The problem was pointed out by Josh
(gitlab.com/krazy-j) in merge request !22.
Apparently MT is not very smart about marking
mapblocks dirty to send to clients based on
calling mt.set_node(), i.e. it will mark them
dirty presumptively even if you set the node to
the same value it had already been.
This behavior can be confirmed by registering
an ABM against a common node like grass and
setting action = minetest.set_node. This
causes every mapblock containing that node
to be invalidated every interval, causing a big
spike in the packets received each interval
that you can clearly see on the F5 graph.
Rather than just fixing it for the most easily
observed case (fire checks), add utlity functions
to check this for ALL node change situations,
and apply it more or less universally anywhere
that we are not certain that the node is being
changed and we don't need to worry about the
extra overhead cost of the check.
Note that we don't need a
nodecore.set_loud_check call, as set_loud was
only ever being used already in cases where
we were pretty sure we were actually changing
a node.
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