Propagating parent PID to ./run

[Earl Chew via skaware]

Is there any appetite for providing a way for ./run to know the PID of 
its parent s6-supervise instance?

This information allows the supervised child to know that it has been 
orphaned, and to tie its fate to its parent (eg PDEATHSIG 
https://stackoverflow.com/a/36945270).

Using getppid(2) alone is not reliable because the child might have been 
orphaned between the fork(2) and getppid(2) calls.

Mechanisms that might be used include a) setting an environment variable 
(eg S6_PPID) before executing ./run, or b) passing the PID as an 
argument when executing ./run.

The environment variable approach can be used when s6-supervise is 
deployed in standalone settings (eg exec env S6_PPID=$$ s6-supervise 
servicedir), but this approach not presently a good fit in s6-svscan 
scenarios.

Earl