Use unique_xmalloc_ptr in Python code

This changes some utility functions in the Python code to return
unique_xmalloc_ptr, and then fixes up the callers.

I chose unique_xmalloc_ptr rather than std::string because at a few
call points the xmalloc'd string is released and ownership transferred
elsewhere.

This patch found a few existing memory leaks.  For example,
py-unwind.c called gdbpy_obj_to_string but never freed the result.

Built and regression tested on the buildbot.

2016-11-09  Tom Tromey  <tom@tromey.com>

	* varobj.h (varobj_get_display_hint): Change return type.
	* varobj.c (varobj_get_display_hint): Return unique_xmalloc_ptr.
	(varobj_value_get_print_value): Update.
	* python/python.c (gdbpy_before_prompt_hook, gdbpy_print_stack)
	(gdbpy_apply_type_printers): Update.
	* python/python-internal.h (unicode_to_target_string)
	(python_string_to_target_string, python_string_to_host_string)
	(gdbpy_obj_to_string, gdbpy_exception_to_string)
	(gdbpy_get_display_hint): Change return types.
	* python/py-varobj.c (py_varobj_iter_next): Update.
	* python/py-value.c (valpy_getitem, convert_value_from_python):
	Update.
	* python/py-utils.c (unicode_to_encoded_string)
	(unicode_to_target_string, python_string_to_target_string)
	(python_string_to_host_string, gdbpy_obj_to_string)
	(gdbpy_exception_to_string): Return unique_xmalloc_ptr.
	* python/py-unwind.c (pyuw_parse_register_id): Update.
	* python/py-type.c (typy_getitem): Update.
	* python/py-prettyprint.c (gdbpy_get_display_hint)
	(print_stack_unless_memory_error, print_children)
	(gdbpy_apply_val_pretty_printer): Update.
	* python/py-param.c (set_parameter_value): Update.
	(get_doc_string, call_doc_function): Return unique_xmalloc_ptr.
	(get_set_value, get_show_value, compute_enum_values, parmpy_init):
	Update.
	* python/py-infthread.c (thpy_set_name): Update.
	* python/py-function.c (fnpy_call, fnpy_init): Update.
	* python/py-framefilter.c (extract_sym): Change "name" to
	unique_xmalloc_ptr.
	(enumerate_args, enumerate_locals): Update.
	(py_print_frame): Use unique_xmalloc_ptr.
	* python/py-frame.c (frapy_read_var): Update.  Remove cleanup.
	* python/py-cmd.c (cmdpy_function, cmdpy_completer, cmdpy_init):
	Update.
	* python/py-breakpoint.c (bppy_set_condition): Use
	unique_xmalloc_ptr.
	(bppy_init): Likewise.  Remove cleanup.
	(local_setattro): Update.
	* mi/mi-cmd-var.c (print_varobj, mi_cmd_var_list_children)
	(varobj_update_one): Update.

gdb/python/python-internal.h

@@ -533,14 +533,15 @@ int gdbpy_print_python_errors_p (void);
 void gdbpy_print_stack (void);
 
 PyObject *python_string_to_unicode (PyObject *obj);
-char *unicode_to_target_string (PyObject *unicode_str);
-char *python_string_to_target_string (PyObject *obj);
+gdb::unique_xmalloc_ptr<char> unicode_to_target_string (PyObject *unicode_str);
+gdb::unique_xmalloc_ptr<char> python_string_to_target_string (PyObject *obj);
 PyObject *python_string_to_target_python_string (PyObject *obj);
-char *python_string_to_host_string (PyObject *obj);
+gdb::unique_xmalloc_ptr<char> python_string_to_host_string (PyObject *obj);
 PyObject *host_string_to_python_string (const char *str);
 int gdbpy_is_string (PyObject *obj);
-char *gdbpy_obj_to_string (PyObject *obj);
-char *gdbpy_exception_to_string (PyObject *ptype, PyObject *pvalue);
+gdb::unique_xmalloc_ptr<char> gdbpy_obj_to_string (PyObject *obj);
+gdb::unique_xmalloc_ptr<char> gdbpy_exception_to_string (PyObject *ptype,
+							 PyObject *pvalue);
 
 int gdbpy_is_lazy_string (PyObject *result);
 void gdbpy_extract_lazy_string (PyObject *string, CORE_ADDR *addr,
@@ -555,7 +556,7 @@ PyObject *apply_varobj_pretty_printer (PyObject *print_obj,
 				       struct value **replacement,
 				       struct ui_file *stream);
 PyObject *gdbpy_get_varobj_pretty_printer (struct value *value);
-char *gdbpy_get_display_hint (PyObject *printer);
+gdb::unique_xmalloc_ptr<char> gdbpy_get_display_hint (PyObject *printer);
 PyObject *gdbpy_default_visualizer (PyObject *self, PyObject *args);
 
 void bpfinishpy_pre_stop_hook (struct gdbpy_breakpoint_object *bp_obj);