[patch] libstdc++/57060 cope with invalid thread IDs

[Jonathan Wakely]

This patch ensures that this_thread::get_id() returns a value that is
distinct from the "not a thread" value, and avoids undefined behaviour
in pthread_equal.

Previously programs using glibc but not linking to libpthread would
get the "not a thread" value for std::this_thread::get_id() in main.
We were also using pthread_equal() with the "not a thread" value,
which is undefined.

This change assumes that pthread_t is EqualityComparable, but we
already have to assume it's LessThanComparable and apparently that
works everywhere that std::thread is supported, so this should too.

If it fails then we can do something smarter like dispatch based on
whether __gthread_t is an EqualityComparable scalar type, but I'd
prefer to avoid that complexity if this simpler version works.

Tested powerpc64le-linux, committed to trunk.


commit 64e418cb8e285bfa6079f176105a38ded456a976
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Fri Dec 4 13:58:56 2015 +0000

    PR libstdc++/57060 cope with invalid thread IDs
    
    	PR libstdc++/57060
    	* include/std/thread (operator==(thread::id, thread::id)): Do not use
    	__gthread_equal.
    	(operator<(thread::id, thread::id)): Add comment.
    	(this_thread::get_id()): Do not use __gthread_self for single-threaded
    	programs using glibc.
    	* testsuite/30_threads/this_thread/57060.cc: New.

diff --git a/libstdc++-v3/include/std/thread b/libstdc++-v3/include/std/thread
index 8c01feb..efdd83e 100644
--- a/libstdc++-v3/include/std/thread
+++ b/libstdc++-v3/include/std/thread
@@ -89,11 +89,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       friend bool
       operator==(thread::id __x, thread::id __y) noexcept
-      { return __gthread_equal(__x._M_thread, __y._M_thread); }
+      {
+	// pthread_equal is undefined if either thread ID is not valid, so we
+	// can't safely use __gthread_equal on default-constructed values (nor
+	// the non-zero value returned by this_thread::get_id() for
+	// single-threaded programs using GNU libc). Assume EqualityComparable.
+	return __x._M_thread == __y._M_thread;
+      }
 
       friend bool
       operator<(thread::id __x, thread::id __y) noexcept
-      { return __x._M_thread < __y._M_thread; }
+      {
+	// Pthreads doesn't define any way to do this, so we just have to
+	// assume native_handle_type is LessThanComparable.
+	return __x._M_thread < __y._M_thread;
+      }
 
       template<class _CharT, class _Traits>
 	friend basic_ostream<_CharT, _Traits>&
@@ -269,7 +279,18 @@ _GLIBCXX_END_NAMESPACE_VERSION
 
     /// get_id
     inline thread::id
-    get_id() noexcept { return thread::id(__gthread_self()); }
+    get_id() noexcept
+    {
+#ifdef __GLIBC__
+      // For the GNU C library pthread_self() is usable without linking to
+      // libpthread.so but returns 0, so we cannot use it in single-threaded
+      // programs, because this_thread::get_id() != thread::id{} must be true.
+      // We know that pthread_t is an integral type in the GNU C library.
+      if (!__gthread_active_p())
+	return thread::id(1);
+#endif
+      return thread::id(__gthread_self());
+    }
 
     /// yield
     inline void
diff --git a/libstdc++-v3/testsuite/30_threads/this_thread/57060.cc b/libstdc++-v3/testsuite/30_threads/this_thread/57060.cc
new file mode 100644
index 0000000..c932719
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/this_thread/57060.cc
@@ -0,0 +1,37 @@
+// Copyright (C) 2015 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-do run { target *-*-gnu* } }
+// { dg-options "-std=gnu++11" }
+// { dg-require-gthreads "" }
+
+// N.B. this test intentionally does *not* use -pthread
+
+#include <thread>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  VERIFY( std::this_thread::get_id() != std::thread::id() );
+}
+
+int
+main()
+{
+  test01();
+}