ProcessReaper: single thread reaper

Peter Levart peter.levart at
Mon Apr 14 09:02:28 UTC 2014

Hi Martin, Roger,

Just a thought. Would it be feasible to have two (ore more) built-in 
strategies, selectable by system property? A backwards compatible tread 
per child, using waitpid(pid, ...), a single reaper thread using 
waitpid(-1, ...), maybe also single threaded strategy accessible only on 
Linux/Solaris using waitid(-1, ..., WNOWAIT)... All packed nicely in a 
package-private interface (ProcessReaper) with multiple implementations?

Regards, Peter

On 04/12/2014 01:37 AM, Martin Buchholz wrote:
> Let's step back again and try to check our goals...
> We could try to optimize the one-reaper-thread-per-subprocess thing. 
>  But that is risky, and the cost of what we're doing today is not that 
> high.
> We could try to implement the feature of killing off an entire 
> subprocess tree.  But historically, any kind of behavior change like 
> that has been vetoed.  I have tried and failed to make less 
> incompatible changes.  We would have to add a new API.
> The reality is that Java does not give you real access to the 
> underlying OS, and unless there's a seriously heterodox attempt to 
> provide OS-specific extensions, people will have to continue to either 
> write native code or delegate to an OS-savvy subprocess like a perl 
> script.
> On Fri, Apr 11, 2014 at 7:52 AM, Peter Levart <peter.levart at 
> <mailto:peter.levart at>> wrote:
>     On 04/09/2014 07:02 PM, Martin Buchholz wrote:
>>     On Tue, Apr 8, 2014 at 11:08 PM, Peter Levart
>>     <peter.levart at <mailto:peter.levart at>> wrote:
>>         Hi Martin,
>>         As you might have seen in my later reply to Roger, there's
>>         still hope on that front: setpgid() + wait(-pgid, ...) might
>>         be the answer. I'm exploring in that direction. Shells are
>>         doing it, so why can't JDK?
>>         It's a little trickier for Process API, since I imagine that
>>         shells form a group of processes from a pipeline which is
>>         known in-advance while Process API will have to add processes
>>         to the live group dynamically. So some races will have to be
>>         resolved, but I think it's doable.
>>     This is a clever idea, and it's arguably better to design
>>     subprocesses so they live in separate process groups (emacs does
>>     that), but:
>>     Every time you create a process group, you change the effect of a
>>     user signal like Ctrl-C, since it's sent to only one group.
>>     Maybe propagate signals to the subprocess group?  It's starting
>>     to get complicated...
>     Hi Martin,
>     Yes, shells send Ctrl-C (SIGINT) and other signals initiated by
>     terminal to a (foreground) process group. A process group is
>     formed from a pipeline of interconnected processes. Each pipeline
>     is considered to be a separate "job", hence shells call this
>     feature "job-control". Child processes by default inherit process
>     group from it's parent, so children born with Process API (and
>     their children) inherit the process group from the JVM process.
>     Considering the intentions of shell job-controll, is propagating
>     SIGTERM/SIGINT/SIGTSTP/SIGCONT signals to children spawned by
>     Process API desirable? If so, then yes, handling those signals in
>     JVM and propagating them to current process group that contains
>     all children spawned by Process API and their descendants would
>     have to be performed by JVM. That problem would certainly have to
>     be addressed. But let's first see what I found out about
>     sigaction(SIGCHLD, ...), setpgid(pid, pgid), waitpid(-pgid, ...),
>     etc...
>     waitpid(-pgid, ...) alone seems to not be enough for our task.
>     Mainly because a process can re-assign it's group and join some
>     other group. I don't know if this is a situation that occurs in
>     real world, but imagine if we have one live child process in a
>     process group pgid1 and no unwaited exited children. If we issue:
>         waitpid(-pgid1, &status, 0);
>     Then this call blocks, because at the time it was given, there
>     were >0 child processes in the pgid1 group and none of them has
>     exited yet. Now if this one child process changes it's process
>     group with:
>         setpgid(0, pgid2);
>     Then the waitpid call in the parent does not return (maybe this is
>     a bug in Linux?) although there are no more live child processes
>     in the pgid1 group any more. Even when this child exits, the call
>     to waitpid does not return, since this child is not in the group
>     we are waiting for when it exits. If all our children "escape" the
>     group in such way, the tread doing waiting will never unblock. To
>     solve this, we can employ signal handlers. In a signal handler for
>     SIGCHLD signal we can invoke:
>         waitpid(-pgid1, &status, WNOHANG); // non-blocking call
> loop until it either returns (0) which means that there're
>     no more unwaited exited children in the group at the momen or (-1)
>     with errno == ECHILD, which means that there're no more children
>     in the queried group any more - the group does not exist any more.
>     Since signal handler is invoked whith SIGCHLD being masked and
>     there is one bit of pending signal state in the kernel, no child
>     exit can be "skipped" this way. Unless the child "escapes" by
>     changing it's group. I don't know of a plausible reason for a
>     program to change it's process group. If a program executing as
>     JVM child wants to become a background daemon it usually behaves
>     as follows:
>     - fork()s a grand-child and then exit()s (so we get notified via
>     signal and waitpid(-pgid, ...) successfully for it's exitstatus)
>     - the grand-child then changes it's session and group (becomes
>     session and group leader), closes file descriptors, etc. The
>     responsibility for waiting on the grand-child daemon is
>     transferred to the init process (pid=1) since the grand-child
>     becomes an orphan (has no parent).
>     Ignoring this still unsolved problem of possible ill-behaved child
>     program that changes it's process group, I started constructing a
>     proof-of-concept prototype. What I will do in the prototype is
>     start throwing IllegalStateException from the methods of the
>     Process API that pertain to such children. I think this is reasonable.
>     Stay tuned,
>     Peter

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