[libcxx] Only declare contents of threading API when _LIBCPP_HAS_THREAD_API_EXTERNAL is not defined.

[DavidSpickett]

Summary

When it is defined they will be declared by the __external_threading header instead.

[michaelplatings]

It looks like if _LIBCPP_HAS_THREAD_API_PTHREAD is defined then with this change you'll be missing everything between lines 115 and 210.

[DavidSpickett]

Addressed Michael's comment by not moving the initial:
#endif // defined(_LIBCPP_HAS_THREAD_API_PTHREAD)

Instead adding a separate block with:
#if !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
further down.

So that these things are still present with pthreads.

[mclow.lists]

This seems to work for me (pthreads and nothreads).
In general, I think we're playing "whack-a-mole' with this chunk of code, and I've become fairly unhappy with it.

I'll be doing some cleanup here after the release goes out.

[EricWF]

I don't fully understand this patch. The point of always declaring the interface was to enforce that it matches exactly what we expect. I'll grant that probably wasn't working, but can you explain more about?

[michaelplatings]

I can see that this patch doesn't make much sense in isolation. This patch plus D66480 make an alternative to D66301. Hopefully the explanation in D66301 makes it clear that we're merely restoring previous behaviour.

libcxx/include/__threading_support

 #define _LIBCPP_NULL_THREAD 0U
 
 typedef pthread_t __libcpp_thread_t;
 
 // Thread Local Storage
 typedef pthread_key_t __libcpp_tls_key;
 
 #define _LIBCPP_TLS_DESTRUCTOR_CC
-#else
+#elif !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 // Mutex
 typedef void* __libcpp_mutex_t;
 #define _LIBCPP_MUTEX_INITIALIZER 0
 
 #if defined(_M_IX86) || defined(__i386__) || defined(_M_ARM) || defined(__arm__)
 typedef void* __libcpp_recursive_mutex_t[6];
 #elif defined(_M_AMD64) || defined(__x86_64__) || defined(_M_ARM64) || defined(__aarch64__)
 typedef void* __libcpp_recursive_mutex_t[5];

 #define _LIBCPP_NULL_THREAD 0U
 
 typedef void* __libcpp_thread_t;
 
 // Thread Local Storage
 typedef long __libcpp_tls_key;
 
 #define _LIBCPP_TLS_DESTRUCTOR_CC __stdcall
-#endif // defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
+#endif // !defined(_LIBCPP_HAS_THREAD_API_PTHREAD) && !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 
+#if !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 // Mutex
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_recursive_mutex_init(__libcpp_recursive_mutex_t *__m);
 
 _LIBCPP_THREAD_ABI_VISIBILITY _LIBCPP_NO_THREAD_SAFETY_ANALYSIS
 int __libcpp_recursive_mutex_lock(__libcpp_recursive_mutex_t *__m);
 
 _LIBCPP_THREAD_ABI_VISIBILITY _LIBCPP_NO_THREAD_SAFETY_ANALYSIS
int __libcpp_tls_create(__libcpp_tls_key* __key,
                         void(_LIBCPP_TLS_DESTRUCTOR_CC* __at_exit)(void*));
 
 _LIBCPP_THREAD_ABI_VISIBILITY
 void *__libcpp_tls_get(__libcpp_tls_key __key);
 
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_tls_set(__libcpp_tls_key __key, void *__p);
 
+#endif // !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
+
 #if (!defined(_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL) || \
      defined(_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL)) && \
     defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
 
 int __libcpp_recursive_mutex_init(__libcpp_recursive_mutex_t *__m)
 {
   pthread_mutexattr_t attr;
   int __ec = pthread_mutexattr_init(&attr);