Hi Peter,
I don't know the history behind the stream draining in ProcessImpl.
I understood it to be a performance/scalability issue.
Maybe Martin, Alan, or someone else can fill in the history.
Roger
On 4/15/2015 2:31 AM, Peter Levart wrote:
Hi Roger,
So I started new thread...
On 04/14/2015 11:33 PM, Roger Riggs wrote:
On 4/14/2015 11:47 AM, Peter Levart wrote:
I have been thinking of another small Process API update. Some
people find it odd how redirected in/out/err streams are exposed:
http://blog.headius.com/2013/06/the-pain-of-broken-subprocess.html
yep, I've read that several times.
To be fair, it's mostly, but not entirely correct. The part that says:
" So when the child process exits, the any data waiting to be read
from its output stream is drained into a buffer. All of it. In memory.
Did you launch a process that writes a gigabyte of data to its output
stream and then terminates? Well, friend, I sure hope you have a
gigabyte of memory, because the JDK is going to read that sucker in
and there's nothing you can do about it. And let's hope there's not
more than 2GB of data, since this code basically just grows a byte[],
which in Java can only grow to 2GB. If there's more than 2GB of data
on that stream, this logic errors out and the data is lost forever."
...is exaggeration. This does not happen as the pipe has a bounded
buffer. When subprocess exits, there is at most that much data left in
the buffer (64k typically) and only that much is sucked into the Java
process and the underlying handle closed.
They basically don't like:
- that exposed Input/Output streams are buffered
- that underlying streams are File(Input/Output)Streams which,
although the backing OS implementation are not files but pipes,
don't expose selectable channels so that non-blocking event-based IO
could be performed on them.
- that exposed IO streams are automatically "managed" in UNIX
variants of ProcessImpl which needs subtle "hacks" to do it in a
perceptively transparent way (delayed close, draining input on exit
and making it available after the underlying handle is already
closed, ...)
So I've been playing with the idea of exposing the "real" pipe
channels in last couple of days. Here's the prototype I came up with:
http://cr.openjdk.java.net/~plevart/jdk9-sandbox/JDK-8046092-branch/Process.PipeChannel/webrev.01/
This adds new Redirect type to the API and 3 new methods to Process
that return Pipe channels when this new Redirect type is used. It's
interesting that no native code changes were necessary. The behavior
of pipes on Windows is a little different (perhaps because the Pipe
NIO API uses sockets under the hood on Windows - why is that?
Windows does have a pipe equivalent). What bothers me is that file
handles opened on files (when redirecting to/from File) can be
closed as soon as the subprocess is started and the subprocess is
still able to read/write from the files (like with UNIX). It's not
the same with pipe (i.e. socket) handles on Windows. They must be
closed only after subprocess exits.
If this subtle difference between file handles and socket handles on
Windows could be dealt with (perhaps some options exist that affect
subprocess spawning), then the extra waiting thread would not be
needed on Windows.
So what do you think of this API update?
Definitely worthy of a separate thread. It looks promising and
addresses some of the issues
raised, while moving other problems from the implementation to the
application.
Such as closing of the channels and cleanup. I worry about how the
resources are freed
if the code spawning the app doesn't do the cleanup. Will it require
hooks (like a finalizer)
to do the cleanup?
Also, it doesn't help with Martin's goal of being able to implement
emacs in Java since it doesn't provide pty control.
As you are aware the complexity in Process is to ensure a timely
cleanup and
allowing the Process to terminate and release the process resources
when it was done and not having to wait for the stdout/stderr consumer.
I wonder how this automatic stream cleanup really helps in real-world
programs. It doesn't help the Process to terminate and release the
process resources any sooner as the process terminates on it's own
(unless killed) and OS releases it's resources without the outside
help anyway. Draining and closing the stream after the process has
already exited just releases one file handle (the consuming side of
the pipe) in a promptly manner. This could be left to the user and/or
finalizer. Draining after the process has already exited does not help
the process to exit any sooner as it happens after the fact. A program
that doesn't consume the stream can cause the process to hang forever
as the pipe's buffer is bounded (64k typically). So draining and
closing after the process has exited only potentially helps for the
last 64k of the stream and only to release one file handle in a
potentially more timely manner.
OTOH now that ProcessImpl for UNIX does that (and why does Windows
implementation not do that?) sloppy programs might exist that would
potentially break if the status quo is not maintained.
I think Windows use of handles makes sure they are open for as long as
any process
holds a handle, so they don't get prematurely closed.
But new functionality need not be so permissive. I'll take a look at
how and if Channel(s) do any kind of automatic cleanup based on
reachability and whether this can be bolted on for Process use. I
doubt it is possible to drain and close a Channel without disturbing
the ongoing Selector IO processing...
Regards, Peter
Thanks, Roger
