MIDI device change causes crash in clap-host

[AleksandrMol]

MIDI device change causes crash in clap-host

OS: Fedora KDE
MIDI device: Yamaha PSR-R300

Steps to reproduce

1. Launch clap-host.
2. Go to File → Settings.
3. Under MIDI → Device, switch the device to USB MIDI Interface.
4. Click OK.

The host aborts with SIGABRT.

Possible cause

I traced the crash to Engine::audioCallback() in host/engine.cc.

The code checks whether the MIDI buffer is empty, but then unconditionally accesses midiBuf[1] and midiBuf[2].

My understanding is that RtMidiIn::getMessage() can return MIDI messages shorter than 3 bytes. In that case, accessing these indices can trigger the std::vector::operator[] bounds assertion and abort the process.

The relevant code is:

if (midiBuf.empty())
    break;

uint8_t eventType = midiBuf[0] >> 4;
uint8_t channel = midiBuf[0] & 0xf;
uint8_t data1 = midiBuf[1];
uint8_t data2 = midiBuf[2];

Possible fix

I tested the following guard locally:

if (midiBuf.size() < 3)
    continue;

placed before accessing midiBuf[1] and midiBuf[2].

With this change, the crash no longer occurs in my setup.

I would appreciate someone familiar with the project/RtMidi integration checking whether this is the appropriate way to handle shorter MIDI messages, or whether the MIDI input should be handled differently.