In UNIX, any zombies without parent process get re-parented to process
with PID 1 (usually running init), and PID 1 process should take care of
them (usually simply clean them up). Cleaning up zombies is important,
as they still take kerner resources, and having enormous amount of
zombie processes signifcantly degrades system performance.
For Talos, PID 1 process is machined, and machined itself forks to run
other processes in process runner and `pkg/cmd` one-time commands. Naive
solution of running `wait()` loop doesn't work as it might race with
`Process.Wait()` and clean up zombie which wasn't re-parented which
leads to process execution false failure.
After considering other solutions, we decided to go with the simple
approach: machined runs global zombie process reaper which publishes
information about reaped zombies. Any call to `Process.Wait()` (or
`Command.Wait()` which calls it) should be replaced with listening to
reaper's channel for notifications to catch info about the process which
was created in this call.
There are several changes in this PR:
1. Reaper implementation itself, started from machined.
2. Process runner and `pkg/cmd` can either use regular `Command.Wait()`
or use reaper notifications depending on reaper status (running/not
running). This allows using this code outside of machined.
3. Small bug fixes with process log which was affecting the tests.
Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>
