Wrapper for DBusMessageIter doesn't reference the DBusMessage used to initialize it

[ziyao233]

After a DBusMessageIter is initialized with a DBusMessage through dbus_message_iter_init(), the iterator actually references the message: usage of the iterator requires the message to stay valid.

ldbus doesn't claim the reference, so it's possible that a message gets garbage collected while some iterators still reference it, thus usage of such a iterator triggers a segfault, for example,

local ldbus		= require "ldbus";
collectgarbage("stop");
local msg = ldbus.message.new_method_call("org.freedesktop.DBus",
			"/org/freedesktop/DBus",
			"org.freedesktop.DBus.Debug.Stats",
			"GetAllMatchRules");
local iter = ldbus.bus.get("session"):
		send_with_reply_and_block(msg):
		iter_init();
collectgarbage(); -- Commenting this line prevents the segfault
print(iter:get_arg_type());

segfaults during iter:get_arg_type() (ldbus master with Lua 5.4), since the message returned by send_with_reply_and_block() has been collected, which makes iter a dangdling reference.

Suggest fix: when creating/cloning new iterators, reference the underlying dbus.message with luaL_ref() in the registry table, and unref the message during __gc.

[daurnimator]

huh. Seems weird to me that the DBusMessageIter doesn't itself have a ref to the DBusMessage.

Suggest fix: when creating/cloning new iterators, reference the underlying dbus.message with luaL_ref() in the registry table, and unref the message during __gc.

There should be no need for that level of indirection:
Option 1. increase the DBusMessage ref when we create a DBusMessageIter; and unref on __gc; however I can't seem to see a way to get the DBusMessage from the DBusMessageIter.
Option 2. keep the DBusMessage in the ldbus_DBusMessageIter userdata's uservalue.

[ziyao233]

huh. Seems weird to me that the DBusMessageIter doesn't itself have a ref to the DBusMessage.

Same, since there's already some type of reference counting existing for DBusMessage. But the RC cannot be done with current API as you're allowed to memcpy an iterator. I think it's a design mistake.

Suggest fix: when creating/cloning new iterators, reference the underlying dbus.message with luaL_ref() in the registry table, and unref the message during __gc.

There should be no need for that level of indirection:
Option 1. increase the DBusMessage ref when we create a DBusMessageIter; and unref on __gc; however I can't seem to see a way to get the DBusMessage from the DBusMessageIter.
Option 2. keep the DBusMessage in the ldbus_DBusMessageIter userdata's uservalue.

Maybe 2 is the only option.