gdb: use make_scoped_restore to restore gdbpy_current_objfile

The current mechanism by which the Python gdb.current_objfile is
maintained does not allow for nested auto-load events.  It is assumed
that once an auto-load script has finished loading then the current
objfile should be set back to NULL.  In a nested situation, we should
be restoring the previous value.

We already have an RAII class to handle save/restore type behaviour,
so lets just switch to use that.

The test is a little contrived, but is simple enough, and triggers the
bug.  The real use case might involve the auto-load script calling
functions (either in the just-loaded object file, or in the main
executable), which in turn trigger further auto-loads to occur.

gdb/ChangeLog:

	* python/python.c (gdbpy_source_objfile_script): Use
	make_scoped_restore to restore gdbpy_current_objfile.
	(gdbpy_execute_objfile_script): Likewise.

gdb/testsuite/ChangeLog:

	* gdb.python/py-auto-load-chaining-f1.c: New file.
	* gdb.python/py-auto-load-chaining-f1.o-gdb.py: New file.
	* gdb.python/py-auto-load-chaining-f2.c: New file.
	* gdb.python/py-auto-load-chaining-f2.o-gdb.py: New file.
	* gdb.python/py-auto-load-chaining.c: New file.
	* gdb.python/py-auto-load-chaining.exp: New file.

1 files changed, 24 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.python/py-auto-load-chaining-f1.c b/gdb/testsuite/gdb.python/py-auto-load-chaining-f1.c
new file mode 100644
index 0000000..77250bd
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-auto-load-chaining-f1.c
@@ -0,0 +1,24 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2021 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/>.  */
+
+/* Just a dummy function.  */
+
+int
+f1 ()
+{
+  return 1;
+}