[Linux AppImage] support type 2 image format

Just a heads-up that we are working on the type 2 image format for AppImage. It will have a number of advantages:

• Uses more mordern, more flexible squashfs rather than outdated zisofs
• Optionally, can use different compressors
• Optionally, can embed digital signatures in an ELF section inside the AppImage
• Does not require fixed offsets
• Stores optional update information in an ELF section
• Is more future-proof

Since electron-builder is an important stakeholder in the AppImage ecosystem, some aspects of the type 2 image format were specifically designed with tools like electron-builder in mind that might want to use their own tools (e.g., mksquashfs, similar to xorriso) rather than the ones provided by AppImageKit, in fact this has been made super simple:

mksquashfs Your.AppDir Your.squashfs -root-owned -noappend
cat runtime >> Your.AppImage
cat Your.squashfs >> Your.AppImage
chmod a+x Your.AppImage

Note that there are no fixed offsets to observe, unlike with type 1 AppImages.

If you want to play with it, here are the tools (will move into AppImageKit when ready): https://github.com/probonopd/appimagetool

Would electron-builder consider to switch to the type 2 image format? If no, what would need to be changed?

[probonopd]

👍

cc @TheAssassin

[develar]

Status — overloaded 😦 Will be done in several weeks as part of Linux auto update.