libstdc++: Fix conformance issues in <stop_token> (PR92895)

Fix synchronization issues in <stop_token>. Replace shared_ptr with
_Stop_state_ref and a reference count embedded in the shared state.
Replace std::mutex with spinlock using one bit of a std::atomic<> that
also tracks whether a stop request has been made and how many
stop_source objects share ownership of the state.

	PR libstdc++/92895
	* include/std/stop_token (stop_token::stop_possible()): Call new
	_M_stop_possible() function.
	(stop_token::stop_requested()): Do not use stop_possible().
	(stop_token::binary_semaphore): New class, as temporary stand-in for
	std::binary_semaphore.
	(stop_token::_Stop_cb::_M_callback): Add noexcept to type.
	(stop_token::_Stop_cb::_M_destroyed, stop_token::_Stop_cb::_M_done):
	New data members for symchronization with stop_callback destruction.
	(stop_token::_Stop_cb::_Stop_cb): Make non-template.
	(stop_token::_Stop_cb::_M_linked, stop_token::_Stop_cb::_S_execute):
	Remove.
	(stop_token::_Stop_cb::_M_run): New member function.
	(stop_token::_Stop_state::_M_stopped, stop_token::_Stop_state::_M_mtx):
	Remove.
	(stop_token::_Stop_state::_M_owners): New data member to track
	reference count for ownership.
	(stop_token::_Stop_state::_M_value): New data member combining a
	spinlock, the stop requested flag, and the reference count for
	associated stop_source objects.
	(stop_token::_Stop_state::_M_requester): New data member for
	synchronization with stop_callback destruction.
	(stop_token::_Stop_state::_M_stop_possible()): New member function.
	(stop_token::_Stop_state::_M_stop_requested()): Inspect relevant bit
	of _M_value.
	(stop_token::_Stop_state::_M_add_owner)
	(stop_token::_Stop_state::_M_release_ownership)
	(stop_token::_Stop_state::_M_add_ssrc)
	(stop_token::_Stop_state::_M_sub_ssrc): New member functions for
	updating reference counts.
	(stop_token::_Stop_state::_M_lock, stop_token::_Stop_state::_M_unlock)
	(stop_token::_Stop_state::_M_lock, stop_token::_Stop_state::_M_unlock)
	(stop_token::_Stop_state::_M_try_lock)
	(stop_token::_Stop_state::_M_try_lock_and_stop)
	(stop_token::_Stop_state::_M_do_try_lock): New member functions for
	managing spinlock.
	(stop_token::_Stop_state::_M_request_stop): Use atomic operations to
	read and update state. Release lock while running callbacks. Use new
	data members to synchronize with callback destruction.
	(stop_token::_Stop_state::_M_remove_callback): Likewise.
	(stop_token::_Stop_state::_M_register_callback): Use atomic operations
	to read and update state.
	(stop_token::_Stop_state_ref): Handle type to manage _Stop_state,
	replacing shared_ptr.
	(stop_source::stop_source(const stop_source&)): Update reference count.
	(stop_source::operator=(const stop_source&)): Likewise.
	(stop_source::~stop_source()): Likewise.
	(stop_source::stop_source(stop_source&&)): Define as defaulted.
	(stop_source::operator=(stop_source&&)): Establish postcondition on
	parameter.
	(stop_callback): Enforce preconditions on template parameter. Replace
	base class with data member of new _Cb_impl type.
	(stop_callback::stop_callback(const stop_token&, Cb&&))
	(stop_callback::stop_callback(stop_token&&, Cb&&)): Fix TOCTTOU race.
	(stop_callback::_Cb_impl): New type wrapping _Callback member and
	defining the _S_execute member function.
	* testsuite/30_threads/stop_token/stop_callback/deadlock-mt.cc: New
	test.
	* testsuite/30_threads/stop_token/stop_callback/deadlock.cc: New test.
	* testsuite/30_threads/stop_token/stop_callback/destroy.cc: New test.
	* testsuite/30_threads/stop_token/stop_callback/destructible_neg.cc:
	New test.
	* testsuite/30_threads/stop_token/stop_callback/invocable_neg.cc: New
	test.
	* testsuite/30_threads/stop_token/stop_callback/invoke.cc: New test.
	* testsuite/30_threads/stop_token/stop_source/assign.cc: New test.
	* testsuite/30_threads/stop_token/stop_token/stop_possible.cc: New
	test.

