworkable in practice.

[CaseyCarter]

• Also drop pointless "_Execute_once function" comment.

🔥🔥🔥 Yes, YES! 🔥🔥🔥

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.