The only secure and robust way to check that a userdata is of the
expected type is to check its metatable. Userdata metatables are not
changeable by Lua code without the debug library, so if the metatable is
a certain table, only C code could have made it so. GLib type checking
functions are _not_ a robust or secure way to check that a block of
memory is a specific type of GObject, because Lua code could cause a
type confusion bug and potentially use it to forge pointers.
The intention is to make checks for enabled log topics faster.
Every topic has its own structure that is statically defined in the file
where the logs are printed from. The structure is initialized transparently
when it is first used and it contains all the log level flags for the levels
that this topic should print messages. It is then checked on the wp_log()
macro before printing the message.
Topics from SPA/PipeWire are also handled natively, so messages are printed
directly without checking if the topic is enabled, since the PipeWire and SPA
macros do the checking themselves.
Messages coming from GLib are checked inside the handler.
An internal WpLogFields object is used to manage the state of each log
message, populating all the fields appropriately from the place they
are coming from (wp_log, spa_log, glib log), formatting the message and
then printing it. For printing to the journald, we still use the glib
message handler, converting all the needed fields to GLogField on demand.
That message handler does not do any checks for the topic or the level, so
we can just call it to send the message.
