nptl: Use exit_lock when accessing TID on pthread_getaffinity_np

Also return EINVAL if the thread is already terminated at the time of
the call.  This is slight better than returning the calling thread
affinity (current behaviour), since the thread lifetime is defined.

Checked on x86_64-linux-gnu.

sysdeps/pthread/Makefile

@@ -217,6 +217,7 @@ tests += \
   tst-pt-vfork1 \
   tst-pt-vfork2 \
   tst-pthread-exit-signal \
+  tst-pthread-exited \
   tst-pthread-mutexattr \
   tst-pthread-mutexattr-2 \
   tst-pthread-raise-blocked-self \

sysdeps/pthread/tst-pthread-exited.c

@@ -0,0 +1,51 @@
+/* Test pthread interface which should return ESRCH when issues along
+   with a terminated pthread_t.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <signal.h>
+#include <stddef.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/xthread.h>
+
+static void *
+noop_thread (void *closure)
+{
+  return NULL;
+}
+
+static int
+do_test (void)
+{
+  pthread_t thr = xpthread_create (NULL, noop_thread, NULL);
+
+  support_wait_for_thread_exit ();
+
+  {
+    cpu_set_t cpuset;
+    int r = pthread_getaffinity_np (thr, sizeof (cpuset), &cpuset);
+    TEST_COMPARE (r, EINVAL);
+  }
+
+  xpthread_join (thr);
+
+  return 0;
+}
+
+#include <support/test-driver.c>