[libc++] Add lit feature for std::signal

[miyuki]

Summary

Some embedded platforms don't implement the C/POSIX function signal().
For such platforms, it would make sense to detect the absence of this
function and mark tests that rely on it as unsupported.

[michaelplatings]

I feel uneasy about this. Different C libraries can choose to skip implementing arbitrary parts of the standard but we should think carefully before adding complexity to libc++ to compensate. From eyeballing the picolibc sources it does support signal(), so perhaps the right answer here is to get other embedded C libs to do likewise?

[Mordante]

I'm also not too convinced of this change. One of the issues we have no CI which tests this configuration so we can't test there are no regressions. I also wonder how much support we want to provide for non-conforming C libraries.

Do you have concrete examples of c libraries not implementing this feature and do you know why it's not there?

[ldionne]

I am sad to have missed this! This is actually something we run into internally on some configurations as well, and having this would simplify our lives. As with any such configuration, it's always hard to determine whether it's worth upstreaming or keeping downstream, however I wouldn't be surprised if that were a fairly common one. To do things consistently, however, we'd need to also add a buildbot that runs without signals and hence probably introduce a LIBCXX_HAS_NO_SIGNALS setting similar to LIBCXX_HAS_NO_LOCALIZATION and friends. I'm not sure that pulls its weight, but I'd be happy to review something like that to evaluate it.

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.
 
+// REQUIRES: has-signal
 // ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 #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/containers/associative/map/at.abort.pass.cpp

 
 // class map
 
 // mapped_type& at(const key_type& k);
 
 // Make sure we abort() when exceptions are disabled and we fetch a key that
 // is not in the map.
 
-// REQUIRES: no-exceptions
+// REQUIRES: no-exceptions && has-signal
 
 #include <csignal>
 #include <cstdlib>
 #include <map>
 
 #include "test_macros.h"
 
 

libcxx/test/libcxx/containers/associative/map/at.const.abort.pass.cpp

 
 // class map
 
 // const mapped_type& at(const key_type& k) const;
 
 // Make sure we abort() when exceptions are disabled and we fetch a key that
 // is not in the map.
 
-// REQUIRES: no-exceptions
+// REQUIRES: no-exceptions && has-signal
 
 #include <csignal>
 #include <cstdlib>
 #include <map>
 
 #include "test_macros.h"
 
 

libcxx/test/libcxx/containers/unord/unord.map/at.abort.pass.cpp

 
 // class unordered_map
 
 // mapped_type& at(const key_type& k);
 
 // Make sure we abort() when exceptions are disabled and we fetch a key that
 // is not in the map.
 
-// REQUIRES: no-exceptions
+// REQUIRES: no-exceptions && has-signal
 // UNSUPPORTED: c++03
 
 #include <csignal>
 #include <cstdlib>
 #include <unordered_map>
 
 #include "test_macros.h"
 
 
 int main(int, char**) {
     std::signal(SIGABRT, [](int) { std::_Exit(EXIT_SUCCESS); });
     std::unordered_map<int, int> map;
     map.at(1);
     return EXIT_FAILURE;
 }

libcxx/test/libcxx/containers/unord/unord.map/at.const.abort.pass.cpp

 
 // class unordered_map
 
 // const mapped_type& at(const key_type& k) const;
 
 // Make sure we abort() when exceptions are disabled and we fetch a key that
 // is not in the map.
 
-// REQUIRES: no-exceptions
+// REQUIRES: no-exceptions && has-signal
 // UNSUPPORTED: c++03
 
 #include <csignal>
 #include <cstdlib>
 #include <unordered_map>
 
 #include "test_macros.h"
 
 
 int main(int, char**) {
     std::signal(SIGABRT, [](int) { std::_Exit(EXIT_SUCCESS); });
     std::unordered_map<int, int> const map;
     map.at(1);
     return EXIT_FAILURE;
 }

libcxx/test/libcxx/input.output/iostreams.base/ios/iostate.flags/clear.abort.pass.cpp

 
 // template <class charT, class traits> class basic_ios
 
 // void clear(iostate state);
 
 // Make sure that we abort() when exceptions are disabled and the exception
 // flag is set for the iostate we pass to clear().
 
-// REQUIRES: no-exceptions
+// REQUIRES: no-exceptions && has-signal
 
 #include <csignal>
 #include <cstdlib>
 #include <ios>
 #include <streambuf>
 
 #include "test_macros.h"
 

libcxx/test/libcxx/localization/locales/locale.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
 //
 //===----------------------------------------------------------------------===//
 
 // <locale>
 
 // class locale;
 
 // explicit locale( const char* std_name );
 
-// REQUIRES: no-exceptions
+// REQUIRES: no-exceptions && has-signal
 
 // Make sure we abort() when we construct a locale with a null name and
 // exceptions are disabled.
 
 #include <csignal>
 #include <cstdlib>
 #include <locale>
 

libcxx/test/libcxx/localization/locales/locale.category.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
 //
 //===----------------------------------------------------------------------===//
 
 // <locale>
 
 // class locale;
 
 // locale(const locale& other, const char* std_name, category cat);
 
-// REQUIRES: no-exceptions
+// REQUIRES: no-exceptions && has-signal
 
 // Make sure we abort() when we construct a locale with a null name and
 // exceptions are disabled.
 
 #include <csignal>
 #include <cstdlib>
 #include <locale>
 

libcxx/test/libcxx/localization/locales/use_facet.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
 //
 //===----------------------------------------------------------------------===//
 
 // <locale>
 
 // template <class Facet> const Facet& use_facet(const locale& loc);
 
-// REQUIRES: no-exceptions
+// REQUIRES: no-exceptions && has-signal
 
 // Make sure we abort() when we pass a facet not associated to the locale to
 // use_facet() and exceptions are disabled.
 
 #include <csignal>
 #include <cstdlib>
 #include <locale>
 

libcxx/utils/libcxx/test/features.py

Feature(name='has-unix-headers',
           when=lambda cfg: sourceBuilds(cfg, """
             #include <unistd.h>
             #include <sys/wait.h>
             int main(int, char**) {
               return 0;
             }
           """)),
 
+  Feature(name='has-signal',
+          when=lambda cfg: sourceBuilds(cfg, """
+            #include <csignal>
+            int main(int, char**) {
+              std::signal(SIGABRT, nullptr);
+              return 0;
+            }
+          """)),
+
   # Whether Bash can run on the executor.
   # This is not always the case, for example when running on embedded systems.
   #
   # For the corner case of bash existing, but it being missing in the path
   # set in %{exec} as "--env PATH=one-single-dir", the executor does find
   # and executes bash, but bash then can't find any other common shell
   # utilities. Test executing "bash -c 'bash --version'" to see if bash
   # manages to find binaries to execute.