Fixing Image Rendering Issues With A 0 Disk Cache

by Alex Johnson 50 views

Decoding the Disk Cache Dilemma: Why Images Vanish with a Zero Disk Cache

Let's dive into a frustrating issue: images failing to render when the disk cache size is set to zero in certain applications, specifically within the Amiga and AWeb3 environments. This problem arises from a conflict between how images are cached and how the rendering engine interacts with those cached resources. Understanding this requires a look into the core mechanics of image loading, caching, and how a zero-sized disk cache disrupts the process. The main issue is that if the disk cache is set to 0 or another very small size, the cache contents are deleted as soon as they are written. Even though the assets are also cached in the ram cache, the DataTypes renderer at least is hardcoded to render from the files directly (even though DataTypes can also render from a ram buffer) and so the images fail to render as if they are missing. Fixing this will require some substantial refactoring.

The Root Cause: A Cascade of Cache Chaos

At the heart of the problem is the immediate deletion of cached image data when the disk cache is disabled. Here's a breakdown of the steps:

  1. Cache Entry Creation: The process begins with the creation of cache entries linked to URLs, even when the disk cache is configured to zero. This initial step sets the stage for the subsequent issues.
  2. Data Writing to Disk: Image data is then written to these cache files, and the cache's size is incremented, indicating that the system recognizes the presence of cached data.
  3. Immediate Deletion: Right after the data is written, a function called Flushexcess() is triggered. This function's purpose is to manage the cache size by removing older or excessive entries. However, when the disk cache is zero, this function acts destructively. It calculates a maximum cache size (max) of zero. Since the cache size (cadisksize) is now greater than zero (because we just wrote data), all cache entries are immediately deleted. This rapid purge is the core of the problem.
  4. The Missing File: When the image loading mechanism tries to access the cached image, it finds that the cache entry has been deleted. It then either falls back to an alternative method, like attempting to load from a file URL, or tries to create a temporary file. However, this fallback mechanism is often unable to locate the image data, leading to a failure in rendering.

The Problematic Code: Where the Logic Breaks Down

The code segment within cache.c (lines 380-390) that controls the cache management is crucial. When the preference for disk cache size (prefs.cadisksize) is set to zero, the max variable is also set to zero. This causes the Flushexcess() function to immediately delete any cache entry where cadisksize is greater than zero. This is a critical point of failure in the system.

Why No RAM Cache Fallback? The Missing Safety Net

One might expect a RAM cache to act as a safety net when the disk cache is disabled. However, the current implementation doesn't provide a robust fallback to the RAM cache. It relies heavily on the presence of disk cache files and temporary files. When the cache entry is deleted, the file path needed to load the image from the disk is lost, and the image can't be retrieved from the RAM cache because the system is designed to load from the file path.

Images and the RAM Cache: A Closer Look

While images are indeed held in RAM by the source driver (AOSRC_Memory), the rendering process is designed to work with files, not raw memory. The system uses a file path to load the image data, which is provided by the cache entry.

How Image Loading Works: A Step-by-Step Guide

  1. Memory Cache: The source driver ensures that image data is stored in memory, using AOSRC_Memory to track the data. This provides a temporary, in-memory storage of the image.
  2. File Path Dependency: The system requires a file path to retrieve the image. The rendering engine within the datatypes library specifically looks for a file path, not direct access to memory. In imgsource.c the process is as follows:
    • Newfile() checks for a cache entry. If found, it uses the cache file path and sets the IMSF_CACHEFILE flag. This means the system assumes the file is already available and doesn't attempt to write data.
    • If no cache entry is found, it creates a temporary file.
  3. Data Flow: In Srcupdateimgsource(), the system checks if the IMSF_CACHEFILE flag is set.
    • If the flag is not set, the data is written to a file.
    • If the flag is set, the data is expected to already exist in a file and isn't written again.

The Zero Disk Cache Effect: A Chain of Failure

When the disk cache size is zero, the following occurs:

  1. A cache entry is made and connected to the URL of the image.
  2. Image data is written to a cache file using Srcupdatecache(), and the cache's size increases.
  3. The Flushexcess() function runs immediately, and because the cadisksize is greater than max (which is zero), the cache entry is immediately deleted.

When the system attempts to load the image:

  1. Newfile() checks for AOURL_Cache, but the entry is gone.
  2. The system creates a temporary file, but the process may not be correctly set up.

The Solution: A Way Forward

The core solution lies in either preventing the Flushexcess() function from running when the disk cache is zero or bypassing the creation of cache entries in the first place. This would avoid the immediate deletion of cache entries, preserving the necessary file paths for the images to render correctly. The system must recognize that the data is also cached in the RAM, and then determine how to load it from the RAM when there is a lack of cache file information.

Summary: The Essence of the Issue

Images are cached in RAM by the source driver, but the rendering mechanism requires a file path. With a zero disk cache, the cache entry is removed immediately after being created, leaving the image loading process unable to find the correct file path. The fix involves ensuring that cache entries aren't deleted when the disk cache is zero or that cache entries are not created in the first place, allowing images to render correctly despite the disk cache configuration.

For additional information, you can check the Amiga Developer Network.