NixOS Cage Startup Failure: Fixing Your 25.11 Upgrade
Hey guys, have you ever run into that super frustrating moment when you upgrade your beloved system, expecting everything to be smooth sailing, only to find a crucial piece of your setup just… stops working? Yeah, we’ve all been there. Today, we're diving deep into a specific issue that some folks running NixOS might have encountered: the mysterious case of cage failing to start after an upgrade from version 25.05 to 25.11. This isn't just a minor glitch; for many, cage is essential for running graphical applications in a minimalist, secure way, often for dedicated purposes like media centers or kiosks. When it breaks, it can really throw a wrench in your plans. We're going to break down what happened, why it's a big deal, and what we can learn from it, all while keeping it casual and easy to understand. So, grab your favorite beverage, and let's unravel this mystery together!
The Heart of the Problem: Cage Fails to Start After NixOS Upgrade
Let's get straight to the point, folks. The core issue many users faced revolved around their cage instances suddenly refusing to launch after a system upgrade. Specifically, this bug cropped up when transitioning their NixOS installations from the 25.05 stable branch to the newer 25.11 release. Imagine having a perfectly configured system, maybe running a mythtv-frontend within cage for your ultimate media experience, and then, after what should be a straightforward upgrade, boom – nothing. Your graphical application, meant to run smoothly in its cage environment, just gives up. This isn't just an annoyance; for dedicated systems, it's a critical failure that can render the entire setup unusable for its intended purpose. The reported problem highlights that services.cage configurations, which were working flawlessly on NixOS 25.05, consistently failed to initialize on the 25.11 branch. The initial investigation even pointed out that core components, like the mythtv application itself, appeared to be the same version across both NixOS releases, making the cage failure even more perplexing. It really makes you scratch your head, right? You'd expect if a dependent application changed, that might be the culprit, but when it hasn't, the focus naturally shifts to the environment hosting it. This kind of behavior immediately forces users into a difficult position: either stick with an older, potentially less secure, or feature-limited OS version, or deal with a broken critical service. The good news (if you can call it that) is that downgrading back to 25.05 immediately resolved the issue for the affected systems, confirming that the 25.11 upgrade itself was indeed the trigger. This temporary fix, while getting things working again, leaves us with the underlying problem unsolved and needing a proper resolution for cage on the latest NixOS release. This kind of experience can really test your patience, especially when you've invested time in building a robust, NixOS-powered setup.
Diving Deeper: Understanding the Error Logs
When cage mysteriously decides to not start, the first place any good troubleshooter looks is the system logs. And boy, do these logs tell a story, even if it's a cryptic one at first glance! Let's break down the console output provided, because understanding these messages is key to diagnosing the problem. We saw lines like these popping up:
Dec 06 19:37:36 minithink systemd[1]: Started cage-tty1.service.
Dec 06 19:37:36 minithink (cage)[776]: pam_unix(cage:session): session opened for user mythtv-frontend(uid=1003) by mythtv-frontend(uid=0)
Dec 06 19:37:45 minithink systemd[1]: cage-tty1.service: Main process exited, code=dumped, status=6/ABRT
Dec 06 19:37:45 minithink systemd[1]: cage-tty1.service: Failed with result 'core-dump'.
Dec 06 19:37:45 minithink systemd[1]: cage-tty1.service: Consumed 12ms CPU time, 4.7M memory peak, 2.5M read from disk.
Now, let's dissect this, piece by piece. The initial message, Started cage-tty1.service., actually sounds promising, right? It tells us that systemd, the system and service manager that NixOS (and most modern Linux distros) uses, successfully initiated the cage-tty1.service. Great start! Following that, we see pam_unix(cage:session): session opened for user mythtv-frontend. This indicates that the Pluggable Authentication Modules (PAM) successfully handled the session opening for the mythtv-frontend user within cage. So far, so good – everything seems to be setting up correctly. It implies that cage itself started and was able to authenticate a user, meaning the initial launch process wasn't the immediate failure point. However, just nine seconds later, things take a dramatic turn: Main process exited, code=dumped, status=6/ABRT. This, folks, is where the party stops. The Main process exited part is self-explanatory: cage itself, or rather the process it was managing, crashed. The code=dumped and status=6/ABRT are highly indicative of a segmentation fault or a similar critical, unhandled error that caused the program to abort and generate a core dump. A core-dump is essentially a snapshot of the program's memory at the time of the crash, which is invaluable for developers trying to debug what went wrong. The ABRT status (which stands for abort) confirms that the program terminated abnormally. Finally, Failed with result 'core-dump' simply summarizes that the service failed because of this core dump, and Consumed 12ms CPU time, 4.7M memory peak, 2.5M read from disk. gives us some quick metrics about its brief, ill-fated execution. What these logs don't tell us directly is why the core dump occurred, but they very clearly point to cage itself (or its immediate environment after user session initiation) being the source of the crash, rather than systemd failing to start it or PAM failing to authenticate. This points towards a compatibility issue or a broken dependency within the 25.11 environment that cage relies on to function correctly after its initial setup. It's like the initial setup was fine, but as soon as cage tried to do its actual job, it hit a brick wall and just gave up.
