Issue when writing a duplex ALSA_seq driver with fluid-style routing

[albedozero]

Hello - I've been experimenting with writing my own duplex ALSA sequencer client that could filter/transform MIDI events, then echo the routed events to other subscribed ports and/or pass them on to a fluid_synth, for purposes similar to what @denybear outlined in #1198. I cribbed heavily from fluid_alsa.c and fluid_midi_router.c, so I thought perhaps someone here might have insight into the issue I encountered.

The issue is that I can output routed events to the buffer with snd_seq_event_output(), but trying to drain the buffer to the sequencer with snd_seq_drain_output() returns -EINVAL. Looking at the alsa lib code, I think this is supposed to mean the output buffer is somehow invalid or inaccessible. A simple test output program I wrote in the source folder linked above works fine, so the only thing I can think of is that because the function to output MIDI events to the sequencer must be called while looping through the router rules, the rules_mutex is somehow blocking access to the appropriate memory location. Anyone here have the knowledge/insight to confirm or set me straight? Thanks for your time!