docs: terminals: mention subreaper requirement

I realised that the terminal documentation which covers detached
terminals fails to mention that callers need to make themselves a
subreaper. Probably a good idea to mention this. I've also included a
minor comparison to LXC.

Signed-off-by: Aleksa Sarai <asarai@suse.de>
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Aleksa Sarai 2020-04-28 22:53:59 +10:00
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@ -228,6 +228,19 @@ Unfortunately using detached mode is a bit more complicated and requires more
care than the foreground mode -- mainly because it is now up to the caller to care than the foreground mode -- mainly because it is now up to the caller to
handle the `stdio` of the container. handle the `stdio` of the container.
Another complication is that the parent process is responsible for acting as
the subreaper for the container. In short, you need to call
`prctl(PR_SET_CHILD_SUBREAPER, 1, ...)` in the parent process and correctly
handle the implications of being a subreaper. Failing to do so may result in
zombie processes being accumulated on your host.
These tasks are usually performed by a dedicated (and minimal) monitor process
per-container. For the sake of comparison, other runtimes such as LXC do not
have an equivalent detached mode and instead integrate this monitor process
into the container runtime itself -- this has several tradeoffs, and runc has
opted to support delegating the monitoring responsibility to the parent process
through this detached mode.
#### Detached Pass-Through #### #### Detached Pass-Through ####
In detached mode, pass-through actually does what it says on the tin -- the In detached mode, pass-through actually does what it says on the tin -- the