Limit the MPD input buffer to less than the available memory.

[the_bertrum]

An essential feature to prevent idiots like me from having to debug why their player stopped responding halfway through an album.

Turns out a Pi3A+ with an input buffer of 512 completely locks up and emits random cheeps every so often as the system gets totally starved of memory.
So if the input cache selection could only show values less than say 75% of the available memory that would protect me from my own stupidity