When developing complex systems, encountering errors that leave your project in a frozen state can bring much sadness and consternation. For developers working with Squeak/Smalltalk, a frozen image refers to a situation where the system becomes unresponsive. This can happen due to various issues, including deadlocks, infinite loops, or recursive operations in Smalltalk code, which can cause the VM to freeze, hang, or lock up.

A recent discussion in the Squeak community highlighted how one such incident, caused by a stuck semaphore during an image load, turned into an opportunity for both recovery and valuable practices in safeguarding against future mishaps. You can explore the full conversation in the Squeak-dev mailing list thread (here).

The Problem: Semaphore Deadlock in the Image File

In this case, a developer encountered an issue while attempting to draw in multiple background threads, using a chain of semaphores to manage the UI thread. After resolving an error in one of the background threads, the drawing process was resumed. However, it was found that the image would fail to load. Instead of opening, the image became unresponsive and effectively “frozen.” It had likely become stuck due to a semaphore waiting for a signal that would never come. This left the image unresponsive and effectively unusable – or so it would have seemed.

Recovery Options: DoItFirst and Debugging

The good news is that there are several options available for recovering from such failures.

For developers dealing with frozen images at startup, Squeak provides a mechanism called DoItFirst. This is a class that is processed at the very beginning of the image startup sequence, allowing developers to inject custom code or actions before the main system fully loads. By using command-line arguments like --debug, it is possible to force the image to enter the debugger at the earliest point in the startup process, allowing issues to be diagnosed and fixed before the image fully loads.

Additionally, for situations where the issue is well-understood, the --doit option allows Smalltalk expressions to be passed and executed before any problematic behavior occurs, giving developers a chance to resolve issues before they impact the image during loading.

Another option is to open the Virtual Machine (VM) in a debugger. In the specific case of a stuck semaphore, you can set a breakpoint in interpret. Run the VM until it reaches interpret a second time (the first invocation does some initialization). At this point, you can use printAllProcesses to locate the stuck process. This should display the semaphore the process is waiting on. Once identified, you can invoke synchronousSignal with that semaphore to unstick it and restore functionality without losing significant progress. While this technique can specifically handle a stuck semaphore, it could also be helpful for other cases of system hangs or unresponsiveness.

The Importance of Backups and Versioning

While debugging and recovery tools are invaluable, it is clear that prevention is equally important. Routine backups and effective versioning are essential practices that can mitigate the risks associated with frozen images and other failures. One practice that proved helpful in this case was maintaining regular system backups, which made it possible to quickly restore a previous version of the image with minimal data loss. However, more granular versioning, especially in environments with frequent risky changes, would provide an additional layer of safety.

Specifically, incremental versioning of the image can be a lifesaver when things go wrong. By saving a new version of the image each time a meaningful step in development is reached, developers can create a clear history of their work. This allows them to revert to earlier versions of the image if issues arise, minimizing the impact of failed changes and speeding up the recovery process. Incremental image saves help ensure that even if an image becomes frozen or corrupted, a previous, stable version can be restored with minimal effort.

The Robustness of Squeak: Multiple Recovery Options

One of the key takeaways from this experience is how robust Squeak is in terms of recovery options. Even when a frozen image occurs or the system becomes unresponsive, Squeak offers multiple layers of recovery mechanisms. Whether using DoItFirst to inject custom code during startup or interacting with the VM through a debugger to manually resolve issues with semaphores, there are a variety of ways to recover from errors and get back on track.

Additionally, Squeak’s development environment is designed to be highly flexible, encouraging experimentation while ensuring that recovery is always possible. One of the key advantages of Squeak is that all the code executed or saved during development is stored in the .changes file. This makes it possible to recover from almost any issue, as the system is built to retain all your code. Even if an image becomes frozen or corrupted, the code itself is never truly lost. With the resources mentioned in this article, developers can restore their environment and continue their work. In Squeak, code is safe; no matter what happens to the image, your code can always be recovered.

Conclusion

In conclusion, while Squeak’s development environment offers tools to recover from serious errors, the most effective defense is good preventive practice: maintain regular backups, version your work consistently, and always be mindful of the unexpected.

For more detailed information on using DoItFirst to debug and recover your Squeak image, visit the DoItFirst Swiki page (here). If you encounter issues like crashes or freezes in Squeak, there are various levels of potential failure to consider, and helpful guidance is available in the What to do if Squeak crashes or freezes resource (here), which offers insight into diagnosing and resolving different types of system freezes or crashes.

Have a great time with Smalltalk and keep on Squeaking!