Fix segfault when invoking -var-info-path-expression on a dynamic varobj

Invoking -var-info-path-expression on a dynamic varobj lead either in wrong
(nonsense) result or to a segmentation fault in cplus_describe_child().
This was caused by the fact that varobj_get_path_expr() called
cplus_path_expr_of_child() ignoring the fact the parent of the variable
is dynamic. Then, cplus_describe_child() accessed the underlaying C type
members by index, causing (i) either wrong (nonsense) expression being
returned (since dynamic child may be completely arbibtrary value)
or (ii) segmentation fault (in case the index higher than number of
underlaying C type members.

This fixes the problem by checking whether a varobj is a child of a dynamic
varobj and, if so, reporting an error as described in documentation.

gdb/ChangeLog:

	* varobj.c (varobj_get_path_expr_parent): Report an error if
	parent is a dynamic varobj.

gdb/testsuite/Changelog:

	* gdb.python/py-mi-var-info-path-expression.c: New file.
	* gdb.python/py-mi-var-info-path-expression.py: New file.
	* gdb.python/py-mi-var-info-path-expression.exp: New file.

1 files changed, 57 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.python/py-mi-var-info-path-expression.py b/gdb/testsuite/gdb.python/py-mi-var-info-path-expression.py
new file mode 100644
index 0000000..6fe0b52
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-mi-var-info-path-expression.py
@@ -0,0 +1,57 @@
+# Copyright (C) 2018 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/>.
+
+import sys
+import gdb
+import gdb.types
+
+# Following is for Python 3 compatibility...
+if sys.version_info[0] > 2:
+    long = int
+
+
+class cons_pp(object):
+  def __init__(self, val):
+    self._val = val
+
+  def to_string(self):
+    if long(self._val) == 0:
+      return "nil"
+    elif long(self._val['type']) == 0:
+      return "( . )"
+    else:
+      return "%d" % self._val['atom']['ival']
+
+  def children(self):
+    if long(self._val) == 0:
+      return []
+    elif long(self._val['type']) == 0:
+      return [
+        ('atom', self._val['atom'])
+      ]
+    else:
+      return [
+        ('car' ,  self._val["slots"][0]),
+        ('cdr' ,  self._val["slots"][1]),
+      ]
+
+def cons_pp_lookup(val):
+  if str(val.type) == 'struct cons *':
+    return cons_pp(val)
+  else:
+    return None
+
+del gdb.pretty_printers[1:]
+gdb.pretty_printers.append(cons_pp_lookup)