Simple way to get a fd polled in glib?

[Steven Brown <swbrown variadic org>]

I've been going through the glib docs trying to find a simple way to get
my fd polled by glib's main loop and call me back, but I've not found
anything near simple.  Is there such a way?  E.g., something like
gdk_input_add (which says it's deprecated but doesn't say by what).

I see that it could be implemented by a custom g_source_new and such,
but doing so is a pretty huge amount of code for what should be a
one-liner.  I must be missing something obvious.