Prevent heavy work in maybeCachedCheckingQueue

[onevcat]

This tries to fix #2362

And also try to improve the iterator perform since Swift now has a better & modern API.

[gemini-code-assist]

Hello @onevcat, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

Summary of Changes

This pull request addresses issue #2362 and aims to improve the performance of the iterator within the DiskStorage class. The primary change involves modifying the setupCacheChecking function to prevent heavy work from being performed on the maybeCachedCheckingQueue. Instead of directly populating the maybeCached set within the queue's asynchronous block, the code now retrieves all files from the directory, creates a set from them, and then asynchronously updates maybeCached on the maybeCachedCheckingQueue. This change also moves the initial file system read to a global dispatch queue to avoid blocking the maybeCachedCheckingQueue during initialization.

Highlights

• Performance Improvement: Moves the potentially heavy file system operation (contentsOfDirectory) to a global dispatch queue to prevent blocking the maybeCachedCheckingQueue.
• Asynchronous Update: Updates the maybeCached set asynchronously on the maybeCachedCheckingQueue after retrieving the file list.
• Error Handling: Ensures that even if the file system operation fails, the maybeCached property is properly handled (set to nil) within the maybeCachedCheckingQueue.

Changelog

• Sources/Cache/DiskStorage.swift
  • Modified setupCacheChecking to perform file system operations on a global dispatch queue.
  • Updated setupCacheChecking to asynchronously update maybeCached on maybeCachedCheckingQueue after retrieving the file list.
  • Ensured error handling within setupCacheChecking updates maybeCached on maybeCachedCheckingQueue.

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---

In queues of code, where tasks reside,
A heavy load, we must divide.
Async calls, a gentle hand,
To keep the system, smooth and grand.

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[gemini-code-assist]

Code Review

The pull request aims to prevent heavy work in the maybeCachedCheckingQueue and improve iterator performance with Swift's modern API. The changes involve moving the file enumeration to a background queue and updating the maybeCached set asynchronously. Overall, the changes seem reasonable, but there are a few points to consider.

Summary of Findings

• Asynchronous Updates to maybeCached: The maybeCached property is now updated asynchronously. Ensure that all accesses to this property are thread-safe and consider the implications of potential race conditions.
• Error Handling: The error handling logic disables the maybeCached functionality. Consider logging the error for debugging purposes.
• Queue Usage: The code uses DispatchQueue.global(qos: .default) for file enumeration. Consider using a more specific QoS level if the file enumeration is performance-critical.

Merge Readiness

The changes seem to address the intended issue and improve performance. However, it's crucial to ensure thread safety when accessing maybeCached and consider logging the error in the catch block. I am unable to approve this pull request, and recommend that others review and approve this code before merging. At a minimum, the high severity issues should be addressed before merging.