ProcessReaper: single thread reaper (original) (raw)

Peter Levart peter.levart at gmail.com
Fri Apr 11 14:52:03 UTC 2014

On 04/09/2014 07:02 PM, Martin Buchholz wrote:

On Tue, Apr 8, 2014 at 11:08 PM, Peter Levart <peter.levart at gmail.com_ _<mailto:peter.levart at gmail.com>> 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

...in 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:

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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