Fix macOS SIGSEGV crash in live webcam mode (tkinter thread-safety)

[llulioscesar]

Summary

• Fix SIGSEGV crash (Tk_FreeGC → TkButtonWorldChanged → ConfigureButton) that occurs on macOS when starting live webcam mode
• Replace direct tkinter widget updates from background threads with ROOT.after(0, ...) to schedule UI changes on the main Tk event loop
• Remove ROOT.update() call from update_status() which re-enters the Tk event loop from background threads

Problem

On macOS (tested on Apple Silicon M2 Max, macOS 26.3.1), clicking "Live" in the webcam preview causes an immediate crash with:

Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Thread 18 Crashed:
0 libtcl9tk9.0.dylib Tk_FreeGC + 24
1 libtcl9tk9.0.dylib TkButtonWorldChanged + 104
2 libtcl9tk9.0.dylib ConfigureButton + 2200

This happens because update_status(), update_pop_status(), and update_pop_live_status() call widget.configure() directly from background threads. Tkinter is not thread-safe on macOS, and modifying widgets from non-main threads causes segfaults in Tk's graphics context management.

Fix

• Use ROOT.after(0, callback) to schedule all widget updates on the main Tk thread
• Add null/existence guards to prevent crashes if widgets are destroyed before the callback executes
• Remove ROOT.update() from update_status() (unnecessary with ROOT.after() and dangerous from background threads)

Test plan

• Launch python run.py on macOS
• Select a source face image
• Click "Live" → should no longer crash
• Verify status messages still update in the UI
• Test on Linux/Windows to confirm no regression

Summary by Sourcery

Route tkinter status label updates through the main Tk event loop to prevent macOS crashes when updating the UI from background threads.

Bug Fixes:

• Prevent SIGSEGV crashes on macOS live webcam mode caused by updating tkinter widgets from background threads.

Enhancements:

• Schedule status and popup label updates via the Tk root event loop and add existence checks to avoid updating destroyed widgets.

[sourcery-ai]

Reviewer's Guide

Refactors tkinter status update helpers to marshal all widget configuration calls onto the main Tk thread via ROOT.after, preventing macOS SIGSEGV crashes caused by updating widgets from background threads, and adds existence guards while removing a risky ROOT.update() call.

Sequence diagram for thread-safe status updates via ROOT.after

sequenceDiagram
    actor User
    participant BackgroundThread
    participant ui_module
    participant ROOT
    participant TkMainLoop
    participant status_label

    User->>BackgroundThread: trigger_long_running_task
    BackgroundThread->>ui_module: update_status(text)
    activate ui_module
    ui_module->>ui_module: translated = _(text)
    alt ROOT_is_not_none
        ui_module->>ROOT: after(0, _do)
        ROOT->>TkMainLoop: schedule_callback(_do)
        activate TkMainLoop
        TkMainLoop->>ui_module: execute _do()
        activate ui_module
        alt status_label_is_not_none
            ui_module->>status_label: configure(text=translated)
        end
        deactivate ui_module
        deactivate TkMainLoop
    else ROOT_is_none
        ui_module-->>BackgroundThread: no_ui_update
        deactivate ui_module
    end

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Sequence diagram for thread-safe popup live status updates

sequenceDiagram
    actor User
    participant BackgroundThread
    participant ui_module
    participant ROOT
    participant TkMainLoop
    participant POPUP_LIVE
    participant popup_status_label_live

    User->>BackgroundThread: start_live_webcam
    BackgroundThread->>ui_module: update_pop_live_status(text)
    activate ui_module
    ui_module->>ui_module: translated = _(text)
    alt ROOT_is_not_none
        ui_module->>ROOT: after(0, _do)
        ROOT->>TkMainLoop: schedule_callback(_do)
        activate TkMainLoop
        TkMainLoop->>ui_module: execute _do()
        activate ui_module
        alt popup_status_label_live_and_POPUP_LIVE_valid
            ui_module->>POPUP_LIVE: winfo_exists()
            POPUP_LIVE-->>ui_module: exists
            ui_module->>popup_status_label_live: configure(text=translated)
        else popup_or_label_invalid
            ui_module-->>TkMainLoop: skip_update
        end
        deactivate ui_module
        deactivate TkMainLoop
    else ROOT_is_none
        ui_module-->>BackgroundThread: no_ui_update
        deactivate ui_module
    end

