support: Add support_wait_for_thread_exit

support/Makefile

@@ -83,9 +83,10 @@ libsupport-routines = \
   support_test_compare_blob \
   support_test_compare_failure \
   support_test_compare_string \
-  support_write_file_string \
   support_test_main \
   support_test_verify_impl \
+  support_wait_for_thread_exit \
+  support_write_file_string \
   temp_file \
   timespec \
   timespec-time64 \

support/support.h

@@ -174,6 +174,10 @@ timer_t support_create_timer (uint64_t sec, long int nsec, bool repeat,
 /* Disable the timer TIMER.  */
 void support_delete_timer (timer_t timer);
 
+/* Wait until all threads except the current thread have exited (as
+   far as the kernel is concerned).  */
+void support_wait_for_thread_exit (void);
+
 struct support_stack
 {
   void *stack;

support/support_wait_for_thread_exit.c

@@ -0,0 +1,72 @@
+/* Wait until all threads except the current thread has exited.
+   Copyright (C) 2021 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 <dirent.h>
+#include <errno.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <unistd.h>
+
+void
+support_wait_for_thread_exit (void)
+{
+#ifdef __linux__
+  DIR *proc_self_task = opendir ("/proc/self/task");
+  TEST_VERIFY_EXIT (proc_self_task != NULL);
+
+  while (true)
+    {
+      errno = 0;
+      struct dirent *e = readdir (proc_self_task);
+      if (e == NULL && errno != 0)
+        FAIL_EXIT1 ("readdir: %m");
+      if (e == NULL)
+        {
+          /* Only the main thread remains.  Testing may continue.  */
+          closedir (proc_self_task);
+          return;
+        }
+
+      if (strcmp (e->d_name, ".") == 0 || strcmp (e->d_name, "..") == 0)
+        continue;
+
+      int task_tid = atoi (e->d_name);
+      if (task_tid <= 0)
+        FAIL_EXIT1 ("Invalid /proc/self/task entry: %s", e->d_name);
+
+      if (task_tid == gettid ())
+        /* The current thread.  Keep scanning for other
+           threads.  */
+        continue;
+
+      /* task_tid does not refer to this thread here, i.e., there is
+         another running thread.  */
+
+      /* Small timeout to give the thread a chance to exit.  */
+      usleep (50 * 1000);
+
+      /* Start scanning the directory from the start.  */
+      rewinddir (proc_self_task);
+    }
+#else
+  /* Use a large timeout because we cannot verify that the thread has
+     exited.  */
+  usleep (5 * 1000 * 1000);
+#endif
+}