After the latest update of Carla on Fedora 43 (Carla-2.5.10-4), the application consistently freezes when attempting to load a Carla project with plugins. This includes both previously existing projects and newly created ones. The freeze occurs during the project loading process causing the application to become unresponsive and does not produce any logs or crash reports and the process remains running but appears to be stuck indefinitely. To rule out project or plugin-related issues, I performed the following checks using the updated version: - Manually recreated the same configuration(adding VST plugins and setting up audio routing) from scratch and it works without issues - Then save the previous configuration as a new Carla project and load it, causing the program to freeze and stop responding This indicates the issue is specifically triggered by the project loading mechanism rather than plugin functionality. Reproducible: Always Steps to Reproduce: 1. Install or update Carla to the latest version(Carla-2.5.10-4) 2. Launch Carla normally. 3. Create a new project with any vst plugin. 4. Save the project to disk. 5. Close the project (or restart the application). 6. Attempt to load the previously saved project. Actual Results: Carla freezes and becomes unresponsive. No logs or error messages are produced, and the process does not crash. Expected Results: The Carla project should be loaded successfully and the application should not freeze. Additional Information: My system specs are: Fedora 43 (KDE Plasma, Wayland session) running on an AMD Ryzen 5 5600 (Zen 3) with an NVIDIA GTX 1080 Ti (proprietary drivers via akmod). Following the repo maintainer guidance (see https://bodhi.fedoraproject.org/updates/FEDORA-2026-0b24ba2c80), I ran Carla under GDB (`carla --gdb`), reproduced the freeze, interrupted execution with Ctrl+C, and collected a full thread backtrace using `thread apply all bt`. I passed the backtrace to an AI to interpret it since I have no technical knowledge and this is what it pointed out: "The trace shows the application blocking inside JUCE’s VST3 handling during plugin activation, specifically: * juce::VST3PluginInstance::prepareToPlay * juce::MessageManagerLock The stack trace indicates the thread is waiting on a condition variable (`futex`), suggesting a potential deadlock involving the message/UI thread. The hang also involves the PipeWire JACK layer, specifically during a buffer size callback: * CarlaBackend::CarlaEngineJack::carla_jack_bufsize_callback_plugin * pipewire-jack (data-loop / buffer size handling) This suggests the freeze may involve an interaction between the audio processing thread (PipeWire/JACK) and JUCE’s MessageManagerLock, potentially indicating a threading or synchronization issue." As a workaround, downgrading to the previous version of Carla(Carla-2.5.10-1) restores normal behavior, confirming this is a regression introduced by the latest update. I've attached the full debug trace and I can provide additional system details if needed.
AI is not a reliable diagnostic tool and should not be used in bug reports, please always provide the raw information. If it is long, you can save it to a file and attach it instead of pasting it directly.
FWIW I cannot reproduce the problem as described (I wrote the patch and obviously it works for me with normal project loading). It's interesting that it hangs with an empty project, since the code path I changed was in plugin to JACK bridging (JACK multi client mode), and with an empty project there should be no plugin clients! But maybe I missed something... One thing I wonder is whether the difference is some Carla setting, so if you could attach your `~/.config/falkTX/Carla2.conf` it might give us a clue.
Created attachment 2136771 [details] Full gdb debug log
Thanks for taking a look at this. I wanted to provide a quick update after re-testing since I had already downgraded and removed the test projects but I’m currently no longer able to reproduce the issue with a completely empty project. It now only occurs when loading projects that contain plugins. With that being said it is possible my earlier report about empty projects was inaccurate or affected by inconsistent reproduction on my side since I was trying to recreate my settings and fiddle around with the plugins to see if they had something to do with the issue, apologies for any confusion caused. That said, the issue remains consistent when plugins are involved, and the behavior is unchanged which causes the application to freeze without logs or crashing. I also re-ran the debugging steps (carla --gdb → run → reproduce → Ctrl+C → thread apply all bt), to obtain the full log and the backtrace still points to the same area as before (JUCE VST3 handling and MessageManagerLock, along with PipeWire/JACK interaction). I've attached the full log and updated the bug report to match more accurately the issue. Please let me know if my Carla settings file is still needed or maybe any other kind of details from my end. Thanks once again and apologies for any trouble caused.
I think the issue is probably VST3 plugins (I only tested LV2/LADSPA plugins). I don't understand why it deadlocks though. I will forward this upstream and see if falkTX has any idea. Can you confirm that LV2 plugins work?
