testscript: add kill command

[kortschak]

This allows sending a termination signal to backgrounded commands.

Please take a look.

Closes #242

[ldemailly]

Might be more generic to be able to choose the signal? (hup vs int vs kill...)

[kortschak]

Yeah, I was thinking that. I was wondering about the API for that. Would you be happy with kill [-SIG] [name]? (SIG would be numeric only in the first instance on second thought, named signal would be better). With that, the default would probably be SIGKILL.

[kortschak]

ping

As noted in the slack conversation about this PR, if adding a kill command is too much, an alternative would be to expose TestScript.findBackground so that client code can build the command within its test code.

[mvdan]

I know @rogpeppe isn't sure whether this is the perfect solution, but I'm perfectly comfortable to choose a solution that's good enough over no solution at all :)

[mvdan]

can you please add failure test cases? in particular:

• trying to kill a running background process that was started with & rather than &word& (i.e. using a background process name that doesn't exist)
• trying to kill a background process that already stopped or was already killed

Please also cover the kill and kill name forms, as otherwise we can't be sure they will keep working in the future.

[kortschak]

These negated tests are not currently possible since the kill command is not negatable. This is consistent with wait.

The positive tests for without name are possible.

[mvdan]

Thanks for adding more tests!

I see your point about consistency with wait not being negatable. I personally think both should be, and both should fail if given an unknown/exited process, but we can always add that later - especially once we have ? to ignore the result of a command.

[mvdan]

As noted in the slack conversation about this PR, if adding a kill command is too much, an alternative would be to expose TestScript.findBackground so that client code can build the command within its test code.

I would personally also be fine with this, but given that the majority of users would presumably use that to write their own kill commands, I'm fine with adding the command directly. We can always choose to expose the Go API later as well.

[mvdan]

Thanks! LGTM