Re-acquire GIL earlier in gdbpy_parse_and_eval

Tom de Vries filed a bug about an intermittent gdb DAP failure -- and
coincidentally, at the same time, Alexandra Hájková sent email about a
somewhat similar failure.

After looking into this for a while (with no results) using ASan and
valgrind, I found that setting PYTHONMALLOC=malloc_debug found the bug
instantly.

The problem is that gdbpy_parse_and_eval releases the GIL while
calling parse_and_eval, but fails to re-acquire it before calling
value_to_value_object.  This is easily fixed by introducing a new
scope.

I wonder whether the test suite should unconditionally set
PYTHONMALLOC=malloc_debug.

Tested-by: Tom de Vries <tdevries@suse.de>
Reviewed-By: Tom de Vries <tdevries@suse.de>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30686

gdb/python/python.c

diff --git a/gdb/python/python.c b/gdb/python/python.c
index 505fc44..6a978d6 100644
--- a/gdb/python/python.c
+++ b/gdb/python/python.c
@@ -994,9 +994,17 @@ gdbpy_parse_and_eval (PyObject *self, PyObject *args, PyObject *kw)
   PyObject *result = nullptr;
   try
     {
-      gdbpy_allow_threads allow_threads;
       scoped_value_mark free_values;
-      struct value *val = parse_and_eval (expr_str, flags);
+      struct value *val;
+      {
+	/* Allow other Python threads to run while we're evaluating
+	   the expression.  This is important because the expression
+	   could involve inferior calls or otherwise be a lengthy
+	   calculation.  We take care here to re-acquire the GIL here
+	   before continuing with Python work.  */
+	gdbpy_allow_threads allow_threads;
+	val = parse_and_eval (expr_str, flags);
+      }
       result = value_to_value_object (val);
     }
   catch (const gdb_exception &except)