[Won't Fix][P2][Performance] CZDiskCacheManager.setCacheFile() - WriteFile threadsafety ioQueue.async(flags: .barrier)

[geekaurora]

Parent Ticket: #35

Description

In CZDiskCacheManager.setCacheFile(),

• ioQueue.async(flags: .barrier): will cause performance issue as many image files are being written to disk when scrolling. (it was ioQueue.async(flags: .barrier), removed for performance reason)

p.s. Files writing is discrete - mostly won't conflict.

Issue (OK Now)

• Same urls written to disk at same time: possible to fail to write - but won't crash.
  • OK: Cached in memory - Next time it will be downloaded again.
• data.write(to: fileURL, options: [.atomic]): atomic - won't Crash.

Solutions - [Won't Fix]

[Risky - OK]: if two threads are writing data to the same temporary file - won't damage but data isn't corrent?

The reason to abandon .barrier flag: it blocks other operations if any writing is on going. (Files writing is discrete: mostly won't conflict)
[Won't Fix] It won't cause issues:

1. As data.write(to:) is called with .atomic option, which won't write the target file with temp file if writing fails.
2. If writing fails, the downloader will download the file again as it doesn't find file in mem / disk cache.

atomic: An option to write data to an auxiliary file first and then replace the original file with the auxiliary file when the write completes.
https://developer.apple.com/documentation/foundation/nsdata/writingoptions/1411764-atomic

1. Add ioQueue.async(flags: .barrier) back: FPS test [Cleaner: same - .atomic write]

• Don't override the same file

2. Or add writingFiles Set: skip if file is being written

• No blocking: Set updating is O(1) - fast

Related ticket

Crash caused by thread safety - #37

[geekaurora]

Solution2. [Not Work] writingFilePaths Set to de-dup.

setCacheFile(withUrl:): Add writingFilePaths set. If filePath in process, skip duplicate writing.

NO:
Later task will be skipped - UI won’t refresh (earlier task: is possible out of screen)