Draft: copy host file to flatpak sandbox

This is a draft for discussion only.

@cullmann @akselmo

Context

From Kate flatpak application, we cannot open some files if they are in blacklisted folders (/lib, /lib32, /lib64, /bin, /sbin, /usr, /boot, /root, /tmp, /etc, /app, /run, /proc, /sys, /dev, /var).

It's annoying when using "Go to definition" and others LSP features on system symbols because it opens a blank file instead of the wanted system header.

To reproduce this problem, we can create a minimal hello world C++ app :

#include <iostream>

int main() {
    std::cout << "hello world" << std::endl;
    return 0;
}

When Ctrl+clicking on std::cout it should open the header file of the host system (/usr/include/c++/10/iostream on my system).

Beginnings of a solution

There is an interesting command that is available from the flatpak sandbox : flatpak-spawn. It allows to execute commands in the host from the flatpak sandbox.

Based on this flatpak-spawn command, this draft shows a hacky solution, just enough to show that it could work.

To test it :

git clone --branch flatpak_playground git@invent.kde.org:remipch/kate.git

cd kate/flatpak_playground

./build_kate_flatpak.sh

./run_kate_flatpak.sh

Questions

• Is it ok to integrate such a solution ?
• Where is the right place to integrate it ? It could be done in the framework to benefit to all applications.
• What are the other limitations of kate flatpak ?
• Side bug : when I run the flatpak, the menus are not ordered correctly, any ideas ?

TODO (if we go further in this direction)

• Use kcoreaddons/ksandbox.cpp instead of calling flatpak-spawn directly
• First try to open the file locally, if it fails and if we're sandboxed, then make a local copy.
• Add a filename cache to avoid making several copies of the same file. Something like a std::map<QUrl host_url, QUrl local_copy_url>
• Integrate cleanly in the right place.