libstdc++-v3/ChangeLog

 2020-01-29  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/92895
+	* include/std/stop_token (stop_token::stop_possible()): Call new
+	_M_stop_possible() function.
+	(stop_token::stop_requested()): Do not use stop_possible().
+	(stop_token::binary_semaphore): New class, as temporary stand-in for
+	std::binary_semaphore.
+	(stop_token::_Stop_cb::_M_callback): Add noexcept to type.
+	(stop_token::_Stop_cb::_M_destroyed, stop_token::_Stop_cb::_M_done):
+	New data members for symchronization with stop_callback destruction.
+	(stop_token::_Stop_cb::_Stop_cb): Make non-template.
+	(stop_token::_Stop_cb::_M_linked, stop_token::_Stop_cb::_S_execute):
+	Remove.
+	(stop_token::_Stop_cb::_M_run): New member function.
+	(stop_token::_Stop_state::_M_stopped, stop_token::_Stop_state::_M_mtx):
+	Remove.
+	(stop_token::_Stop_state::_M_owners): New data member to track
+	reference count for ownership.
+	(stop_token::_Stop_state::_M_value): New data member combining a
+	spinlock, the stop requested flag, and the reference count for
+	associated stop_source objects.
+	(stop_token::_Stop_state::_M_requester): New data member for
+	synchronization with stop_callback destruction.
+	(stop_token::_Stop_state::_M_stop_possible()): New member function.
+	(stop_token::_Stop_state::_M_stop_requested()): Inspect relevant bit
+	of _M_value.
+	(stop_token::_Stop_state::_M_add_owner)
+	(stop_token::_Stop_state::_M_release_ownership)
+	(stop_token::_Stop_state::_M_add_ssrc)
+	(stop_token::_Stop_state::_M_sub_ssrc): New member functions for
+	updating reference counts.
+	(stop_token::_Stop_state::_M_lock, stop_token::_Stop_state::_M_unlock)
+	(stop_token::_Stop_state::_M_lock, stop_token::_Stop_state::_M_unlock)
+	(stop_token::_Stop_state::_M_try_lock)
+	(stop_token::_Stop_state::_M_try_lock_and_stop)
+	(stop_token::_Stop_state::_M_do_try_lock): New member functions for
+	managing spinlock.
+	(stop_token::_Stop_state::_M_request_stop): Use atomic operations to
+	read and update state. Release lock while running callbacks. Use new
+	data members to synchronize with callback destruction.
+	(stop_token::_Stop_state::_M_remove_callback): Likewise.
+	(stop_token::_Stop_state::_M_register_callback): Use atomic operations
+	to read and update state.
+	(stop_token::_Stop_state_ref): Handle type to manage _Stop_state,
+	replacing shared_ptr.
+	(stop_source::stop_source(const stop_source&)): Update reference count.
+	(stop_source::operator=(const stop_source&)): Likewise.
+	(stop_source::~stop_source()): Likewise.
+	(stop_source::stop_source(stop_source&&)): Define as defaulted.
+	(stop_source::operator=(stop_source&&)): Establish postcondition on
+	parameter.
+	(stop_callback): Enforce preconditions on template parameter. Replace
+	base class with data member of new _Cb_impl type.
+	(stop_callback::stop_callback(const stop_token&, Cb&&))
+	(stop_callback::stop_callback(stop_token&&, Cb&&)): Fix TOCTTOU race.
+	(stop_callback::_Cb_impl): New type wrapping _Callback member and
+	defining the _S_execute member function.
+	* testsuite/30_threads/stop_token/stop_callback/deadlock-mt.cc: New
+	test.
+	* testsuite/30_threads/stop_token/stop_callback/deadlock.cc: New test.
+	* testsuite/30_threads/stop_token/stop_callback/destroy.cc: New test.
+	* testsuite/30_threads/stop_token/stop_callback/destructible_neg.cc:
+	New test.
+	* testsuite/30_threads/stop_token/stop_callback/invocable_neg.cc: New
+	test.
+	* testsuite/30_threads/stop_token/stop_callback/invoke.cc: New test.
+	* testsuite/30_threads/stop_token/stop_source/assign.cc: New test.
+	* testsuite/30_threads/stop_token/stop_token/stop_possible.cc: New
+	test.
+
 	* libsupc++/compare (__detail::__3way_builtin_ptr_cmp): Use
 	three_way_comparable_with.
 	(__detail::__3way_cmp_with): Remove workaround for fixed bug.