Mentioned upstream here: https://github.com/falkTX/Carla/pull/2040
Hello and thanks again for the prompt response. I ran the suggested test by creating a new project with just a singular LV2 plugin and it did work fine. Interestingly enough I tried loading a project with just a VST2 plugin and it also worked fine, however, attempting the same with any VST3 plugin caused the freeze during the project load. This should confirm your suspicion and most likely the reason why you can't reproduce the bug. I have no technical knowledge about the differences behind each type of plugin or how they work in the background but in case that it may help, the project that I normally run to perform real-time audio processing is composed mostly of VST3 plugins, which explains why it would keep freezing regardless of my plugin configuration. With that being said, I really hope that this information is useful and thank you so much again for the quick responses and for forwarding this issue. I'll remain attentive to any updates.
I'm still a bit confused about the Carla VST3 implementations, but it seems there's two of them, and I found something interesting. Can you try setting the `CARLA_DO_NOT_USE_JUCE_FOR_VST3=1` environment variable? I think Carla has a non-JUCE native VST3 implementation and JUCE seems to be gone in the Carla main branch. I need to get confirmation from FalkTX, but maybe the solution here is just to disable the JUCE path at build time. I'm just not sure if it's fully ready in the release branch.
Hello there. I ran Carla with the environmental variable as suggested and it did load my old project with plugins without freezing as you mentioned, however, the plugins themselves do not work as I do not get any audio output from the VST3 ones. Additionally, when I try to open the UIs from the plugins, the LV2 and VST2 plugins open their respective UI windows without issue, but the VST3 ones don't open anything and have the configuration icon disabled/grayed out and I obtain a message inside the logs window talking about some qt/wayland exception. (See below) ======= Starting engine ======= ======= Engine started ======== Carla engine started, details: Driver name: JACK Sample rate: 48000 Process mode: Multi client [carla] Carla assertion failure: "plugin.get() != nullptr && plugin->isEnabled()" in file CarlaEngineJack.cpp, line 4493 [carla] Carla assertion failure: "plugin.get() != nullptr && plugin->isEnabled()" in file CarlaEngineJack.cpp, line 4493 [carla] Carla assertion failure: "false" in file CarlaPlugin.cpp, line 1933 [carla] Carla assertion failure: "plugin.get() != nullptr && plugin->isEnabled()" in file CarlaEngineJack.cpp, line 4493 [carla] Carla assertion failure: "v3_cpp_obj(fV3.component)->get_bus_info(fV3.component, V3_AUDIO, V3_OUTPUT, j, &busInfo) == V3_OK" in file CarlaPluginVST3.cpp, line 1228 [carla] Carla assertion failure: "false" in file CarlaPlugin.cpp, line 1933 qt.qpa.wayland: Wayland does not support QWindow::requestActivate() <- This is the message I get every time I try to open the UI from a VST3 plugin
Right, I think non-JUCE VST3 support is incomplete in the released version of Carla (it works in git main). Shame... @mgansser I think we might need to revert the fix until upstream does a release of the main branch, unless falkTX has an idea of how to fix JUCE...
FEDORA-2026-847d758056 (Carla-2.5.10-5.fc45) has been submitted as an update to Fedora 45. https://bodhi.fedoraproject.org/updates/FEDORA-2026-847d758056
FEDORA-2026-63c3ef0659 (Carla-2.5.10-5.fc43) has been submitted as an update to Fedora 43. https://bodhi.fedoraproject.org/updates/FEDORA-2026-63c3ef0659
FEDORA-2026-946319129f (Carla-2.5.10-5.fc44) has been submitted as an update to Fedora 44. https://bodhi.fedoraproject.org/updates/FEDORA-2026-946319129f
FEDORA-2026-847d758056 (Carla-2.5.10-5.fc45) has been pushed to the Fedora 45 stable repository. If problem still persists, please make note of it in this bug report.
FEDORA-2026-946319129f has been pushed to the Fedora 44 testing repository. Soon you'll be able to install the update with the following command: `sudo dnf upgrade --enablerepo=updates-testing --refresh --advisory=FEDORA-2026-946319129f` You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2026-946319129f See also https://fedoraproject.org/wiki/QA:Updates_Testing for more information on how to test updates.
FEDORA-2026-63c3ef0659 has been pushed to the Fedora 43 testing repository. Soon you'll be able to install the update with the following command: `sudo dnf upgrade --enablerepo=updates-testing --refresh --advisory=FEDORA-2026-63c3ef0659` You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2026-63c3ef0659 See also https://fedoraproject.org/wiki/QA:Updates_Testing for more information on how to test updates.
FEDORA-2026-63c3ef0659 (Carla-2.5.10-5.fc43) has been pushed to the Fedora 43 stable repository. If problem still persists, please make note of it in this bug report.
FEDORA-2026-946319129f (Carla-2.5.10-5.fc44) has been pushed to the Fedora 44 stable repository. If problem still persists, please make note of it in this bug report.