Hi Thomas,
I'd be interested in hearing more about the use cases.
There seem to be many cases where containers are doing the management
of groups of processes.
The function will need to have an equivalent on Windows.
The expressed use case is taking advantage of Posix/Unix signal behavior.
But there are oh so many issues with signals, its likely to be a big can
of worms.
You mention a desire for other Posix functions, please elaborate.
If there's a way to scope it reasonably then a JEP is the process to pursue.
Thanks, Roger
On 11/12/2018 12:29 PM, Thomas Stüfe wrote:
Dear all,
may I please hear your thoughts about the following proposal?
We would like to add support for process groups to the JDK: the
ability to put child processes into new or pre-existing process
groups. We added this feature to our proprietary port some time ago
and has been very useful in cases where the VM acts in a process
scheduling role.
With process groups we mean of course standard Unix process groups.
There exists a similar concept on Windows, Job Objects, so at least a
subset of what we propose could be done in a platform independent way.
----
Motivation:
Most importantly, the ability to safely terminate a group of processes.
The established way to do this is, since Java 9, to iterate over a
process tree, calling Process.children() or Process.descendants() on
the root Process, and killing them using Process.destroy().
In practice, that approach is not always a good fit. It leaves out any
orphaned processes; any deceased non-leaf process in the tree makes
its children unreachable. Worst case, if the root process dies, all
children are orphaned and cannot be reached. Another limitation is
that this only works for process trees - parent-child relationships -
but not for unrelated processes one might want to group together. It
also becomes a bit inefficient with many processes, requiring one JNI
call/system call per process to kill.
Process groups, OTOH, would allow us to group together any number of
unrelated processes. We can then send them bulk signals, eg
SIGTERM/SIGKILL with only one system call. And for that to work, the
parent relationships do not matter, so we also reach processes which
have been orphaned.
There are more things one could do with process groups besides killing
them: suspend/resume them together (SIGSTOP/CONT), or to send them to
the background of the controlling terminal.
In fact, one could write its own shell in Java :)
----
I drew up a tiny patch to demonstrate how this could look. This is
just an example, to have something to play with and talk about:
http://cr.openjdk.java.net/~stuefe/webrevs/processgroup-support/webrev.01/webrev/index.html
and here is a small usage example:
https://github.com/tstuefe/ojdk-repros/blob/master/src/other/RuntimeExecSimpleTestWithProcessGroup.java
The suggested API changes are small:
- A new class ProcessGroup as the platform's notion of a process
group. In this patch, it offers four functions:
- destroy()/destroyForcibly() terminate or kill the whole process group
- suspend()/resume() puts them to sleep and wakes them up.
More functionality could be added if needed. This mostly depends on
how tightly we want to be bound by platform limitations on Windows,
where process groups cannot be translated 1:1 to Job Objects.
- ProcessBuilder has now two new attributes:
- createProcessGroup() is a boolean flag directing the builder to
let sub processes create their own process group, with themselves
being the leader.
- processGroup() is a reference to a ProcessGroup object; when not
null, subprocesses will join that process group.
- The Process class gets a new query method to retrieve a ProcessGroup
object linked to its process group id.
Using these building stones, a typical pattern could be:
<example>
ProcessBuilder processBuilder = new ProcessBuilder(cmd);
processBuilder.createProcessGroup(true); <-- next process is pg leader
Process leader = processBuilder.start();
ProcessGroup pgr = leader.processGroup(); <-- retrieve newly
created process group
processBuilder.processGroup(pgr); <-- next processes shall be
members of this process group too
processBuilder.start();
processBuilder.start();
....
</example>
and then call operations on the ProcessGroup object.
----
It is clear to me that this kind of change would require probably a
JEP, if it is desired at all. With this mail I just wanted to gauge
interest.
What do you think?
Thanks & Best Regards, Thomas
