[libc++] Allow enabling assertions when back-deploying

[ldionne]

Summary

When back-deploying to older platforms, we can still provide assertions,
but we might not be able to provide a great implementation for the verbose
handler. Instead, we can just call ::abort().

[ldionne]

Availability markup is not needed since we provide a fallback.

libcxx/docs/UsingLibcxx.rst

 if the user defines ``_LIBCPP_ENABLE_ASSERTIONS=1`` (the same is true for the inverse case
 where the static or shared library was compiled **with** assertions but the user tries to
 disable them). However, most of the code in libc++ is in the headers, so the user-selected
 value for ``_LIBCPP_ENABLE_ASSERTIONS`` (if any) will usually be respected.
 
 When an assertion fails, the program is aborted through a special verbose termination function. The
 library provides a default function that prints an error message and calls ``std::abort()``. Note
 that this function is provided by the static or shared library, so it is only available when deploying
-to a platform where the compiled library is sufficiently recent. However, users can also override that
-function with their own, which can be useful to provide custom behavior, or when deploying to older
-platforms where the default function isn't available.
-
-Replacing the default verbose termination function is done by defining the following function:
+to a platform where the compiled library is sufficiently recent. On older platforms, the program will
+terminate in an unspecified unsuccessful manner, but the quality of diagnostics won't be great.
+However, users can also override that function with their own, which can be useful to either provide
+custom behavior or when deploying to an older platform where the default function isn't available.
+
+Replacing the default verbose termination function is done by defining the
+``_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED`` macro in all translation units of your program
+and defining the following function in exactly one translation unit:
 
 .. code-block:: cpp
 
   void __libcpp_verbose_abort(char const* format, ...)
 
 This mechanism is similar to how one can replace the default definition of ``operator new``
 and ``operator delete``. For example:
 

 Also note that the verbose termination function should never return. Since assertions in libc++
 catch undefined behavior, your code will proceed with undefined behavior if your function is called
 and does return.
 
 Furthermore, exceptions should not be thrown from the function. Indeed, many functions in the
 library are ``noexcept``, and any exception thrown from the termination function will result
 in ``std::terminate`` being called.
 
-Back-deploying with a custom verbose termination function
----------------------------------------------------------
-When deploying to an older platform that does not provide a default verbose termination function,
-the compiler will diagnose the usage of ``std::__libcpp_verbose_abort`` with an error. This is done
-to avoid the load-time error that would otherwise happen if the code was being deployed on older
-systems.
-
-If you are providing a custom verbose termination function, this error is effectively a false positive.
-To let the library know that you are providing a custom function in back-deployment scenarios, you must
-define the ``_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED`` macro, and the library will assume that
-you are providing your own definition. If no definition is provided and the code is back-deployed to an older
-platform, it will fail to load when the dynamic linker fails to find a definition of the function, so you
-should only remove the guard rails if you really mean it!
-
 Libc++ Configuration Macros
 ===========================
 
 Libc++ provides a number of configuration macros which can be used to enable
 or disable extended libc++ behavior, including enabling "debug mode" or
 thread safety annotations.
 
 **_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS**:

libcxx/include/__availability

// #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_format
     // user has provided their own).
     //
     // Users can pass -D_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED
     // to the compiler to tell the library to ignore the fact that the
     // default function isn't available on their deployment target. Note that
     // defining this macro but failing to define a custom function will lead to
     // a load-time error on back-deployment targets, so it should be avoided.
 #   define _LIBCPP_AVAILABILITY_DEFAULT_VERBOSE_ABORT
+// #   define _LIBCPP_HAS_NO_VERBOSE_ABORT_IN_LIBRARY
 
 #elif defined(__APPLE__)
 
 #   define _LIBCPP_AVAILABILITY_SHARED_MUTEX                                    \
         __attribute__((availability(macos,strict,introduced=10.12)))            \
         __attribute__((availability(ios,strict,introduced=10.0)))               \
         __attribute__((availability(tvos,strict,introduced=10.0)))              \
         __attribute__((availability(watchos,strict,introduced=3.0)))

 #   endif
 
 #   define _LIBCPP_AVAILABILITY_FORMAT                                          \
         __attribute__((unavailable))
 #   define _LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_format
 
 #   define _LIBCPP_AVAILABILITY_DEFAULT_VERBOSE_ABORT                  \
         __attribute__((unavailable))
+#   define _LIBCPP_HAS_NO_VERBOSE_ABORT_IN_LIBRARY
+
 #else
 
 // ...New vendors can add availability markup here...
 
