Hi Peter,
We do know the PIDs of the processes that we care about but are unwilling
to pay the cost of waiting for them individually.
For the escapees, Process could resort to an individual thread invoking
waitpid(n).
Thanks, Roger
On 4/11/2014 10:52 AM, Peter Levart wrote:
On 04/09/2014 07:02 PM, Martin Buchholz wrote:
On Tue, Apr 8, 2014 at 11:08 PM, Peter Levart <peter.lev...@gmail.com
<mailto:peter.lev...@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:
- 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
