ProcessReaper: single thread reaper

Peter Levart peter.levart at
Tue Mar 25 16:50:57 UTC 2014

On 03/25/2014 05:25 PM, Roger Riggs wrote:
> Hi Peter,
> Thanks for the comments, I'll fix the races conditions, I intended
> to go back to the dedicated thread but wanted to check for scalability
> in the case of a large number of threads and non-trivial termination 
> overheads
> where the streams had to be drained.

Why does stream need to be drained right away (in reaper thread)? Isn't 
it enough for child pid to be waited upon and let the client of the 
stream pull from the stream until it reaches the EOF or not if it 
doesn't need the data... Is this just to be able to reclaim the file 
descriptor of the pipe right away? Wouldn't FileInputStream's close() 
method do the job (or finalize() if user forgets to call close())? There 
would be a native resource leakage if user forgets to call close() AND 
retains the reference to InputStream, but this is the same with all 
normal streams like FileInputStream...

Regards, Peter

> Thanks, Roger
> On 3/25/14 7:05 AM, Peter Levart wrote:
>> On 03/24/2014 10:05 PM, roger riggs wrote:
>>> Hi Rob, Martin,
>>> I've prototyped (for 9) a thread reaper[1] that uses a single thread 
>>> to wait for exiting
>>> processes and calling back to the process with the exit status.
>>> the interesting part is getting the exit status back to the Process 
>>> that needs it
>>> It needs more testing and hardening.
>>> I had not considered using a signal handler for SIGCHLD but that's 
>>> an option,
>>> though we need to be very careful about thread usage.
>>> Roger
>>> p.s. comments on the single thread reaper appreciated (in a new thread)
>>> [1]
>> Hi Roger,
>> I think I found a little race. Suppose reaper task is still alive and 
>> that all consumers have been serviced (consumerCount is 0). The 
>> reaper task waits for 500 millis for next consumer to be registered, 
>> but times out. Before calling "reaperThread.release()", new consumer 
>> comes around and registers itself, also calling runReaper(), but 
>> since reaperThread.release() has not yet been called by old reaper 
>> task, new reaper task is not submitted to commonPool. The old reaper 
>> task finishes, leaving one consumer on the waitingList with no reaper 
>> task to service it. If no new consumers get registered, the waiting 
>> consumer will never be notified...
>> The simplest solution for this race, I think, would be to have a 
>> dedicated long-running thread. It could be spawned lazily, but then 
>> it would never finish.
>> Otherwise a nice solution with two lists (exitList/waitList) and 
>> avoidance of race with reversed orders between
>> - consumer registration: register on waitList 1st then check 
>> exitList, and
>> - exit event dispatch: register on exitList 1st then check waitList
>> ...but the check you use with consumeCount local variable to detect 
>> processes spawned by other means (for purposes of logging only) has a 
>> race:
>> thread1: Suppose a new consumer is being registered with 
>> onExitCall(...), is added on the waitList, but before checking 
>> exitList().size() and iterating the exitList, ...
>> thread2: the reaper task detects that the very same process has 
>> finished (gets it's pid from waitpid) and adds it's pid to exitList. 
>> Then before iterating waitList, ...
>> thread1: iterates the exitList, finds a match and consumes the pid, 
>> removing the matching entries from both exitList and waitList. Then ...
>> thread2: the reaper task iterates waitList, doesn't find a matching 
>> entry for exitPid, doesn't increment consumeCount and voila: debug 
>> log("Unexpected process exit for pid:...").
>> That's enough races for today.
>> Regards, Peter
>>> On 3/24/2014 12:38 AM, Rob McKenna wrote:
>>>> Hi folks,
>>>> Roger Riggs (cc'd) may want to chip in here as he's looking at the 
>>>> reaper thread arrangement in 9 at the moment.
>>>> On another note, I too support the merging of those files. I didn't 
>>>> think there was much appetite for it at the time so I must admit 
>>>> this fell down my todo list. Looking at this bug did remind me that 
>>>> its something worth trying though. As per Alan's mail, I'm going to 
>>>> tackle it separately if you folks don't mind. I'll have a look at 
>>>> Peter's changes (thanks Peter!) as soon as I can and see about 
>>>> getting them in.
>>>>     -Rob
>>>> On 23/03/14 22:30, Martin Buchholz wrote:
>>>>> On Sun, Mar 23, 2014 at 2:34 AM, Martin Buchholz 
>>>>> <martinrb at <mailto:martinrb at>> wrote:
>>>>>     We have also thought about whether having reaper threads is
>>>>>     necessary.  The Unix rule is that child processes should be
>>>>>     waited for, and some thread needs to do that.  There's no way
>>>>>     to wait for a set of child pids, or to specify a "completion
>>>>>     handler".  Well, you might be able to get the newish waitid()
>>>>>     to do what you want, but it looks like it's not sufficient
>>>>>     when java is running inside a process that might do
>>>>>     independent subprocess creation outside of the JVM.
>>>>> Actually, I take it back.  With sufficient work, it looks like you 
>>>>> can get SIGCHLD to give you pid information in siginfo_t si_pid, 
>>>>> and that can be used to trigger the reaping.  It looks like 
>>>>> waitpid is "async-signal-safe", so we can call it from our signal 
>>>>> handler.
>>>>> While we're at it we can fix SIGCHLD handling to do signal 
>>>>> chaining, as with other signals.

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