run0 cannot be overridden despite insufficient checks whether it works, and no fallback is attempt when it fails

[creshal]

Affected Version

Any version supporting run0

Describe the bug

1. run0 "support" is detected by checking for the existence of the binary. Unlike with doas/sudo/pkexec, this check is insufficient to ensure a working configuration: The systemd package creates the symlink no matter whether you have polkit installed or not, and without polkit installed run0 just crashes
2. If run0 crashes, no other elevator is attempted
3. Additionally, the sudo config file setting is ignored
4. That means yay cannot not crash on a system with systemd installed but not polkit; unless you manually pass --sudo on every yay invocation

Reproduction Steps

1. Set up Arch with only base installed (=no polkit)
2. Install yay
3. Watch yay crash with Failed to start transient service unit: Access denied
4. Try setting "sudo": "sudo", or any other working configuration in your config file
5. Watch yay crash anyway

Expected behavior

1. sudo config file option actually works
2. yay does not crash without manual configuration

Possible mechanisms:

1. Just try elevation mechanisms in order – would still throw an error, but a harmless one
2. De-prioritize run0 – easiest solution, but probably sucks for people who do have polkit?
3. Check for pkexec existence, prioritize run0 if pkexec exists – slightly more complicated, but would keep run0 as default for polkit-enabled systems

Output

> python -mjson.tool ~/.config/yay/config.json && echo " --> valid json file"
{
    "buildDir": "/home/creshal/.cache/yay/",
    "editor": "",
    "makepkgbin": "/usr/bin/makepkg",
    "pacmanbin": "/usr/bin/pacman",
    "pacmanconf": "/etc/pacman.conf",
    "tarbin": "/usr/bin/bsdtar",
    "requestsplitn": 150,
    "sortmode": 0,
    "sudo": "/usr/bin/doas",
    "sudoloop": false,
    "timeupdate": false,
    "devel": false,
    "cleanAfter": false
}
 --> valid json file
> yay --debug
[DEBUG:runner] running /usr/sbin/run0 /usr/bin/pacman --debug -S -y --config /etc/pacman.conf --
Failed to start transient service unit: Access denied
 -> error refreshing databases - exit status 1
> yay --debug --sudo /usr/bin/doas
[DEBUG:runner] running /usr/bin/doas /usr/bin/pacman --debug -S -y --config /etc/pacman.conf --
doas (creshal@localhost) password:

(It would be helpful if --debug did debug config file parsing. Or was documented in the manpage.)

[jdholtz]

The config to specify the sudo binary for yay to use is sudobin, not sudo. This doesn't necessarily fix the underlying issue of the weak run0 detection but you don't need to specify --sudo on the command line.

It's recommended to modify the Yay config via yay rather than directly editing the config to avoid mistakes like this. You can do so for sudo in the following way:
yay -Y --sudo /usr/bin/doas --save

[jdholtz]

1. Just try elevation mechanisms in order – would still throw an error, but a harmless one

I think there could be issues with this. Yay would need to know if run0 errors or the underlying command does in order to avoid rerunning commands (refreshing databases is harmless, but this could be an issue if the command is removing a package).

3. Check for pkexec existence, prioritize run0 if pkexec exists – slightly more complicated, but would keep run0 as default for polkit-enabled systems

I don't see this being very complicated. When checking for the run0, we just check that the pkexec binary exists as well. Am I missing something here?

[creshal]

I don't see this being very complicated. When checking for the run0, we just check that the pkexec binary exists as well. Am I missing something here?

No, that's about it. I just wasn't sure how complex you want the checks to be.