 #   error "It looks like you're trying to enable vendor availability markup, but you haven't defined the corresponding macros yet!"
 
 #endif
 

libcxx/include/__verbose_abort

 
 #include <__availability>
 #include <__config>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
+// Provide a default implementation of __libcpp_verbose_abort if we know that neither the built
+// library not the user is providing one. Otherwise, just declare it and use the one from the
+// built library or the one provided by the user.
+//
+// We can't provide a great implementation because it needs to be pretty much
+// dependency-free (this is included everywhere else in the library).
+#if defined(_LIBCPP_HAS_NO_VERBOSE_ABORT_IN_LIBRARY) && !defined(_LIBCPP_AVAILABILITY_CUSTOM_VERBOSE_ABORT_PROVIDED)
+
+extern "C" void abort();
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+_LIBCPP_ATTRIBUTE_FORMAT(__printf__, 1, 2) _LIBCPP_HIDE_FROM_ABI inline
+void __libcpp_verbose_abort(const char *, ...) {
+    ::abort();
+}
+
+_LIBCPP_END_NAMESPACE_STD
+
+#else
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_AVAILABILITY_VERBOSE_ABORT _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 1, 2)
 void __libcpp_verbose_abort(const char *__format, ...);
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif
+
 #endif // _LIBCPP___VERBOSE_ABORT

libcxx/test/libcxx/assertions/default_verbose_abort.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // Test that the default verbose termination function aborts the program.
 
 // ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
-// We flag uses of the verbose termination function in older dylibs at compile-time to avoid runtime
-// failures when back-deploying.
-// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11.0|12.0}}
-
 #include <csignal>
 #include <cstdlib>
 
 void signal_handler(int signal) {
   if (signal == SIGABRT)
     std::_Exit(EXIT_SUCCESS);
   std::_Exit(EXIT_FAILURE);
 }
 
 int main(int, char**) {
   if (std::signal(SIGABRT, signal_handler) != SIG_ERR)
     _LIBCPP_ASSERT(false, "foo");
   return EXIT_FAILURE;
 }

libcxx/test/libcxx/assertions/headers_declare_verbose_abort.sh.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // Test that all public C++ headers define the verbose termination function.
 
-// We flag uses of the verbose termination function in older dylibs at compile-time to avoid runtime
-// failures when back-deploying.
-// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11.0|12.0}}
-
 // The system-provided <uchar.h> seems to be broken on AIX, which trips up this test.
 // XFAIL: LIBCXX-AIX-FIXME
 
 /*
 BEGIN-SCRIPT
 
 for i, header in enumerate(public_headers):
     # Skip C compatibility headers.

libcxx/test/libcxx/assertions/single_expression.sh.cpp

 // RUN: %{run}
 
 // RUN: %{build} -Wno-macro-redefined -D_LIBCPP_ENABLE_ASSERTIONS=0
 // RUN: %{run}
 
 // RUN: %{build} -Wno-macro-redefined -D_LIBCPP_ENABLE_ASSERTIONS=0 -D_LIBCPP_ASSERTIONS_DISABLE_ASSUME
 // RUN: %{run}
 
-// We flag uses of the assertion handler in older dylibs at compile-time to avoid runtime
-// failures when back-deploying.
-// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11.0|12.0}}
-
 #include <__assert>
 #include <cassert>
 
 void f() {
   int i = (_LIBCPP_ASSERT(true, "message"), 3);
   assert(i == 3);
   return _LIBCPP_ASSERT(true, "message");
 }
 
 int main(int, char**) {
   f();
   return 0;
 }

libcxx/utils/ci/buildkite-pipeline.yml

- label: "Apple back-deployment macosx11.0 arm64"
         os: "macos"
         arch: "arm64"
       retry:
         automatic:
           - exit_status: -1  # Agent was lost
             limit: 2
       timeout_in_minutes: 120
 