libstdc++-v3/testsuite/30_threads/stop_token/stop_callback/deadlock-mt.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a -pthread"  }
+// { dg-require-effective-target c++2a }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+#include <stop_token>
+#include <memory>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::stop_source ssrc;
+  std::stop_token stok = ssrc.get_token();
+  using F = void(*)();
+  std::unique_ptr<std::stop_callback<F>> pcb;
+  auto dereg = [&pcb] { pcb.reset(); };
+  std::stop_callback cb1(stok, dereg);
+  pcb = std::make_unique<std::stop_callback<F>>(stok, []{});
+  std::stop_callback cb2(stok, dereg);
+
+  // PR libstdc++/92895
+  // Making a stop request runs the callbacks. Whichever of cb1 and cb2
+  // runs first will destroy *pcb, which will try to unregister it.
+  // This recursive access to the shared stop state within a callback must
+  // work without deadlock.
+  ssrc.request_stop();
+}
+
+int main()
+{
+  test01();
+}

libstdc++-v3/testsuite/30_threads/stop_token/stop_callback/deadlock.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <stop_token>
+#include <memory>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::stop_source ssrc;
+  std::stop_token stok = ssrc.get_token();
+  using F = void(*)();
+  std::unique_ptr<std::stop_callback<F>> pcb;
+  auto dereg = [&pcb] { pcb.reset(); };
+  std::stop_callback cb1(stok, dereg);
+  pcb = std::make_unique<std::stop_callback<F>>(stok, []{});
+  std::stop_callback cb2(stok, dereg);
+
+  // PR libstdc++/92895
+  // Making a stop request runs the callbacks. Whichever of cb1 and cb2
+  // runs first will destroy *pcb, which will try to unregister it.
+  // This recursive access to the shared stop state within a callback must
+  // work without deadlock.
+  ssrc.request_stop();
+}
+
+int main()
+{
+  test01();
+}

libstdc++-v3/testsuite/30_threads/stop_token/stop_callback/destroy.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a -pthread"  }
+// { dg-require-effective-target c++2a }
+// { dg-require-effective-target pthread }
+// { dg-require-gthreads "" }
+
+#include <stop_token>
+#include <atomic>
+#include <thread>
+#include <testsuite_hooks.h>
+
+struct F
+{
+  static std::atomic<int> stage;
+
+  F(int) { }
+
+  ~F()
+  {
+    // PR libstdc++/92895
+    // Callback function must not be destroyed while still executing.
+    VERIFY( stage == 4 );
+  }
+
+  void operator()() const noexcept
+  {
+    stage = 2; // trigger destructor of stop_callback that owns *this
+    while (stage == 2)
+      std::this_thread::sleep_for(std::chrono::milliseconds(10));
+    std::this_thread::sleep_for(std::chrono::milliseconds(500));
+    stage = 4; // destructor checks for this
+  }
+};
+
+std::atomic<int> F::stage{0};
+
+void
+test01()
+{
+  std::stop_source ssrc;
+  std::stop_token stok = ssrc.get_token();
+  std::thread t1([&ssrc] {
+    while (F::stage == 0)
+      std::this_thread::sleep_for(std::chrono::milliseconds(10));
+    ssrc.request_stop();
+    while (F::stage != 5)
+      std::this_thread::sleep_for(std::chrono::milliseconds(10));
+  });
+
+  std::thread t2([&ssrc] {
+    std::stop_callback<F> cb(ssrc.get_token(), 0);
+    F::stage = 1; // trigger stop request in other thread, which runs callback
+    while (F::stage == 1)
+      std::this_thread::sleep_for(std::chrono::milliseconds(10));
+    F::stage = 3;
+    // stop_callback destructor should block until callback completes
+  });
+
+  t2.join();
+  F::stage = 5; // allow first thread to exit
+  t1.join();
+}
+
+int main()
+{
+  test01();
+}

