(Refactoring) Simplify RMD child process disposal

[ElianHugh]

Minor refactor to the R Markdown preview/knit processes, cleaning up the disposal of child processes.

Changes:

• Contributes a helper function for creating disposables from objects
• Simplifies disposal of child processes
• Do not print lines to output when the process has already been terminated

Both knitting and previewing should function as before

[renkun-ken]

I try the following Rmd

```{r}
x <- 1
Sys.sleep(100)
```

The kniting will take 100s to finish. When I click "Cancel", the progress window is closed but the R process is still running until it is finished. Shouldn't the process be killed once it is cancelled?

[ElianHugh]

Does the process not receive an exit signal in your output stream? This is what I see when I try it:

Edit: oh, I see what you mean, despite SIGKILL, the process output a file after 100 seconds. Bizarre, I'll try to see why

[renkun-ken]

It does receive a SIGKILL signal

[VSC-R] terminating R process
[VSC-R] doc4.Rmd process exited from signal 'SIGKILL'

but I'm monitoring all R processes and the process is not actually terminated.

[ElianHugh]

I believe this is caused by the use of exec rather than spawn, as the shell appears to not close until completion

[ElianHugh]

Last push appears to have fixed the termination issue on my end

[renkun-ken]

Thanks for the quick fix. It works on my end too.

[renkun-ken]

LGTM