Repository: yt-dlp
Issue: ffmpeg's `-movflags +faststart` fails on gvfs
Labels: external issue,regression,core:downloader

### Checklist

- [X] I'm reporting a broken site
- [X] I've verified that I'm running yt-dlp version **2022.01.21**. ([update instructions](https://github.com/yt-dlp/yt-dlp#update))
- [X] I've checked that all provided URLs are alive and playable in a browser
- [X] I've checked that all URLs and arguments with special characters are [properly quoted or escaped](https://github.com/ytdl-org/youtube-dl#video-url-contains-an-ampersand-and-im-getting-some-strange-output-1-2839-or-v-is-not-recognized-as-an-internal-or-external-command)
- [X] I've searched the [bugtracker](https://github.com/yt-dlp/yt-dlp/issues?q=) for similar issues including closed ones. DO NOT post duplicates
- [X] I've read the [guidelines for opening an issue](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#opening-an-issue)
- [X] I've read about [sharing account credentials](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#are-you-willing-to-share-account-details-if-needed) and I'm willing to share it if required

### Region

France (maybe EU?)

### Description

Trying to fetch https://www.france.tv/france-5/ernest-et-celestine/saison-1/3068333-pendant-que-l-orage-gronde.html with no particular CLI options, the downloaded file cannot be played with any of the players I have on my system. `mpv` notably reports
```
$ mpv Ernest\ et\ Célestine\ -\ Pendant\ que\ l\'orage\ gronde\ \[bc5bfd1c-2813-48d6-8c0e-524581bfce46\].mp4.old 
[ffmpeg/demuxer] mov,mp4,m4a,3gp,3g2,mj2: moov atom not found
[lavf] avformat_open_input() failed
[ffmpeg/demuxer] mov,mp4,m4a,3gp,3g2,mj2: moov atom not found
[lavf] avformat_open_input() failed
Failed to recognize file format.

Exiting... (Errors when loading file)
```

Reverting back to `yt-dlp==2021.10.22` allows the downloaded file to be played properly.

I tried diffing the files (my MPEG4 skills are rusty) and it looks like some chucks are missing at the very end of the file.
```
-rw-r--r-- 1 remi users 182480138 25 janv. 12:57 'Ernest et Célestine - Pendant que l'\''orage gronde [bc5bfd1c-2813-48d6-8c0e-524581bfce46].mp4'
-rw-r--r-- 1 remi users 182068891 25 janv. 12:57 'Ernest et Célestine - Pendant que l'\''orage gronde [bc5bfd1c-2813-48d6-8c0e-524581bfce46].mp4.2022.1.21'
```
In fact, the files are identical up to their common size. So really, the newer version of yt-dlp omits the end of the file, which probably include the index bits (is that what it's called? sorry if I get the terminology wrong)
```
$ dd bs=182068891 count=1 if=Ernest\ et\ Célestine\ -\ Pendant\ que\ l\'orage\ gronde\ \[bc5bfd1c-2813-48d6-8c0e-524581bfce46\].mp4 | sha256sum 
1+0 enregistrements lus
1+0 enregistrements écrits
182068891 octets (182 MB, 174 MiB) copiés, 3,60738 s, 50,5 MB/s
cbdf4aa2343c555311c14e8ccb13122944ec381fc8cc9ca9649d4e2b9d7cb6a7  -
$ dd bs=182068891 count=1 if=Ernest\ et\ Célestine\ -\ Pendant\ que\ l\'orage\ gronde\ \[bc5bfd1c-2813-48d6-8c0e-524581bfce46\].mp4.2022.1.21 | sha256sum 
1+0 enregistrements lus
1+0 enregistrements écrits
182068891 octets (182 MB, 174 MiB) copiés, 3,49349 s, 52,1 MB/s
cbdf4aa2343c555311c14e8ccb13122944ec381fc8cc9ca9649d4e2b9d7cb6a7  -
```

I'll try to bisect later, that's all I have for now.

