New testcase gdb.threads/leader-exit-attach.exp (PR threads/8153)

Add a new testcase for exercising attaching to a process after its
main thread has exited.

This is not possible on Linux, the kernel does not allow attaching to
a zombie task, so the test is kfailed there.  It is possible however
on Windows at least, and was the scenario addressed by the Windows
backend fix in
https://sourceware.org/legacy-ml/gdb-patches/2003-12/msg00479.html,
nowadays PR threads/8153, back in 2003.

Passes cleanly on Cygwin.
KFAILed on GNU/Linux native and gdbserver.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=8153
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31554
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31555
Change-Id: Ib554f92f68c965bb4603cdf2aadb55ca45ded53b

gdb/testsuite/gdb.threads/leader-exit-attach.exp

@@ -0,0 +1,87 @@
+# Copyright (C) 2024 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/>.
+
+# Test attaching to a program after its main thread has exited.
+
+require can_spawn_for_attach
+
+standard_testfile leader-exit.c
+
+if {[build_executable "failed to prepare" $testfile $srcfile {debug pthreads}] == -1} {
+    return
+}
+
+set escapedbinfile [string_to_regexp ${binfile}]
+
+set test_spawn_id [spawn_wait_for_attach $binfile]
+set testpid [spawn_id_get_pid $test_spawn_id]
+
+# Wait a bit for the leader thread to exit, before attaching.
+sleep 2
+
+clean_restart ${binfile}
+
+# Save this early as we may not be able to talk with GDBserver anymore
+# when we need to check it.
+set is_gdbserver [target_is_gdbserver]
+
+# True if successfully attached.
+set attached 0
+
+gdb_test_multiple "attach $testpid" "attach" {
+    -re "Attaching to process $testpid failed.*" {
+	# GNU/Linux gdbserver.  Linux ptrace does not let you attach
+	# to zombie threads.
+	setup_kfail "gdb/31555" *-*-linux*
+	fail $gdb_test_name
+    }
+    -re "warning: process $testpid is a zombie - the process has already terminated.*" {
+	# Native GNU/Linux.  Linux ptrace does not let you attach to
+	# zombie threads.
+	setup_kfail "gdb/31555" *-*-linux*
+	fail $gdb_test_name
+    }
+    -re "Attaching to program: $escapedbinfile, process $testpid.*$gdb_prompt $" {
+	pass $gdb_test_name
+	set attached 1
+    }
+}
+
+# With gdbserver, after we failed to attach, we hit PR server/31554:
+#  print $_inferior_thread_count
+#  Remote connection closed
+#  (gdb) KFAIL: gdb.threads/leader-exit-attach.exp: get valueof "$_inferior_thread_count"
+if {!$attached && $is_gdbserver} {
+    setup_kfail "server/31554" "*-*-*"
+}
+
+set thread_count [get_valueof "" "\$_inferior_thread_count" -1]
+
+if {$thread_count == -1} {
+    kill_wait_spawned_process $test_spawn_id
+    return
+}
+
+if {$attached} {
+    # Check that we have at least one thread.  We can't assume there
+    # will only be exactly one thread, because on some systems, like
+    # Cygwin, the runtime spawns extra threads.  Also, on Windows,
+    # attaching always injects one extra thread.
+    gdb_assert {$thread_count >= 1}
+} else {
+    gdb_assert {$thread_count == 0}
+}
+
+kill_wait_spawned_process $test_spawn_id