[libc++] Add an option to disable the use of __ulock_wait

[Mordante]

Summary

__ulock_wait / __ulock_wake are considered private APIs and
their use will result in rejection from the App Store.
libc++ comes natively on Apple platforms, but some projects
prefer to static link their own libc++ builds.

So, add a CMake option to disable the use of __ulock_wait
on Apple platforms.

Also fix an issue where __ulock_wait code-path is actually not used
due to a missing include.

[ldionne]

When shipping on the App Store, you should be using the system-provided libc++. There are various reasons for that, but long story short it'll make your app a good citizen in the ecosystem.

Is there a reason why you're going for a static link instead? If you are not careful, I can imagine various ways this could behave unexpectedly.

[bc-lee]

In D143689#4118849, @ldionne wrote:

When shipping on the App Store, you should be using the system-provided libc++. There are various reasons for that, but long story short it'll make your app a good citizen in the ecosystem.

Is there a reason why you're going for a static link instead? If you are not careful, I can imagine various ways this could behave unexpectedly.

As I wrote in the issue (https://github.com/llvm/llvm-project/issues/60466), there are already some projects that do this. But I'm not speaking on behalf of those projects.

@thakis , can you explain why chromium uses static libc++ on Apple platforms?

[Mordante]

This patch is inactive for quite a while. We're moving to GitHub and want to clean up the patch review queue. If you want to pursue this patch, please file a new patch as GitHub PR. I'll close the patch for now,

libcxx/CMakeLists.txt

 option(LIBCXX_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
 option(LIBCXX_HAS_WIN32_THREAD_API "Ignore auto-detection and force use of win32 thread API" OFF)
 option(LIBCXX_HAS_EXTERNAL_THREAD_API
   "Build libc++ with an externalized threading API.
    This option may only be set to ON when LIBCXX_ENABLE_THREADS=ON." OFF)
 option(LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY
     "Build libc++ with an externalized threading library.
      This option may only be set to ON when LIBCXX_ENABLE_THREADS=ON" OFF)
+option(LIBCXXX_DISABLE_APPLE_ULOCK_WAIT
+    "Build libc++ without __ulock_wait on Apple platforms" OFF)
 
 # Misc options ----------------------------------------------------------------
 # FIXME: Turn -pedantic back ON. It is currently off because it warns
 # about #include_next which is used everywhere.
 option(LIBCXX_ENABLE_PEDANTIC "Compile with pedantic enabled." OFF)
 option(LIBCXX_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 option(LIBCXX_DISABLE_MACRO_CONFLICT_WARNINGS "Disable #warnings about conflicting macros." OFF)
 

 config_define_if_not(LIBCXX_ENABLE_WIDE_CHARACTERS _LIBCPP_HAS_NO_WIDE_CHARACTERS)
 config_define_if_not(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
 config_define_if(LIBCXX_ENABLE_DEBUG_MODE _LIBCPP_ENABLE_DEBUG_MODE)
 if (LIBCXX_ENABLE_ASSERTIONS)
   config_define(1 _LIBCPP_ENABLE_ASSERTIONS_DEFAULT)
 else()
   config_define(0 _LIBCPP_ENABLE_ASSERTIONS_DEFAULT)
 endif()
+if (APPLE)
+  config_define_if(LIBCXXX_DISABLE_APPLE_ULOCK_WAIT _LIBCXXX_DISABLE_ULOCK_WAIT)
+endif()
 
 if (LIBCXX_ABI_DEFINES)
   set(abi_defines)
   foreach (abi_define ${LIBCXX_ABI_DEFINES})
     if (NOT abi_define MATCHES "^_LIBCPP_ABI_")
       message(SEND_ERROR "Invalid ABI macro ${abi_define} in LIBCXX_ABI_DEFINES")
     endif()
     list(APPEND abi_defines "#define ${abi_define}")

libcxx/include/__config_site.in

 #cmakedefine _LIBCPP_HAS_NO_FILESYSTEM_LIBRARY
 #cmakedefine _LIBCPP_HAS_PARALLEL_ALGORITHMS
 #cmakedefine _LIBCPP_HAS_NO_RANDOM_DEVICE
 #cmakedefine _LIBCPP_HAS_NO_LOCALIZATION
 #cmakedefine _LIBCPP_HAS_NO_FSTREAM
 #cmakedefine _LIBCPP_HAS_NO_WIDE_CHARACTERS
 #cmakedefine01 _LIBCPP_ENABLE_ASSERTIONS_DEFAULT
 #cmakedefine _LIBCPP_ENABLE_DEBUG_MODE
+#ifdef __APPLE__
+#cmakedefine _LIBCXXX_DISABLE_ULOCK_WAIT
+#endif
 
 // __USE_MINGW_ANSI_STDIO gets redefined on MinGW
 #ifdef __clang__
 #  pragma clang diagnostic push
 #  pragma clang diagnostic ignored "-Wmacro-redefined"
 #endif
 
 @_LIBCPP_ABI_DEFINES@
 @_LIBCPP_EXTRA_SITE_DEFINES@
 
 #ifdef __clang__
 #  pragma clang diagnostic pop
 #endif
 
 #endif // _LIBCPP___CONFIG_SITE

libcxx/src/atomic.cpp

 #include <sys/syscall.h>
 
 // libc++ uses SYS_futex as a universal syscall name. However, on 32 bit architectures
 // with a 64 bit time_t, we need to specify SYS_futex_time64.
 #if !defined(SYS_futex) && defined(SYS_futex_time64)
 # define SYS_futex SYS_futex_time64
 #endif
 
+#elif defined(__APPLE__)
+#include "include/apple_availability.h"
+
 #elif defined(__FreeBSD__)
 
 #include <sys/types.h>
 #include <sys/umtx.h>
 
 #else // <- Add other operating systems here
 
 // Baseline needs no new headers

libcxx/src/include/apple_availability.h

 #define _LIBCPP_USE_UTIMENSAT
 #endif
 #elif defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__)
 #if __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ >= 40000
 #define _LIBCPP_USE_UTIMENSAT
 #endif
 #endif // __ENVIRONMENT_.*_VERSION_MIN_REQUIRED__
 
+#if !defined(_LIBCXXX_DISABLE_ULOCK_WAIT)
 #if defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)
 #if __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 101500
 #define _LIBCPP_USE_ULOCK
 #endif
 #elif defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__)
 #if __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 130000
 #define _LIBCPP_USE_ULOCK
 #endif
 #elif defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__)
 #if __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ >= 130000
 #define _LIBCPP_USE_ULOCK
 #endif
 #elif defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__)
 #if __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ >= 60000
 #define _LIBCPP_USE_ULOCK
 #endif
 #endif // __ENVIRONMENT_.*_VERSION_MIN_REQUIRED__
+#endif // _LIBCXXX_DISABLE_ULOCK_WAIT
 
 #endif // __APPLE__
 
 #endif // _LIBCPP_SRC_INCLUDE_APPLE_AVAILABILITY_H