The function fluid_player_join in the library is broken beyond compare for the
usecases it was used for (see their #872). It does not wait until it is safe
to delete the player, so it is up to the end user to ensure that.
For OpenTTD we acquire a lock before fluid_synth_write_s16 and we acquire the
same lock in the stop function. So, only one of the functions can be doing its
thing, meaning we do not need to wait for the player to be stopped as it
cannot be doing anything as we prevent that by the lock.
In FluidSynth 2.2.0 an extra state was added to denote stopping. To transition
from this state to a stopped state the rendering needs to be running. Since
04ce1f07 locking was added that skipped the rendering when something else held
a lock, so the state would never get to stopped and join would never return.
Debian now provides a default soundfont for FluidSynth via its alternatives system.
In short, FluidSynth is configured to look for `/usr/share/sounds/sf3/default-GM.sf3` as its default soundfont, and each soundfront package (FluidR3, OPL-3, MuseScore...) may provide or override this symlink. By default, FluidSynth is installed on Debian with the `TimGM6mb` soundfont by default due to its limited size.
See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=929185 for further details.
Rubidium