[Feature Request] Merge fragments while the download is ongoing for `-N`

Checklist

• I’m reporting a broken site support issue
• I’ve verified that I’m running yt-dlp version 2021.05.20
• I’ve checked that all provided URLs are alive and playable in a browser
• I’ve checked that all URLs and arguments with special characters are properly quoted or escaped
• I’ve searched the bugtracker for similar bug reports including closed ones
• I’ve read bugs section in FAQ

Verbose log

[debug] Encodings: locale cp1252, fs utf-8, out utf-8, pref cp1252
[debug] yt-dlp version 2021.05.20 (exe)
[debug] Python version 3.9.5 (CPython 64bit) - Windows-10-10.0.19043-SP0
[debug] exe versions: ffmpeg N-102620-g3300625c6f, ffprobe N-102620-g3300625c6f

Latest stable versions of Windows 10, Python and dependencies with Yt-dlp built from master as of this post.

Description

When using concurrent fragment option (-N), fragments are saved to disk and not added to the part file until every frag is downloaded despite the documentation saying this shouldn’t happen. The Yt-dlp exe also increases memory proportional to the fragment size every time a new one is downloaded causing it to grow in to the GBs on large files.

Args: https://www.crunchyroll.com/ah-my-buddha/episode-6-dont-transform-713589 -o "D:\Programs\YoutubeDL\crunchyroll.com\%(series)s - S%(season_number)sE%(episode_number)s - %(episode)s - %(epoch)s.%(ext)s" --embed-subs --sub-lang en --cookies cookies-crunchyroll-com.txt --user-agent "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:88.0) Gecko/20100101 Firefox/88.0" --fragment-retries "infinite" -N 3 --no-mtime --download-archive archive.txt

[pukkandan]

Honestly it wouldn’t be a big deal if the Windows file explorer wasn’t a buggy pile of garbage… Unfortunately it is and every time a fragment is downloaded it un-selects the file you have selected, scrolls to the top, stops you editing file name or doing anything else with another file ect. every 500ms or so making it hard to do anything when 700 fragments are needed even for a tiny 350mb file.

This is the exact reason I implemented --path temp:. I was tired of windows explorer refreshing every time something downloaded

[candrapersada]

why Merge fragments error ERROR: unable to download video data: [WinError 2] The system cannot find the file specified: '.part-Frag162.part' and how to use Auto Redownload for youtube playlist?