Fix download issue for games with large number of savefiles and improve performance

[danez]

While testing with a game that has ~650 savefiles, I noticed that Legendary wasn't downloading any data — the download process completed instantly without actually retrieving any files.

Root Cause

Upon investigation, I discovered that the Epic Games Store (EGS) API limits the number of files returned in the metadata JSON to 1000 entries. For games with many files, such as save-heavy ones, essential files like the manifest chunks were missing from the response due to this limit.

Fix

To resolve this, I:

• Modified the download logic to request manifest data separately, bypassing the 1000-file limitation.
• Then used the manifest contents to selectively request only the necessary chunks from the API.
  • In my test case, this resulted in ~1600 chunk files to be downloaded.

Enhancements

To improve download performance and reliability:

• Implemented parallel chunk downloads, significantly reducing overall download time (for my example):
  • Before: 11 minutes 40 seconds
  • After: 1 minute 6 seconds
• Added a retry mechanism for failed chunk requests, as parallel requests occasionally encounter transient errors.

legendary-gl#709

Possible fix Heroic-Games-Launcher/HeroicGamesLauncher#3708

[danez]

Any Love? :) @imLinguin @CommandMC

[CommandMC]

Feel free to bug us on Discord next time to get things reviewed faster

[danez]

Thanks will do.

I applied the nit.

[danez]

Thanks for merging.