I'm not sure I'm supposed to engage with throwaway accounts but here goes.
Why can't a program on the user bus send a message to a program on the system bus and vice versa? Because dbus messages cannot transit between buses. That is why.
The workaround of straddling both buses is something I recently had a good look for. To see if that was a reasonable thing to do. I couldn't find a single example of a program doing this. I think that's because it's a security bodge trying to evade the intentional isolation built into dbus. Although if I'm honest I think it's an OK workaround as you can restrict the system bus access.
The problem I keep genuinely running into is the existing programs I want to talk to (and don't control) only sit on the user bus, very very rarely the system bus because they are gui programs tied into the user session. So my program is then forced to run as the same user. And even that's not enough because they need to run under the same user session, not just the same user. This restriction is a deal-breaker for my system service-type programs and is totally avoidable when using other communication methods (which those existing programs also provide, presumably because they realise dbus is often totally unusable).
The simple way is to use a Unix socket and that's why people continue to "litter" the filesystem with them. You can even get a bit of auth with them.
Why can't a program on the user bus send a message to a program on the system bus and vice versa? Because dbus messages cannot transit between buses. That is why.
The workaround of straddling both buses is something I recently had a good look for. To see if that was a reasonable thing to do. I couldn't find a single example of a program doing this. I think that's because it's a security bodge trying to evade the intentional isolation built into dbus. Although if I'm honest I think it's an OK workaround as you can restrict the system bus access.
The problem I keep genuinely running into is the existing programs I want to talk to (and don't control) only sit on the user bus, very very rarely the system bus because they are gui programs tied into the user session. So my program is then forced to run as the same user. And even that's not enough because they need to run under the same user session, not just the same user. This restriction is a deal-breaker for my system service-type programs and is totally avoidable when using other communication methods (which those existing programs also provide, presumably because they realise dbus is often totally unusable).
The simple way is to use a Unix socket and that's why people continue to "litter" the filesystem with them. You can even get a bit of auth with them.