Reproducing the Bug: Your Guide to Seeing It Firsthand
Alright, for all you brave souls out there who want to confirm this issue or help with debugging, reproducing a bug is often the first critical step towards a solution. The process for recreating this specific cage failure after a NixOS upgrade is thankfully quite straightforward, assuming you have a suitable environment. Here’s how you can potentially run into this problem yourself, following the steps outlined by the original report. First off, you need to start with a working cage instance on NixOS 25.05. This means you should have a configuration.nix file where services.cage is enabled and configured to run a graphical application, such as mythtv-frontend, or any other application you prefer to launch via cage. Ensure that this setup is fully operational and that cage launches your chosen application without any hitches on the 25.05 release. This baseline functionality is crucial, as it confirms that your configuration itself isn't inherently flawed and that cage was working as expected at one point. Once you've verified your 25.05 setup is solid, the next step is to initiate the upgrade to NixOS 25.11. This typically involves updating your nixpkgs channel to point to the 25.11 stable release and then performing a nixos-rebuild switch. For many users, this command is almost ritualistic, a gateway to the latest and greatest features and fixes. After the upgrade process completes and you reboot your system (or restart the cage service), you will then attempt to start your cage instance. This is where the expected behavior – cage launching your application as smoothly as it did on 25.05 – tragically diverges from the actual behavior. Instead of seeing your application gracefully appear, you'll likely encounter the service failing to start, accompanied by those tell-tale core-dump and ABRT messages in your systemd journal. The whole point here is to demonstrate that a simple, canonical upgrade path from one stable NixOS version to the next is sufficient to trigger this bug, ruling out highly custom or obscure configurations as the primary cause. This clear reproducibility is invaluable for developers, as it gives them a reliable way to test fixes and ensure the issue is truly resolved across a common upgrade scenario. So, if you're in the mood for some debugging, setting up this scenario will give you a front-row seat to the problem!
Why This Matters: The Impact on Your NixOS Experience
Let's be real, folks, when an essential service like cage starts to fail after an upgrade, it's more than just a minor inconvenience; it can significantly impact your entire NixOS experience. One of the biggest selling points of NixOS is its promise of reproducibility and atomic upgrades. You expect that when you upgrade, either everything works perfectly, or if something breaks, you can easily roll back to a known working state. While rolling back to 25.05 was a viable workaround here, the fact that a core component like cage breaks during an upgrade to a newer stable version undermines that expectation of seamless progression. For users who rely on cage for specific, often critical functions—think dedicated media center PCs running mythtv-frontend, kiosk systems, or specialized embedded setups—this issue can be a showstopper. These aren't just casual desktop environments; they're often built with a singular purpose in mind, and cage provides that lightweight, secure, and focused graphical environment. When it suddenly produces a core-dump and ABRT errors, it means the entire purpose of that machine is compromised. The user can't access their media, the kiosk can't display information, and the embedded system can't run its GUI. This forces users into a difficult choice: either delay upgrades indefinitely, risking security vulnerabilities and missing out on new features, or spend significant time troubleshooting an issue that shouldn't exist in a stable release. Moreover, such issues can erode trust in the upgrade path. If one core service breaks, what else might? This uncertainty can lead to upgrade anxiety, which is the last thing you want in a system designed for predictable and reliable updates. The beauty of NixOS lies in its ability to manage dependencies and environments precisely, ensuring that applications run with the exact libraries they need. When cage crashes with a core-dump despite its application (like mythtv) remaining the same version, it strongly suggests a subtle but significant change in the underlying NixOS environment, perhaps a library conflict or an ABI break that cage isn't gracefully handling. This isn't just about a single user; it highlights a potential systemic issue that could affect many NixOS deployments relying on cage for their graphical needs. So, fixing this isn't just about getting cage to work again; it's about upholding the integrity and reliability that makes NixOS such a powerful and appealing operating system for a wide range of applications.
What's Next? Troubleshooting and Community Collaboration
Okay, so we've identified the problem: cage is failing to start after the NixOS 25.11 upgrade, leading to those nasty core-dump errors. Now, what do we do about it? This is where the power of the NixOS community and systematic troubleshooting really comes into play. If you're encountering this, or similar issues, here are some actionable steps and considerations. First, while downgrading is a good immediate fix, it's not a long-term solution. To move forward, a deeper investigation is warranted. For those with debugging skills, attaching a debugger (like gdb) to the cage process when it crashes and analyzing the core-dump can provide incredibly detailed insights into exactly where in the code the failure occurs. This level of detail is gold for developers. You might also want to check for any related package changes between 25.05 and 25.11 that cage might depend on. Even minor version bumps in a dependency could introduce a breaking change that causes cage to crash. Tools like nix-diff (though sometimes complex to interpret for system-wide changes) or simply comparing package definitions in nixpkgs might reveal clues. Additionally, ensuring your Nixpkgs source is completely up-to-date and that your system is in a consistent state (nix-store --verify --check-validity can sometimes catch subtle corruption, though less likely for this type of issue) is always a good practice. The original bug report provided excellent system metadata, which is crucial for debugging: system: "x86_64-linux", host os: Linux 6.12.55, NixOS, 25.05 (Warbler), 25.05.20251027.daf6dc4, multi-user?: yes, sandbox: yes, version: nix-env (Nix) 2.28.5, and nixpkgs: /nix/store/l6dvcwx15645vi6dj9i8b3h7w4dzai0p-source. This information helps developers replicate the exact environment where the bug occurred. For an issue like this, community collaboration is paramount. The report noted