Linux/ALSA Native MIDI backend #637
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This does not do anything yet, but does set up the necessary build changes so that we will link against the libasound library.
We don't write any MIDI data to the sequencer yet but this sets up the basic foundations. For now, the playback thread just exits immediately, so `playmus` completes successfully.
We try various different ports that are usually the "default"; we need to provide a mechanism to specify it explicitly but for now this will do. These are the same "default" ports that DOSbox's ALSA code tries.
This converts all the common MIDI events to the ALSA event types. The converted events are not yet sent to ALSA.
With this change in place the module actually works! There's more still to finish but this is actually usable.
The ALSA device we're sending messages to will continue playing them even if the program quits, and we don't want to leave notes hanging. So when stopping the current track, send the "notes all off" and "reset all controllers" messages to every channel, and also send an ALSA reset event as well for good measure (the Timidity++ daemon seems to respond to it)
When stopping a song we must wait for the playback thread to terminate. However, the thread may currently be doing a blocking write of some MIDI events and we don't actually know how long this may take to complete. Instead, we can use non-blocking mode and the poll() system call. This allows us to sleep when the output event buffer is full, but also allows us to wake it back up when it's time for the song to stop. We accomplish this by creating a pipe that we close on shutdown.
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This (finally!) adds a native MIDI backend for Linux using the ALSA sequencer interface. I've tested this with Chocolate Doom and the Timidity++ ALSA daemon and it works pretty reliably; I haven't gotten around to testing with actual hardware yet. There are still a couple of features missing but it should be complete enough now for people to start using it.