[libc++] Don't try to wait on a thread that hasn't started in std::async (#125433)

If the creation of a thread fails, this causes an idle loop that will
never end because the thread wasn't started in the first place.

Fixes #125428

Author: philnik777

libcxx/include/future

@@ -865,7 +865,8 @@ void __async_assoc_state<_Rp, _Fp>::__execute() {
 
 template <class _Rp, class _Fp>
 void __async_assoc_state<_Rp, _Fp>::__on_zero_shared() _NOEXCEPT {
-  this->wait();
+  if (base::__state_ & base::__constructed)
+    this->wait();
   base::__on_zero_shared();
 }
 
@@ -902,7 +903,8 @@ void __async_assoc_state<void, _Fp>::__execute() {
 
 template <class _Fp>
 void __async_assoc_state<void, _Fp>::__on_zero_shared() _NOEXCEPT {
-  this->wait();
+  if (base::__state_ & base::__constructed)
+    this->wait();
   base::__on_zero_shared();
 }
 

libcxx/test/std/thread/futures/futures.async/thread_create_failure.pass.cpp

@@ -0,0 +1,60 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: no-threads, no-exceptions
+
+// ASan seems to try to create threadsm which obviouly doesn't work in this test.
+// UNSUPPORTED: asan
+
+// UNSUPPORTED: c++03
+
+// There is no way to limit the number of threads on windows
+// UNSUPPORTED: windows
+
+// AIX and macOS seem to limit the number of processes, not threads via RLIMIT_NPROC
+// XFAIL: target={{.+}}-aix{{.*}}
+// XFAIL: target={{.+}}-apple-{{.*}}
+
+// This test makes sure that we fail gracefully in care the thread creation fails. This is only reliably possible on
+// systems that allow limiting the number of threads that can be created. See https://llvm.org/PR125428 for more details
+
+#include <cassert>
+#include <future>
+#include <system_error>
+
+#if __has_include(<sys/resource.h>)
+#  include <sys/resource.h>
+#  ifdef RLIMIT_NPROC
+void force_thread_creation_failure() {
+  rlimit lim = {1, 1};
+  assert(setrlimit(RLIMIT_NPROC, &lim) == 0);
+}
+#  else
+#    error "No known way to force only one thread being available"
+#  endif
+#else
+#  error "No known way to force only one thread being available"
+#endif
+
+int main(int, char**) {
+  force_thread_creation_failure();
+
+  try {
+    std::future<int> fut = std::async(std::launch::async, [] { return 1; });
+    assert(false);
+  } catch (const std::system_error&) {
+  }
+
+  try {
+    std::future<void> fut = std::async(std::launch::async, [] { return; });
+    assert(false);
+  } catch (const std::system_error&) {
+  }
+
+  return 0;
+}