Enable libc++ error message in debug builds

Fixes: #24541

Author: sbc100

system/lib/libcxx/include/__verbose_abort

@@ -43,8 +43,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 // make sure that the program terminates but without taking any complex dependencies in this header.
 #if !defined(_LIBCPP_VERBOSE_ABORT)
 
-// XXX EMSCRIPTEN __libcpp_verbose_abort creases code size too much
-#  if !_LIBCPP_AVAILABILITY_HAS_VERBOSE_ABORT || defined (__EMSCRIPTEN__)
+// XXX EMSCRIPTEN avoid  __libcpp_verbose_abort in release builds due to code
+// size
+#  if !_LIBCPP_AVAILABILITY_HAS_VERBOSE_ABORT || (defined(__EMSCRIPTEN__) && defined(NDEBUG))
 // The decltype is there to suppress -Wunused warnings in this configuration.
 void __use(const char*, ...);
 #    define _LIBCPP_VERBOSE_ABORT(...) (decltype(::std::__use(__VA_ARGS__))(), __builtin_abort())

system/lib/libcxx/src/verbose_abort.cpp

@@ -11,6 +11,7 @@
 #include <cstdarg>
 #include <cstdio>
 #include <cstdlib>
+#include <cstring>
 
 #ifdef __BIONIC__
 #  include <syslog.h>
@@ -30,6 +31,12 @@ _LIBCPP_WEAK void __libcpp_verbose_abort(char const* format, ...) _LIBCPP_VERBOS
     va_list list;
     va_start(list, format);
     std::vfprintf(stderr, format, list);
+    // TODO(sbc): Add newline here unconditionally. libc++ seems inconsistent about strings
+    // passed to __libcpp_verbose_abort. The _LIBCPP_VERBOSE_ABORT macro seems to never use
+    // newlines, but _LIBCPP_ASSERTION_HANDLER does include a newline.
+    if (format[strlen(format) - 1] != '\n') {
+      std::fprintf(stderr, "\n");
+    }
     va_end(list);
   }
 

test/test_other.py

@@ -16219,3 +16219,20 @@ def test_unsupported_min_version_when_unsupported_env(self, env):
   })
   def test_automatic_env_worker(self, env, emcc_args):
     self.emcc(test_file('hello_world.c'), [f'-sENVIRONMENT={env}'] + emcc_args)
+
+  def test_libcxx_errors(self):
+    create_file('main.cpp', '''
+    #include <thread>
+    void func() {
+    }
+
+    int main() {
+      std::thread t(func);
+      t.join();
+    }
+    ''')
+
+    # Since we aare building without -pthread the thread constructor will fail,
+    # and in debug mode at leasts we expect to see the error message from libc++
+    expected = 'system_error was thrown in -fno-exceptions mode with error 6 and message "thread constructor failed"'
+    self.do_runf('main.cpp', expected, assert_returncode=NON_ZERO);

tools/system_libs.py

@@ -1665,7 +1665,7 @@ def get_files(self):
         filenames=filenames)
 
 
-class libcxx(ExceptionLibrary, MTLibrary):
+class libcxx(ExceptionLibrary, MTLibrary, DebugLibrary):
   name = 'libc++'
 
   cflags = [