JEP 102 Process Updates revised API draft

[Peter Levart]

On 02/10/2015 02:02 PM, Peter Levart wrote:
> On 02/10/2015 12:35 PM, Peter Levart wrote:
>> ProcessHandle.completableFuture().cancel(true) forcibly destorys 
>> (destroyForcibly()) the process *and* vice versa: destory[Forcibly]() 
>> cancels the CompletableFuture. I don't know if this is the best way - 
>> can't decide yet. In particular, in the implementation it would be 
>> hard to achieve the atommicity of both destroying the process and 
>> canceling the future. Races are inevitable. So it would be better to 
>> think of a process (and a ProcessHandle representing it) as the 1st 
>> stage in the processing pipeline, where 
>> ProcessHandle.completableFuture() is it's dependent stage which 
>> tracks real changes of the process. Which means the behaviour would 
>> be something like the following:
>>
>> - ProcessHandle.destroy[Forcibly]() triggers destruction of the 
>> process which in turn (when successful) triggers completion of 
>> CompletableFuture, exceptionally with CompletionException, wrapping 
>> the exception indicating the destruction of the process 
>> (ProcessDestroyedException?).
>>
>> - ProcessHandle.completableFuture().cancel(true/false) just cancels 
>> the CompletableFuture and does not do anything to the process itself.
>>
>> In that variant, then perhaps it would be more appropriate for 
>> ProcessHandle.completableFuture() to be a "factory" for 
>> CompletableFuture(s) so that each call would return new independent 
>> instance.
>>
>> What do you think? 
>
> Contemplating on this a little more, then perhaps the singleton-per 
> pid CompletionStage could be OK if it was a "mirror" of real process 
> state. For that purpose then, instead of .completableFuture() the 
> method would be:
>
> public CompletionStage<ProcessHandle> completionStage()
>
> Returns a CompletionStage<ProcessHandle> for the process. The 
> CompletionStage provides supporting dependent functions and actions 
> that are run upon process completion.
>
> Returns:
>     a CompletionStage<ProcessHandle> for the ProcessHandle; the same 
> instance is returned for each unique pid.
>
>
> This would provide the most clean API I think, as CompletionStage does 
> not have any cancel(), complete(), obtrudeXXX() or get() methods. One 
> could still obtain a CompletableFuture by calling 
> .toCompletableFuture() on the CompletionStage, but that future would 
> be a 2nd stage future (like calling .thenApply(x -> x)) which would 
> not propagate cancel(true) to the process destruction.
>
> The implementation could still use CompletableFuture under the hood, 
> but exposed wrapped in a delegating CompletionStage proxy.
>
> So the javadoc might be written as:
>
>
> public abstract void destroy()
>
> Kills the process. Whether the process represented by this Process 
> object is forcibly terminated or not is implementation dependent. If 
> the process is not alive, no action is taken.
>
> If/when the process dies as the result of calling destroy(), the 
> completionStage() completes exceptionally with CompletionException, 
> wrapping ProcessDestroyedException.
>
>
> public abstract ProcessHandle destroyForcibly()
>
> Kills the process. The process represented by this ProcessHandle 
> object is forcibly terminated. If the process is not alive, no action 
> is taken.
>
> If/when the process dies as the result of calling destroyForcibly(), 
> the completionStage() completes exceptionally with 
> CompletionException, wrapping ProcessDestroyedException.
>
>
> But I'm still unsure of whether it would be better for the 
> completionStage() to complete normally in any case. Unless the fact 
> that the process died as a result of killing it could be reliably 
> communicated regardless of who was responsible for the killing (either 
> via ProcessHandle.destoroy() or by a KILL/TERMINATE signal originating 
> from outside the VM).
>
> Peter
>
>

Hi Roger,

I checked out your branch in jdk9-sandbox and studied current 
implementation.

One problem with this approach (returning a singleton-per-pid 
CompletableFuture or CompletionStage) is that current 
processReaperExecutor is using threads with very small stack size (32 K) 
and the returned CompletableFuture could be instructed to append a 
continuation that executes synchronously:

     CompletionStage.thenApply(), CompletionStage.handle(), etc...

... so user code would execute by such thread and probably get 
StackOverflowException...

Also, multiple ProcessHandle objects together with a Process object for 
the same pid each return a separate CompletableFuture instance (not what 
spec. says). Each of them also spawns it's own thread to wait for 
process termination.

Here's a better approach (a diff to your branch):

http://cr.openjdk.java.net/~plevart/jdk9-sandbox/JDK-8046092-branch/webrev.01/

...it contains a global registry of internal CompletionStage(s) - one 
per pid. They are not exposed to users, just used internally (in Unix 
ProcessImpl) to execute cleanup and in .completionStage() to append an 
asynchronous stage which is returned by the method on each call. So 
users appending synchronous continuations to returned CompletionStage 
would execute them in a common FJ pool.

I haven't tested this yet. It's just an idea to share.

Regards, Peter