### Verbose log

```shell
$ yt-dlp -vU "https://www.france.tv/france-5/ernest-et-celestine/saison-1/3068333-pendant-que-l-orage-gronde.html"
[debug] Command-line config: ['-vU', 'https://www.france.tv/france-5/ernest-et-celestine/saison-1/3068333-pendant-que-l-orage-gronde.html']
[debug] Encodings: locale UTF-8, fs utf-8, out utf-8, err utf-8, pref UTF-8
[debug] yt-dlp version 2022.01.21 [f20d607b0]
[debug] Python version 3.9.10 (CPython 64bit) - Linux-5.12.6-gentoo-x86_64-Intel-R-_Core-TM-_i5-4670_CPU_@_3.40GHz-with-glibc2.34
[debug] exe versions: ffmpeg 4.4.1 (setts), ffprobe 4.4.1
[debug] Optional libraries: Cryptodome, mutagen, sqlite, websockets
[debug] Proxy map: {}
Latest version: 2022.01.21, Current version: 2022.01.21
yt-dlp is up to date (2022.01.21)
[debug] [FranceTVSite] Extracting URL: https://www.france.tv/france-5/ernest-et-celestine/saison-1/3068333-pendant-que-l-orage-gronde.html
[FranceTVSite] 3068333-pendant-que-l-orage-gronde: Downloading webpage
[debug] [FranceTV] Extracting URL: francetv:bc5bfd1c-2813-48d6-8c0e-524581bfce46
[FranceTV] bc5bfd1c-2813-48d6-8c0e-524581bfce46: Downloading desktop video JSON
[FranceTV] bc5bfd1c-2813-48d6-8c0e-524581bfce46: Downloading mobile video JSON
[FranceTV] bc5bfd1c-2813-48d6-8c0e-524581bfce46: Downloading signed dash manifest URL
[FranceTV] bc5bfd1c-2813-48d6-8c0e-524581bfce46: Downloading MPD manifest
[FranceTV] bc5bfd1c-2813-48d6-8c0e-524581bfce46: Downloading signed hls manifest URL
[FranceTV] bc5bfd1c-2813-48d6-8c0e-524581bfce46: Downloading m3u8 information
[debug] Formats sorted by: hasvid, ie_pref, lang, quality, res, fps, hdr:12(7), vcodec:vp9.2(10), acodec, filesize, fs_approx, tbr, vbr, abr, asr, proto, vext, aext, hasaud, source, id
[debug] Default format spec: bestvideo*+bestaudio/best
[info] bc5bfd1c-2813-48d6-8c0e-524581bfce46: Downloading 1 format(s): hls-2218+dash-audio_fre=96000
[debug] Invoking downloader on "https://cloudreplay.ftven.fr/11d11784efe16/1086049405_france-domtom_TA.ism/ZXhwPTE2NDM0MzI3MjV+YWNsPSUyZjExZDExNzg0ZWZlMTYlMmYxMDg2MDQ5NDA1X2ZyYW5jZS1kb210b21fVEEuaXNtKn5obWFjPWEwNDdiMzllZjZjM2IyZjJlY2ZjZTI0NzdkZjMyN2I2ZGZjZWI3ZmFhY2M2MzNmOGVjZjg3N2RiM2NjNDU4MjI=/1086049405_france-domtom_TA-video=2000000.m3u8"
[hlsnative] Downloading m3u8 manifest
[hlsnative] Total fragments: 91
[download] Destination: Ernest et Célestine - Pendant que l'orage gronde [bc5bfd1c-2813-48d6-8c0e-524581bfce46].fhls-2218.mp4
[download] 100% of 171.06MiB in 00:36
[debug] Invoking downloader on "https://cloudreplay.ftven.fr/11d11784efe16/1086049405_france-domtom_TA.ism/manifest.mpd?hdnea=ip=78.196.20.108~exp=1643411725~acl=%2f11d11784efe16%2f1086049405_france-domtom_TA.ism*~hmac=faf3bffd25fe71d0f1e38101506550299a8c5c092ccbc44bf14a8db0f3b9e46f"
[dashsegments] Total fragments: 365
[download] Destination: Ernest et Célestine - Pendant que l'orage gronde [bc5bfd1c-2813-48d6-8c0e-524581bfce46].fdash-audio_fre=96000.m4a
[download] 100% of 8.52MiB in 00:42
[Merger] Merging formats into "Ernest et Célestine - Pendant que l'orage gronde [bc5bfd1c-2813-48d6-8c0e-524581bfce46].mp4"
[debug] ffmpeg command line: ffmpeg -y -loglevel repeat+info -i 'file:Ernest et Célestine - Pendant que l'"'"'orage gronde [bc5bfd1c-2813-48d6-8c0e-524581bfce46].fhls-2218.mp4' -i 'file:Ernest et Célestine - Pendant que l'"'"'orage gronde [bc5bfd1c-2813-48d6-8c0e-524581bfce46].fdash-audio_fre=96000.m4a' -c copy -map 0:v:0 -map 1:a:0 -movflags +faststart 'file:Ernest et Célestine - Pendant que l'"'"'orage gronde [bc5bfd1c-2813-48d6-8c0e-524581bfce46].temp.mp4'
Deleting original file Ernest et Célestine - Pendant que l'orage gronde [bc5bfd1c-2813-48d6-8c0e-524581bfce46].fdash-audio_fre=96000.m4a (pass -k to keep)
Deleting original file Ernest et Célestine - Pendant que l'orage gronde [bc5bfd1c-2813-48d6-8c0e-524581bfce46].fhls-2218.mp4 (pass -k to keep)
```