Fix aio_error thread-safety.

* sysdeps/pthread/aio_error.c: New file
* sysdeps/pthread/aio_misc.c: Remove optimistic comment about
synchronization.

sysdeps/pthread/aio_error.c

@@ -0,0 +1,49 @@
+/* Return error status of asynchronous I/O request.
+   Copyright (C) 1997-2015 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by Ulrich Drepper <drepper@cygnus.com>, 1997.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+
+/* We use an UGLY hack to prevent gcc from finding us cheating.  The
+   implementation of aio_error and aio_error64 are identical and so
+   we want to avoid code duplication by using aliases.  But gcc sees
+   the different parameter lists and prints a warning.  We define here
+   a function so that aio_error64 has no prototype.  */
+#define aio_error64 XXX
+#include <aio.h>
+/* And undo the hack.  */
+#undef aio_error64
+
+#include <aio_misc.h>
+
+
+int
+aio_error (aiocbp)
+     const struct aiocb *aiocbp;
+{
+  int ret;
+
+  /* Acquire the mutex to make sure all operations for this request are
+     complete.  */
+  pthread_mutex_lock(&__aio_requests_mutex);
+  ret = aiocbp->__error_code;
+  pthread_mutex_unlock(&__aio_requests_mutex);
+
+  return ret;
+}
+
+weak_alias (aio_error, aio_error64)