AppImage takes longer to start since switching to velopack

[peppy]

As reported by at least one user

Likely related to xz being used instead of zstd:

https://github.com/AppImage/appimagetool/blob/81ee1db4946780292cd046d4c269a9b4fd7bc29d/src/appimagetool.c#L139-L141

[caesay]

I tried switching the default compression to zstd, but it broke in my test environment (failed to start). I then found several other threads discussing how xz and gzip are really the only two widely supported compression formats and there are many circumstances in which the others break.

• Flameshot AppImage doesn't start, squashfs image uses (null) compression TheAssassin/AppImageLauncher#602
• AppImage files no longer runnable after upgrading to Electron-Builder@23.0.0 electron-userland/electron-builder#6678

So, I made compression configurable in velopack/velopack#238

Feel free to switch to zstd if you understand that it may not work for some people.

[smoogipoo]

Thanks @caesay.

failed to start

Actually, I was already working on this in a branch (https://github.com/smoogipoo/velopack/commits/use-zstd/) and reached the same issue.
Likely the reason it failed to start is because Velopack uses AppImageKit which is the old cross-platform toolkit that doesn't support zstd (see: AppImage/AppImageKit#1091), as compared to appimagetool that is the intended upgrade path which does support it but is not cross-platform. That issue prevented me from going further with this, because the Velopack wiki states that Linux can be cross-compiled to.

In the case of the current tool, it will appear to work but fail to decompress because it can't recognise the zstd header. Unfortunately, using zstd with your change is not going to work for any system. It was working for us previously because we were using appimagetool.

I would suggest reverting your change, as using zstd would break installations on the next update.

[caesay]

I'm surprised by this. 'appimagetool' seems to use the same runtimes as appimagekit? (https://github.com/AppImage/type2-runtime) And we don't use these tools directly, we build the AppImage by creating a squashfs filesystem manually and prepending the runtime so that we can also build packages on windows.

[caesay]

I stand corrected, they do have different runtimes. So I think fixing this up will just require replacing our runtimes with those available here https://github.com/AppImage/type2-runtime/releases

[smoogipoo]

Also, I used the default params for squashfs for zstd (https://linuxreviews.org/Squashfs#mksquashfs_.28squashfs-utils.29) which were also changed for appimagetool in AppImage/appimagetool#65

I'm not sure how big of an issue that is, just thought I'd be consistent with what's worked so far 😛

[caesay]

zstd images appear to be working properly in https://github.com/velopack/velopack/releases/tag/0.0.630-g9c52e40