bug: Kivy RecycleView data mutated from background thread in show_debug_log (crash/corruption)

[RockyOmvi]

Describe the bug

In src/buskill_gui.py, the show_debug_log() method modifies self.rv.data (a Kivy RecycleView property) from a background threading.Thread. Kivy's UI properties are not thread-safe — they must only be modified from the main thread. This can cause random crashes, data corruption, or visual glitches.

Code reference

src/buskill_gui.py:1026-1032:

threading.Thread( target=self.show_debug_log ).start()
...

def show_debug_log( self ):
    lines = []
    for line in self.debug_log_contents.splitlines(keepends=False):
        lines.append({'text': line})
    self.rv.data = lines   # <-- UI mutation from background thread

Expected behavior

Use Clock.schedule_once() to schedule the data update on the main thread:

def show_debug_log( self ):
    lines = [{'text': line} for line in self.debug_log_contents.splitlines(keepends=False)]
    from kivy.clock import Clock
    Clock.schedule_once(lambda dt: setattr(self.rv, 'data', lines), 0)

Severity

Medium — can cause hard-to-reproduce crashes and visual corruption in the debug log screen.

[maltfield]

@RockyOmvi thanks for the detailed bug report

Do you have a suggested code fix for when we update a specific element in the array of self.rv.data?

We used RecycleView much heavier in the dev branch, for the fonts sub-Screen of the Settings Screen. For example

buskill-app/src/buskill_gui.py

Lines 926 to 947 in 441578c

	# UPDATE DATA
	# loop through every dict of OptionItem data in the Recycle View's list
	# of dicts
	for n in range(0,len(self.rv.data)):
	# get the value for this specific OptionItem
	this_value = self.rv.data[n]['value']
	if str(this_value) == str(set_value):
	# this is the currently-set option
	# set the radio button icon to "selected"
	self.rv.data[n]['radio_button_icon'] = '[font=mdicons][size=18sp]\ue837[/size][/font] '
	else:
	# this is not the currently-set option
	# set the radio button icon to "unselected"
	self.rv.data[n]['radio_button_icon'] = '[font=mdicons][size=18sp]\ue836[/size][/font] '
	# update RecycleView data in next frame
	# * https://stackoverflow.com/questions/49935190/kivy-how-to-initialize-the-viewclass-of-the-recycleview-dynamically
	Clock.schedule_once(self.rv.refresh_from_data,0)

...or is this not an issue because I already do schedule the updating?

Update: or maybe this isn't an issue at all, because we're not creating the "Font Face" Complex Option Screen inside a thread?

[RockyOmvi]

@maltfield Nah your font code is fine. Two reasons:

1. It's not running in a thread — it's on the main thread, so mutating self.rv.data[n] directly is perfectly safe. The thread-safety rule only applies when you're inside a threading.Thread.
2. You're already calling Clock.schedule_once(self.rv.refresh_from_data, 0) after the mutations, which is exactly what Kivy recommends to refresh the RecycleView. So you're good.

The only problem was show_debug_log — that one's actually inside a thread and was touching self.rv.data directly. I put up a fix in PR #125 that schedules the assignment onto the main thread with Clock.schedule_once.