Godot Mod Loader: Fixing `_rotate_log_file` Web Export Error

Hey guys! Let's dive into a tricky issue encountered in the Godot Mod Loader that causes some serious headaches when dealing with web exports. Specifically, we're going to break down how the _rotate_log_file() function can lead to problems, especially in non-debug web builds. This article will cover the technical details, the error's manifestation, and potential solutions. So, buckle up, and let's get started!

The Issue: _rotate_log_file() and Web Exports

The core problem lies within the _rotate_log_file() function in the Godot Mod Loader, particularly in how it handles scenarios where a log file cannot be created. The issue manifests differently depending on whether you're running a debug or non-debug web export.

The problematic code snippets can be found in these files:

• log.gd#L587
• setup_log.gd#L178

These sections of code use assert statements to check if a log_file can be created. However, they lack an early exit strategy if the assertion fails. This is where things get interesting, especially in the context of web exports.

Non-Debug Web Exports: Obscure Index Out of Bounds Exception

In non-debug web exports, the assert statements are essentially ignored. When the log file creation fails and there’s no proper error handling, it leads to an obscure "index out of bounds" exception. This type of error is notoriously difficult to debug because it doesn't directly point to the root cause – the failed log file creation.

The following image illustrates this issue:

Debug Web Exports: Asserts in the Browser Console

On the flip side, debug web exports behave a bit differently. The assert statements do their job and appear in the browser console, giving you a clue that something went wrong with the log file. However, even in this case, the index out of bounds exception doesn't occur. This discrepancy between debug and non-debug behavior adds to the confusion.

The following image illustrates this behavior:

Why Does This Happen?

The million-dollar question is: why can't the log file be created in a web export, and why does this lead to an exception in non-debug exports but not in debug exports? The exact reasons can be multifaceted and might involve the way web browsers handle file system access or how Godot's file system API interacts with the web environment. Let's break down the possible causes and the differences between debug and non-debug builds.

File System Limitations in Web Exports

Web browsers impose strict security restrictions on file system access to prevent malicious websites from reading or writing arbitrary files on a user's system. This is a crucial security measure, but it can also limit what a web application, including a Godot game, can do with files. In many cases, web applications are restricted to using the browser's virtual file system or IndexedDB for persistent storage.

When a Godot game is exported to the web, it operates within these constraints. The standard file system access methods that work perfectly fine on desktop platforms might not function as expected in a web environment. This could be a primary reason why the log_file creation fails in web exports.

Differences Between Debug and Non-Debug Exports

The behavior discrepancy between debug and non-debug exports often boils down to how the Godot engine handles errors and assertions in different build configurations. Here's a breakdown:

• Debug Exports: Debug builds are designed to provide detailed information about errors and potential issues. When an assert statement fails in a debug build, the engine typically prints an error message to the console, making it easier for developers to identify the problem. Additionally, debug builds often include extra checks and validations that can help catch errors early on.

• Non-Debug Exports: Non-debug (or release) builds, on the other hand, are optimized for performance and are meant to be distributed to end-users. In these builds, assert statements are often disabled to reduce overhead. This means that if a log file creation fails, the assert is ignored, and the code continues to execute without proper error handling. This lack of error handling can then lead to subsequent errors, such as the index out of bounds exception observed in this case.

The Index Out of Bounds Exception: A Consequence of Unhandled Failure

The index out of bounds exception is a secondary error, a symptom of the primary issue (failed log file creation) not being properly handled. When the _rotate_log_file() function fails to create the log file and doesn't exit or return an error, subsequent operations that assume the log file exists can lead to accessing invalid memory locations, resulting in the exception.

In essence, the sequence of events in a non-debug web export is as follows:

1. Attempt to create a log file fails due to file system restrictions.
2. The assert statement is ignored in the non-debug build.
3. The code continues to execute as if the log file was created.
4. Later operations try to write to the non-existent log file.
5. This leads to an invalid memory access, triggering the index out of bounds exception.

Minimal Reproducible Example

To better understand and address this issue, a minimal reproducible example (MRE) has been provided. This project demonstrates the problem in a simplified context, making it easier to debug and test potential solutions.

You can download the MRE here: modloaderwebrelease.zip

By running this project in both debug and non-debug web export configurations, you can observe the different behaviors and confirm the issue.

Potential Solutions

So, what can we do to fix this? Here are a few potential solutions to consider:

1. Early Exit and Error Handling

The most straightforward solution is to add an early exit to the _rotate_log_file() function if the log file cannot be created. Instead of relying solely on assert statements, we should implement proper error handling that prevents the code from proceeding if the file creation fails. This can involve checking the return value of the file creation function and returning an error code or throwing an exception if necessary.

Here’s a conceptual example of how this could be implemented:

func _rotate_log_file():
 var log_file = FileAccess.open(log_path, FileAccess.WRITE)
 if log_file == null:
 print_error("Failed to create log file at: %s" % log_path)
 return # Early exit
 # Rest of the function
 log_file.close()

2. Web-Specific Logging Mechanism

Another approach is to implement a web-specific logging mechanism that works within the constraints of the browser environment. This could involve using the browser's console for logging or storing log messages in the browser's local storage (e.g., IndexedDB). By using web-friendly APIs, we can avoid the issues associated with standard file system access in web exports.

For example, you could use print() to log messages to the browser console or implement a custom logging system that stores messages in a queue and periodically writes them to IndexedDB.

3. Conditional Compilation

Conditional compilation allows you to include or exclude code based on the target platform. We could use this to implement different logging strategies for web and non-web platforms. For web exports, we could use the web-specific logging mechanism, while for desktop platforms, we could continue to use the standard file system-based logging.

Here’s a conceptual example using Godot’s feature tags:

# In log.gd

# Conditional logging based on platform
# Web platform
# Log to console
# Other platforms
# Log to file

# Example for web platform (using print for simplicity)
# Using a more robust web-specific logging mechanism
func _log_message(message: String):
 # Web platform
 if OS.has_feature("web"):
 print("[LOG]: " + message)
 else:
 # Other platforms: Log to file (existing implementation)
 # File logging code here
 pass

4. Patching Godot Mod Loader

Given that this issue occurs within the Godot Mod Loader, a patch to the mod loader itself would be the most direct solution. This patch should include the early exit and error handling described above, as well as any necessary adjustments for web export compatibility.

Conclusion

The _rotate_log_file() function's behavior in web exports highlights the importance of robust error handling, especially when dealing with platform-specific constraints. By adding early exits, implementing web-specific logging, or using conditional compilation, we can mitigate this issue and ensure that Godot Mod Loader works reliably across different platforms. Remember, thorough error handling is key to creating stable and user-friendly applications, especially in environments with varying capabilities and restrictions.

Keep an eye out for updates to the Godot Mod Loader that address this issue, and feel free to contribute to the project if you have ideas or solutions! Happy coding, guys!