Process exit status is leader exit status testcase

This adds a multi-threaded testcase that has all threads in the
process exit with a different exit code, and ensures that GDB reports
the thread group leader's exit status as the whole-process exit
status.  Before this set of patches, this would randomly report the
exit code of some other thread, and thus fail.

Tested on Linux-x86_64, native and gdbserver.

Co-Authored-By: Pedro Alves <pedro@palves.net>
Change-Id: I30cba2ff4576fb01b5169cc72667f3268d919557

gdb/testsuite/gdb.threads/process-exit-status-is-leader-exit-status.c

@@ -0,0 +1,64 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2022 Free Software Foundation, Inc.
+
+   This program 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 of the License, or
+   (at your option) any later version.
+
+   This program 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 program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <pthread.h>
+#include <unistd.h>
+#include <sys/syscall.h>
+
+#define NUM_THREADS 32
+
+pthread_barrier_t barrier;
+
+static void
+do_exit (int exitcode)
+{
+  /* Synchronize all threads up to here so that they all exit at
+     roughly the same time.  */
+  pthread_barrier_wait (&barrier);
+
+  /* All threads exit with SYS_exit, even the main thread, to avoid
+     exiting with a group-exit syscall, as that syscall changes the
+     exit status of all still-alive threads, thus potentially masking
+     a bug.  */
+  syscall (SYS_exit, exitcode);
+}
+
+static void *
+start (void *arg)
+{
+  int thread_return_value = *(int *) arg;
+
+  do_exit (thread_return_value);
+}
+
+int
+main(void)
+{
+  pthread_t threads[NUM_THREADS];
+  int thread_return_val[NUM_THREADS];
+  int i;
+
+  pthread_barrier_init (&barrier, NULL, NUM_THREADS + 1);
+
+  for (i = 0; i < NUM_THREADS; ++i)
+    {
+      thread_return_val[i] = i + 2;
+      pthread_create (&threads[i], NULL, start, &thread_return_val[i]);
+    }
+
+  do_exit (1);
+}

gdb/testsuite/gdb.threads/process-exit-status-is-leader-exit-status.exp

@@ -0,0 +1,46 @@
+# Copyright (C) 2022 Free Software Foundation, Inc.
+
+# This program 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 of the License, or
+# (at your option) any later version.
+#
+# This program 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 program.  If not, see <http://www.gnu.org/licenses/>.
+
+# GDB must always report the process's exit status based on the exit
+# status of the thread group leader thread.  Test that when multiple
+# threads exit simultaneously, GDB doesn't confuse the non-leader
+# threads' exit status for the process's exit status.  GDB used to
+# have a race condition that led to randomly handling this
+# incorrectly.
+#
+# Since the improper behavior is racy in nature, this test is not
+# expected to be able to reproduce the error reliably.  Multiple
+# executions (or increasing the number of iterations) might be
+# required to reproduce the error with a misbehaving GDB.
+
+if { ![istarget "*-*-linux*"] } {
+    return 0
+}
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug pthreads}] == -1} {
+    return -1
+}
+
+for {set iteration 0} {$iteration < 10} {incr iteration} {
+    with_test_prefix "iteration=$iteration" {
+	if {![runto_main]} {
+	    return
+	}
+
+	gdb_test "continue" "\\\[Inferior 1 \\(.*\\) exited with code 01\\\]"
+    }
+}