Handle real-time MIDI SysEx messages

[Sean M. Pappalardo]

According to http://www.midi.org/techspecs/midituning.php, universal real-time MIDI SysEx messages are of the form:

F0 7F ... F7

Add to Mixxx's MIDI handling the ability to accept these at any time (such as in the middle of any other non-real-time message, SysEx or otherwise.)

[RJ Skerry-Ryan (rryan)]

A real-time sysex message can come in the middle of a non-real-time sysex message?

Also, you said "F0 7F ... F0" -- I think you meant: " F0 7F ... F7". The page you linked terminates every message with EOX -- realtime or non-realtime.

If sysex messages cannot interleave then we already interpret the real-time ones as a sysex blob since we match F0 .. F7 pairs.

If they can interleave that's pretty surprising because it implies a stack or at least some slush memory -- which for MIDI processors built onto an ASIC would complicate the circuit quite a bit.

[Sean M. Pappalardo (pegasus-renegadetech)]

Oops, yes I meant ending in F7. Typing too quick and not enough sleep last night. :)
My understanding is that yes indeed any real-time MIDI messages (SysEx and otherwise) can arrive at any time (and are expected to be handled ASAP.) Since the data bytes of a SysEx can not exceed 0x7F, this is possible.
Whether all MIDI devices properly handle them is another question, because many of them don't need to. E.g. controllers likely don't need to _receive_ real-time SysExes, but may _send_ them and expect Mixxx to handle them promptly. Since we do have access to resources to properly handle these cases, I think we should to future-proof ourselves. But it's not a priority at all since I don't know of any current controllers that use them. But once Owen's MIDI clock stuff makes it in, I can see other software intercommunicating with Mixxx via RT SysEx.

[RJ Skerry-Ryan (rryan)]

To follow up -- we determined that PortMIDI delivers realtime messages as a separate PmEvent, not in the middle of a message body. There's now a unit test that verifies we interact correctly with PortMIDI's API/convention.

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/8119