Godot: FileAccess.get_size Returns Old Value After Store

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Godot: FileAccess.get_size Returns Old Value After Store

Hey there, fellow Godot enthusiasts! Let's dive into a quirky issue reported in Godot 4.5 regarding FileAccess.get_size. It seems that get_size might not always give you the most up-to-date information immediately after using store_* functions. Let's break down the problem, how to reproduce it, and what might be happening under the hood. Understanding these nuances is super important, especially when you're dealing with file I/O operations where accuracy is key. The more you know about potential pitfalls, the smoother your game development journey will be!

Issue Description

So, the main thing to keep in mind, guys, is that FileAccess.get_size appears to return a stale size value right after you've written something to a file using store_*. It's like it hasn't quite caught up with the changes yet. The size only seems to refresh after you call seek. This behavior can be a bit unexpected if you're not aware of it, potentially leading to incorrect assumptions about the file's actual size. This can be particularly problematic in scenarios where you're dynamically writing data to a file and need to know the updated size for subsequent operations. Imagine writing a game save system where you need to keep track of the file size to efficiently manage data storage – an outdated size could lead to corruption or data loss. Therefore, it's crucial to understand and account for this behavior to ensure the integrity of your file operations.

Tested Versions & System Information

This issue was observed in:

  • Godot v4.5.stable.official [876b29033]

Here's the system info where the problem was spotted:

  • OS: Windows 10 (build 19045)
  • Display: Multi-window, 1 monitor
  • Graphics API: Vulkan (Mobile)
  • GPU: dedicated NVIDIA GeForce RTX 3060 (NVIDIA; 32.0.15.8142)
  • CPU: Intel(R) Core(TM) i7-4790 CPU @ 3.60GHz (8 threads)
  • Memory: 31.95 GiB

Knowing the specific environment where the issue was reproduced helps in narrowing down potential causes. For instance, the Vulkan (Mobile) graphics API might interact differently with file system operations compared to other APIs. Similarly, the hardware configuration could influence the behavior of file access routines. By providing this detailed system information, it becomes easier for developers to replicate the issue and investigate its root cause more effectively.

Steps to Reproduce

Alright, let's get our hands dirty and reproduce this thing! Here’s the GDScript code snippet that shows the problem:

var file_path:String = "user://test_file.dat"
var file_access: FileAccess = FileAccess.open(file_path, FileAccess.READ_WRITE)
print("Opened ", FileAccess.get_size(file_path)) # Expected: 4 (if the file exists and has 4 bytes), Actual result may vary based on existing file
file_access.resize(0)
print("Resized ", FileAccess.get_size(file_path)) # Expected: 0, Actual: 0
file_access.store_32(0)
print("Stored ", FileAccess.get_size(file_path)) # Expected: 4, Actual: 0 (Incorrect!)
file_access.seek(0)
print("Seeked ", FileAccess.get_size(file_path)) # Expected: 4, Actual: 4 (Correct after seek!)
file_access.close()

Here’s what’s happening step-by-step:

  1. We open a file in READ_WRITE mode.
  2. We print the initial size. If the file exists and contains data (e.g., 4 bytes), it should print 4. If the file doesn't exist, it will print 0.
  3. We resize the file to 0 bytes and print the size again, which correctly shows 0.
  4. We store a 32-bit integer (4 bytes) into the file. Now, the file should be 4 bytes in size.
  5. Here’s the catch: printing the size immediately after store_32 still shows 0! This is the outdated value.
  6. We then call seek(0), which essentially moves the file cursor to the beginning of the file.
  7. After the seek call, printing the size correctly shows 4. Ta-da!

To reproduce this reliably, make sure you either create the file beforehand or handle the case where the file might not exist initially. The key is to observe the discrepancy between the expected size after store_32 and the actual value returned by get_size before and after calling seek. Understanding these steps is crucial for anyone trying to debug file-related issues in Godot, allowing you to pinpoint exactly where the problem lies and implement appropriate workarounds.

Explanation

So, why does this happen? It seems like FileAccess.get_size isn't immediately updating its cached file size after a store_* operation. The seek call likely forces a refresh of this cached value, bringing it in sync with the actual file size on disk. The underlying file system operations might involve buffering or delayed updates, which could explain why the size isn't immediately reflected. It’s possible that Godot's FileAccess class optimizes file operations by caching the file size to avoid frequent system calls, which can be expensive. However, this optimization can lead to inconsistencies if the cached value isn't properly updated after every write operation.

Workaround

The simplest workaround is to call seek(file_access.get_position()) or file_access.seek(0) after each store_* operation if you need the updated file size immediately. This forces the FileAccess object to refresh its size information. Here’s how you can modify the code to include the workaround:

var file_access: FileAccess = FileAccess.open(file_path, FileAccess.READ_WRITE)
print("Opened ", FileAccess.get_size(file_path))
file_access.resize(0)
print("Resized ", FileAccess.get_size(file_path))
file_access.store_32(0)
file_access.seek(file_access.get_position())
print("Stored ", FileAccess.get_size(file_path)) # Correct now!
file_access.close()

By inserting file_access.seek(file_access.get_position()) right after the store_32 call, you ensure that the file size is updated before you attempt to retrieve it. This workaround is particularly useful in scenarios where you need to perform multiple write operations and rely on the updated file size for subsequent calculations or logic. Always remember to close file handles with file_access.close() when you're done with them to prevent resource leaks and ensure data is properly flushed to disk.

Minimal Reproduction Project (MRP)

N/A (The code snippet provided is sufficient to reproduce the issue.)

Conclusion

While this behavior of FileAccess.get_size might seem like a bug, it could be an intentional optimization with unintended side effects. Knowing about it allows you to code defensively and avoid potential issues. Always be mindful of the order of operations when working with file I/O, and don't assume that get_size will always return the absolute latest value immediately after a write operation. Use the seek workaround when necessary to ensure you have the most accurate file size information. Understanding these quirks can save you a lot of debugging time and headache in the long run. Happy coding, and may your file operations always be in sync!