libstdc++-v3/testsuite/30_threads/stop_token/stop_callback/destructible_neg.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <stop_token>
+
+struct F
+{
+  F();
+
+  void operator()() const { }
+
+private:
+  ~F();
+};
+
+auto
+test01(std::stop_token& tok, F& f)
+{
+  auto ok = sizeof(std::stop_callback<F&>);
+  auto bad = sizeof(std::stop_callback<F>); // { dg-error "here" }
+  return ok + bad;
+}
+
+struct G
+{
+  G();
+  ~G() noexcept(false);
+
+  void operator()() const { }
+};
+
+auto
+test02(std::stop_token& tok, G& g)
+{
+  auto ok = sizeof(std::stop_callback<G&>);
+  auto bad = sizeof(std::stop_callback<G>); // { dg-error "here" }
+  return ok + bad;
+}
+
+// { dg-error "static assertion failed" "" { target *-*-* } 0 }

libstdc++-v3/testsuite/30_threads/stop_token/stop_callback/invocable_neg.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <stop_token>
+
+struct F
+{
+};
+
+auto
+test01(std::stop_token& tok, F& f)
+{
+  auto bad1 = sizeof(std::stop_callback<F&>); // { dg-error "here" }
+  auto bad2 = sizeof(std::stop_callback<F>);  // { dg-error "here" }
+  return bad1 + bad2;
+}
+
+// { dg-error "static assertion failed" "" { target *-*-* } 0 }

libstdc++-v3/testsuite/30_threads/stop_token/stop_callback/invoke.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <stop_token>
+#include <testsuite_hooks.h>
+
+int lval[5];
+int rval[5];
+
+void
+test01()
+{
+  std::stop_source ssrc;
+  std::stop_token stok = ssrc.get_token();
+  struct F
+  {
+    void operator()() const & { ++lval[i]; }
+    void operator()() && { ++rval[i]; }
+
+    int i;
+  };
+  std::stop_callback<F> cb0(stok, F{0});
+  std::stop_callback<F> cb1(stok, F{1});
+  std::stop_callback<F> cb2(stok, F{2});
+  F f3{3};
+  std::stop_callback<F&> cb3(stok, f3);
+  std::stop_callback<const F> cb4(stok, F{4});
+
+  // PR libstdc++/92895
+  // Callback should be invoked with correct value category.
+  ssrc.request_stop();
+
+  VERIFY( lval[0] == 0 && lval[1] == 0 && lval[2] == 0 );
+  VERIFY( lval[3] == 1 );
+  VERIFY( lval[4] == 1 );
+  VERIFY( rval[0] == 1 );
+  VERIFY( rval[1] == 1 );
+  VERIFY( rval[2] == 1 );
+  VERIFY( rval[3] == 0 && rval[4] == 0 );
+}
+
+int main()
+{
+  test01();
+}

libstdc++-v3/testsuite/30_threads/stop_token/stop_source/assign.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <stop_token>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::stop_source src1, src2;
+  const std::stop_source orig1(src1);
+  VERIFY( src1 != src2 );
+  src1 = src2;
+  VERIFY( src1 == src2 );
+  VERIFY( src1 != orig1 );
+}
+
+void
+test02()
+{
+  std::stop_source src1, src2;
+  const std::stop_source orig1(src1), orig2(src2), src0(std::nostopstate);
+  src1 = std::move(src2);
+  VERIFY( src1 == orig2 );
+  VERIFY( src2 == src0 );
+  VERIFY( src1 != orig1 );
+  VERIFY( src0 != orig1 );
+}
+
+int main()
+{
+  test01();
+  test02();
+}

libstdc++-v3/testsuite/30_threads/stop_token/stop_token/stop_possible.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <stop_token>
+#include <memory>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::stop_source ssrc;
+  std::stop_token tok = ssrc.get_token();
+  VERIFY(tok.stop_possible());
+
+  ssrc.request_stop();
+  VERIFY(tok.stop_possible());
+}
+
+void
+test02()
+{
+  std::stop_token tok = std::stop_source().get_token();
+  // PR libstdc++/92895
+  // stop_possible() is false when there is no associated stop_source
+  VERIFY(!tok.stop_possible());
+}
+
+int main()
+{
+  test01();
+  test02();
+}