Sonic pi portamento

portamento.rb

STOP_NOTE = 60 # stop on pressing the C note, you can use another MIDI or timer event to stop the synth

live_loop :midi_keys do
  use_real_time
  note, velocity = sync "/midi:*:1/note_*"
  
  if velocity > 0 && note != STOP_NOTE
    if (get :current_note) == nil
      use_synth :dsaw
      x = play note: note, note_slide: 0.3, amp: velocity / 127.0, sustain: 300
      set :current_note, x
    else
      control (get :current_note), note: note
    end
  end
end

live_loop :stop do
  use_real_time
  
  note, velocity = sync "/midi:*:1/note_*"
  
  if note == STOP_NOTE
    control (get :current_note), amp: 0, amp_slide: 0.5
    sleep 0.5
    kill (get :current_note)
    set :current_note, nil
  end
end

Demo: https://youtu.be/_P8UgGLzTWY