[racket] adding other objects to custodian

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Sun Jul 4 16:28:04 EDT 2010

I agree that a parameter with a default is the right thing and that
the "kill the children" options should come with some warnings and
perhaps a pointer to an OS textbook for more details. The current
favorite seems to be this one:



On Sun, Jul 4, 2010 at 10:43 AM, Matthew Flatt <mflatt at cs.utah.edu> wrote:
> At Sat, 03 Jul 2010 22:03:33 -0400, Neil Van Dyke wrote:
>> I want to be sure that a host OS subprocess created with "subprocess" is
>> killed when thread of my program in which it was created terminates.
> This is a common request for built-in default functionality. Each time
> I try to add it, I get stuck on three issues:
>  * Under Unix and Mac OS X, there are multiple ways to terminate a
>    process: SIGTERM (can be ignored), SIGKILL (can't be ignored), and
>    SIGINT (usually ends a non-interactive program, anyway). At first
>    glance, the more custodian-like shutdown is SIGKILL, but if you
>    think of a subprocess as a kind of untrusted code, then SIGTERM or
>    SIGINT may be more appropriate --- so that the program can clean up
>    before exiting. Under Windows, we have only the equivalent of
>    SIGKILL, though.
>  * If you use `process' or `system' instead of `process*' or `system*',
>   then there may be an intermediate shell process between Racket and
>   process that the programmer probably has in mind. Killing the shell
>   process (with either SIGKILL or SIGTERM) doesn't kill the
>   subprocesses, and shells seem to ignore SIGINT without passing it
>   along to a subprocess.
>   At the same time, if the command is simple enough, a Unix `sh' will
>   typically replace the shell process with the target program, so
>   SIGKILL and SIGTERM would work as expected. That behavior of the
>   shell is not gauranteed, as far as I can tell. So, in the common
>   case of `system' and `process' under Unix, custodian-based
>   termination will look like it works better than it actually does,
>   which is misleading in a worrying way.
>  * A subprocess can create any number of sub-subprocesses itself, and
>   there's no way to kill those, which isn't very custodian-like.
> With all that in mind, I don't think there's a good default behavior
> that tries to terminate a subprocess when a custodian is shut down. We
> could add a parameter that determines how `subprocess' interacts with
> the custodian: SIGINT, SIGKILL, or nothing (i.e., the current
> behavior). I think the default should be nothing; having to set a
> parameter encourages a programmer to become informed on the issues of
> OS-level processes --- or, at least, explicitly accept best-effort
> termination that could just as well make things worse instead of
> better.
> Any opinions?
> Meanwhile, on the question of how to do this now:
>> Currently, I create a custodian for the thread and explicitly call
>> "custodian-shutdown-all" at the end of the thread's lifetime (in a
>> "dynamic-wind" cleanup thunk).
>> Ideally, I would like to use this same custodian to kill the subprocess,
>> but I do not see how.  "make-custodian-box" does not seem to involve
>> "custodian-shutdown-all" evaluating a closure in the box or executing a
>> will.
>> If using the current custodian is not possible, then I think I have to
>> do something like make a will executor and execute it from my
>> "dynamic-wind" cleanup thunk.
> That sounds ok. Also, you could use the FFI and scheme_add_managed().
> I started to suggest that you use custodian boxes and have a thread
> that waits until a box is emptied to terminate the corresponding
> process. That would work better if custodian boxes were synchronizable
> events; I will make custodian boxes act as events, so that strategy
> could work in the future.
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