[Bug] QGIS crashes instantly on macOS (arm64) in v5.0.3 – QDialog opened from background thread during dependency install

[zicherino]

[Bug] QGIS crashes instantly on macOS (arm64) in v5.0.3 – QDialog opened from background thread during dependency install

Description

This is a follow-up to #47 (Python interpreter mismatch on macOS arm64), which was fixed in v5.0.3.

After updating to v5.0.3, the interpreter fix works — QGIS now finds the correct Python. However, a new crash has been introduced: QGIS crashes immediately (SIGABRT) when Dzetsaka attempts to install the remaining missing packages. The crash occurs because a QDialog is being opened from within a background QgsTask thread, which violates Qt's rule that all GUI operations must happen on the main thread.

In v5.0.2, this code path was never reached (the wrong interpreter was used, so installation silently failed). Now that the interpreter is correctly detected and installation actually proceeds, the threading bug is exposed.

This is a hard crash with no recovery — QGIS terminates instantly.

Environment

• QGIS Version: 3.44.7
• Dzetsaka Version: 5.0.3 (regression introduced in this version)
• OS: macOS 26.3 (Apple Silicon / ARM64, MacBook Pro)
• Python: 3.12

Steps to Reproduce

1. Install Dzetsaka plugin in QGIS on macOS (Apple Silicon)
2. Open the Dzetsaka panel
3. Dzetsaka detects missing packages and attempts to auto-install them
4. QGIS crashes instantly

Missing packages that triggered the crash:

• lightgbm
• optuna
• shap
• seaborn

Full bundle Dzetsaka attempted to install:
scikit-learn, xgboost, catboost, optuna, shap, seaborn, imbalanced-learn

Expected Behavior

Missing packages are installed silently in the background, or the user is prompted in a thread-safe way (e.g. via a signal/slot mechanism to the main thread).

Actual Behavior

QGIS crashes immediately with EXC_CRASH (SIGABRT).

Crash Analysis

The macOS crash report clearly shows the cause. Thread 18 (a pooled Qt thread running QgsTask) calls QDialog::exec() directly, which is forbidden outside the main thread:

34  _core.so                 sipQgsTask::run()
...
27  QtWidgets.abi3.so        meth_QDialog_exec(...)   ← GUI call from background thread
26  libQt5Widgets.5.15.dylib QDialog::exec()
...
11  AppKit                   -[NSWindow _initContent:styleMask:backing:defer:contentView:]
10  CoreFoundation           -[NSException raise]     ← macOS throws exception
 2  libsystem_c.dylib        abort()                  ← SIGABRT → QGIS terminates

Root cause: The dependency installation routine runs inside a QgsTask and opens a QDialog from that background thread. On macOS (especially Apple Silicon), AppKit enforces strict main-thread-only UI access and throws an NSException, which propagates as a C++ exception and causes abort().

This does not appear to affect Windows or Linux, where the restriction is not enforced at the OS level.

Workaround

Install all required packages manually into QGIS's Python environment before launching QGIS:

/Applications/QGIS.app/Contents/MacOS/bin/python3 -m pip install \
  lightgbm optuna shap seaborn \
  scikit-learn xgboost catboost \
  imbalanced-learn --upgrade

Doesn't work.

[nkarasiak]

Hi! Thanks for the detailed bug report.

We've implemented a new approach to fix this crash in dzetsaka v5.0.5, which is now available in the QGIS plugin repository. The key changes:

• Replaced the cross-thread taskCompleted/taskTerminated signals with a main-thread finished() callback, which avoids the SIGABRT from Qt's
  thread safety checks
• Dependency installation now runs via subprocess in a QgsTask background thread (no more QProcess/QEventLoop from a worker thread)
• Also added pip bootstrapping via get-pip.py and --break-system-packages for PEP 668 compatibility

Since we don't have access to a macOS machine, we couldn't verify the fix ourselves. Could you please update to v5.0.5 and let us know if it
resolves the crash?

You can update from Plugins → Manage and Install Plugins → Upgradeable in QGIS.

[zicherino]

Hi! Thanks for your quick help.

Now the Autoinstallation is Crashing QGIS.

Description:
QGIS crashes with a SIGABRT when clicking the "Auto-install dependencies" button in the dzetsaka classification dialog.
Steps to reproduce:

Open QGIS and activate the dzetsaka plugin
Start a classification with missing ML dependencies
Click "Auto-install dependencies" in the dialog that appears
QGIS crashes immediately

Expected behavior:
Dependencies are installed without crashing.
Actual behavior:
QGIS crashes with EXC_CRASH (SIGABRT).
Root cause:
The crash occurs because dzetsaka attempts to create a UI dialog (NSWindow) from a background thread (QgsTask/thread pool). On macOS, AppKit strictly forbids UI operations outside the main thread and throws an exception, which causes QGIS to abort.
Relevant crash trace:
Thread 30 Crashed (pooled):
AppKit: -[NSWindow _initContent:styleMask:backing:defer:contentView:]
...
sipQgsTask::run()
QgsTask::start()

[nkarasiak]

Hi @zicherino, thanks for the follow-up, helpful to know the crash persisted after v5.0.5 :(

We've just released v5.0.6 which should be available in the QGIS Plugin Manager now. Here's what changed:

The v5.0.5 fix correctly moved GUI calls out of background threads into the finished() callback (which runs on the main thread). However, on macOS ARM64, opening a modal dialog (QMessageBox.exec()) directly inside QGIS's task-manager callback chain still triggers AppKit's assertion.
The fix is to defer all dialog calls via QTimer.singleShot(0, ...) so they execute in the next event-loop iteration, cleanly outside the callback chain.

Could you please update to v5.0.6 and try again?

If the crash persists, we've also added thread-identity diagnostic logging. After triggering the install, check View → Log Messages → dzetsaka
for lines like:
[DependencyInstallTask.run] thread=...
[DependencyInstallTask.finished] thread=...
[on_task_finished] thread=...
[on_task_finished._show_result] thread=...
Sharing those log lines (and any crash trace) would help us pinpoint the exact issue. Thanks again for reporting this!

[nkarasiak]

Hi @zicherino ,
Did this fix work ?
If not, I'll try to test and try on a Mac laptop of a friend.

[zicherino]

Hi, sorry for the late answer.

Unfortunatly it didn‘t work.

QGIS is crashing on the autoinstaller tab or while classifying with the normal Models (like RF). I‘m trying to reinstall QGIS completly to see if the Error still persists.

If I can provide you any Log-Data. Let me now.

[nkarasiak]

No problem @zicherino , thanks for the feedback. I'll try to solve it directly by using a Mac, it will be more reliable :)
I'll let you know when it will be fixed. Sorry for this issue.

[zicherino]

Thanks for your help!

[nkarasiak]

Just a little update @zicherino .
I tried to fix that yesterday using a MacOS system, but after one hour still lot of issues. And lot of issues fixed. I didn't know there were that much of differences between MacOS and Linux (I'm on debian), as it is Unix.
I'll continue tomorrow.

[nkarasiak]

Hello @zicherino, and sorry, I didn't forget you. I'll try to look at that as soon as possible.