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Flow diagram for guarded popup status update logic

flowchart TD
    A["update_pop_status(text) called"] --> B["translated = _(text)"]
    B --> C{"ROOT is not None"}
    C -- "No" --> Z["Return without scheduling UI update"]
    C -- "Yes" --> D["Call ROOT.after(0, _do)"]
    D --> E["Later on Tk main thread: execute _do()"]
    E --> F{"popup_status_label is not None"}
    F -- "No" --> Z
    F -- "Yes" --> G{"POPUP is not None"}
    G -- "No" --> Z
    G -- "Yes" --> H{"POPUP.winfo_exists() is True"}
    H -- "No" --> Z
    H -- "Yes" --> I["popup_status_label.configure(text=translated)"]
    I --> Z["End"]

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File-Level Changes

Change	Details	Files
Make status label updates thread-safe by scheduling them on the main Tk event loop instead of calling widget.configure directly from background threads.	Store the translated status text in a local variable before scheduling the update Wrap the label.configure call in an inner _do() callback Guard the callback with a None-check on the status_label reference Use ROOT.after(0, _do) when ROOT is available to enqueue the UI update on the main thread	modules/ui.py
Harden popup status updates (normal and live) against race conditions and destroyed widgets, while also moving them to the main Tk event loop.	Translate the text once and capture it in a local variable for each update function Introduce inner _do() callbacks for popup_status_label and popup_status_label_live Add guards ensuring the popup windows exist (POPUP/POPUP_LIVE not None and winfo_exists() is true) before configuring labels Use ROOT.after(0, _do) when ROOT is available to perform the updates on the main thread	modules/ui.py
Remove direct re-entry into the Tk event loop from background threads to avoid unsafe concurrent Tk operations.	Delete the ROOT.update() call from update_status() as it is no longer needed with ROOT.after and is unsafe from non-main threads	modules/ui.py

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[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• When scheduling callbacks with ROOT.after, consider also checking ROOT.winfo_exists() (or handling TclError) to avoid crashes if the root window is destroyed before the callback is queued.
• Even with existence checks before configure(), there is still a race if the widget is destroyed between the check and the configure; consider wrapping the configure() calls in a small try/except TclError to make this fully robust against teardown races.
• The three update_* functions now share a very similar pattern (translate → inner _do → ROOT.after); extracting a small helper to schedule safe label updates on the main thread would reduce duplication and keep future changes to this behavior in one place.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- When scheduling callbacks with ROOT.after, consider also checking ROOT.winfo_exists() (or handling TclError) to avoid crashes if the root window is destroyed before the callback is queued.
- Even with existence checks before configure(), there is still a race if the widget is destroyed between the check and the configure; consider wrapping the configure() calls in a small try/except TclError to make this fully robust against teardown races.
- The three update_* functions now share a very similar pattern (translate → inner _do → ROOT.after); extracting a small helper to schedule safe label updates on the main thread would reduce duplication and keep future changes to this behavior in one place.

## Individual Comments

### Comment 1
<location path="modules/ui.py" line_range="760-761" />
<code_context>
+        if status_label is not None:
+            status_label.configure(text=translated)
+
+    if ROOT is not None:
+        ROOT.after(0, _do)


</code_context>
<issue_to_address>
**issue:** Consider guarding against a destroyed ROOT before calling `after` to avoid potential `TclError`.

If `ROOT` has been destroyed but the reference is still non-`None`, `ROOT.after` may raise `TclError`. To harden this, also check `ROOT.winfo_exists()` before scheduling the callback, mirroring the popup checks.
</issue_to_address>

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