Test attaching to a process that isn't a process group leader

The patch at
<https://sourceware.org/ml/gdb-patches/2017-11/msg00039.html> was
proposing to add an assertion to child_terminal_init that turns out
would fail if you tried to attach to a process that isn't a process
group leader.

Since the testsuite failed to catch the problem, this commit adds a
new testcase that would catch it, like:

  (gdb) attach 12415
  Attaching to program: build/gdb/testsuite/outputs/gdb.base/attach-non-pgrp-leader/attach-non-pgrp-leader, process 12415
  src/gdb/inflow.c:180: internal-error: void child_terminal_init(target_ops*): Assertion `getpgid (inf->pid) == inf->pid' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Quit this debugging session? (y or n) FAIL: gdb.base/attach-non-pgrp-leader.exp: child: attach to child (GDB internal error)

gdb/testsuite/ChangeLog:
2017-11-06  Pedro Alves  <palves@redhat.com>

	* gdb.base/attach-non-pgrp-leader.c: New.
	* gdb.base/attach-non-pgrp-leader.exp: New.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2017-11-06  Pedro Alves  <palves@redhat.com>
+
+	* gdb.base/attach-non-pgrp-leader.c: New.
+	* gdb.base/attach-non-pgrp-leader.exp: New.
+
 2017-11-06  Pedro Alves  <palves@redhat.com>
 
 	* configure.ac: No longer check for termio.h and sgtty.h.

gdb/testsuite/gdb.base/attach-non-pgrp-leader.c

@@ -0,0 +1,45 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017 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 <stdlib.h>
+#include <unistd.h>
+
+/* GDB reads this to figure out the child's PID.  */
+pid_t child_pid;
+
+void
+marker (void)
+{
+}
+
+int
+main ()
+{
+  alarm (60);
+
+  child_pid = fork ();
+  if (child_pid == -1)
+    return 1;
+
+  while (1)
+    {
+      marker ();
+      usleep (1000);
+    }
+
+  return 0;
+}

gdb/testsuite/gdb.base/attach-non-pgrp-leader.exp

@@ -0,0 +1,76 @@
+# Copyright 2017 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/>.  */
+
+# Check that we can attach to processes that are not process group
+# leaders.  We test that by attaching to a fork child that doesn't
+# call any of setpgrp/setpgid/setsid to move itself to a new process
+# group.
+
+if {![can_spawn_for_attach]} {
+    return 0
+}
+
+standard_testfile
+
+if { [build_executable ${testfile}.exp ${testfile} $srcfile {debug}] == -1 } {
+    return -1
+}
+
+proc do_test {} {
+    global binfile
+
+    set test_spawn_id [spawn_wait_for_attach $binfile]
+    set parent_pid [spawn_id_get_pid $test_spawn_id]
+
+    # Attach to the parent, run it to a known point, extract the
+    # child's PID, and detach.
+    with_test_prefix "parent" {
+	clean_restart ${binfile}
+
+	gdb_test "attach $parent_pid" \
+	    "Attaching to program.*, process $parent_pid.*" \
+	    "attach"
+
+	gdb_breakpoint "marker"
+	gdb_continue_to_breakpoint "marker"
+
+	set child_pid [get_integer_valueof "child_pid" -1]
+	if {$child_pid == -1} {
+	    return
+	}
+
+	gdb_test "detach" \
+	    "Detaching from program: .*process $parent_pid"
+    }
+
+    # Start over, and attach to the child this time.
+    with_test_prefix "child" {
+	clean_restart $binfile
+
+	gdb_test "attach $child_pid" \
+	    "Attaching to program.*, process $child_pid.*" \
+	    "attach"
+
+	gdb_breakpoint "marker"
+	gdb_continue_to_breakpoint "marker"
+
+	gdb_test "detach" \
+	    "Detaching from program: .*process $child_pid"
+    }
+
+    kill_wait_spawned_process $test_spawn_id
+}
+
+do_test