Previously, I had used integers encoded as pointers. This
introduced a flaw: NULL represented both the integer zero, and the
absence of a setting. This in turn made the checks in
cfg_msg_set_{type,chan}() not actually check for an altered value if
the previous value had been set to zero.
Also, I had previously kept a pointer to a dispatch_fypefns_t rather
than making a copy of it. This meant that if the dispatch_typefns_t
were changed between defining the typefns and creating the
dispatcher, we'd get the modified version.
Found while investigating coverage in pubsub_add_{pub,sub}_()
This module implements a way to send messages from one module to
another, with associated data types. It does not yet do anything to
ensure that messages are correct, that types match, or that other
forms of consistency are preserved.