+    - label: "Apple back-deployment with assertions enabled"
+      command: "libcxx/utils/ci/run-buildbot apple-system-backdeployment-assertions-11.0"
+      artifact_paths:
+        - "**/test-results.xml"
+        - "**/*.abilist"
+      agents:
+        queue: "libcxx-builders"
+        os: "macos"
+      retry:
+        automatic:
+          - exit_status: -1  # Agent was lost
+            limit: 2
+      timeout_in_minutes: 120
+
   - group: "ARM"
     steps:
     - label: "AArch64"
       command: "libcxx/utils/ci/run-buildbot aarch64"
       artifact_paths:
         - "**/test-results.xml"
         - "**/*.abilist"
       agents:

libcxx/utils/ci/run-buildbot

xcrun --sdk macosx                                              \
             --install-dir ${INSTALL_DIR}                            \
             --symbols-dir "${BUILD_DIR}/symbols"                    \
             --architectures "${arch}"                               \
             --version "999.99"
 
     # TODO: It would be better to run the tests against the fake-installed version of libc++ instead
     xcrun --sdk macosx ninja -vC "${BUILD_DIR}/${arch}" check-cxx check-cxxabi check-cxx-abilist
 ;;
+apple-system-backdeployment-assertions-*)
+    clean
+
+    if [[ "${OSX_ROOTS}" == "" ]]; then
+        echo "--- Downloading previous macOS dylibs"
+        PREVIOUS_DYLIBS_URL="https://dl.dropboxusercontent.com/s/gmcfxwgl9f9n6pu/libcxx-roots.tar.gz"
+        OSX_ROOTS="${BUILD_DIR}/macos-roots"
+        mkdir -p "${OSX_ROOTS}"
+        curl "${PREVIOUS_DYLIBS_URL}" | tar -xz --strip-components=1 -C "${OSX_ROOTS}"
+    fi
+
+    DEPLOYMENT_TARGET="${BUILDER#apple-system-backdeployment-assertions-}"
+
+    # TODO: On Apple platforms, we never produce libc++abi.1.dylib or libunwind.1.dylib,
+    #       only libc++abi.dylib and libunwind.dylib. Fix that in the build so that the
+    #       tests stop searching for @rpath/libc++abi.1.dylib and @rpath/libunwind.1.dylib.
+    cp "${OSX_ROOTS}/macOS/libc++abi/${DEPLOYMENT_TARGET}/libc++abi.dylib" \
+       "${OSX_ROOTS}/macOS/libc++abi/${DEPLOYMENT_TARGET}/libc++abi.1.dylib"
+    cp "${OSX_ROOTS}/macOS/libunwind/${DEPLOYMENT_TARGET}/libunwind.dylib" \
+       "${OSX_ROOTS}/macOS/libunwind/${DEPLOYMENT_TARGET}/libunwind.1.dylib"
+
+    arch="$(uname -m)"
+    PARAMS="target_triple=${arch}-apple-macosx${DEPLOYMENT_TARGET}"
+    PARAMS+=";cxx_runtime_root=${OSX_ROOTS}/macOS/libc++/${DEPLOYMENT_TARGET}"
+    PARAMS+=";abi_runtime_root=${OSX_ROOTS}/macOS/libc++abi/${DEPLOYMENT_TARGET}"
+    PARAMS+=";unwind_runtime_root=${OSX_ROOTS}/macOS/libunwind/${DEPLOYMENT_TARGET}"
+    PARAMS+=";use_system_cxx_lib=True"
+    PARAMS+=";enable_assertions=True"
+    # TODO: Enable experimental features during back-deployment -- right now some of the availability
+    #       annotations are incorrect, leading to test failures that could be avoided.
+    PARAMS+=";enable_experimental=False"
+
+    generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Apple.cmake" \
+                   -DLIBCXX_TEST_CONFIG="apple-libc++-backdeployment.cfg.in" \
+                   -DLIBCXXABI_TEST_CONFIG="apple-libc++abi-backdeployment.cfg.in" \
+                   -DLIBUNWIND_TEST_CONFIG="apple-libunwind-backdeployment.cfg.in" \
+                   -DLIBCXX_TEST_PARAMS="${PARAMS}" \
+                   -DLIBCXXABI_TEST_PARAMS="${PARAMS}" \
+                   -DLIBUNWIND_TEST_PARAMS="${PARAMS}"
+
+    check-runtimes
+;;
 apple-system-backdeployment-*)
     clean
 
     if [[ "${OSX_ROOTS}" == "" ]]; then
         echo "--- Downloading previous macOS dylibs"
         PREVIOUS_DYLIBS_URL="https://dl.dropboxusercontent.com/s/gmcfxwgl9f9n6pu/libcxx-roots.tar.gz"
         OSX_ROOTS="${BUILD_DIR}/macos-roots"
         mkdir -p "${OSX_ROOTS}"