On 04/15/2015 07:59 PM, Martin Buchholz wrote:
I was at least partly responsible for the pipe buffer cleanup code.
Subprocess terminates, but may have written some data to the pipe
buffer (typically 4k on Linux). Usually the pipe buffer is empty, but
in case it's not, you don't want to lose the straggler data, you want
to drain it and close the file descriptor, because it's easier to
manage the memory than the fd. Messy, but I didn't see a better way.
But the data would stay there (in the pipe's buffer) until it is read by
the user. The producing end of pipe may already be closed, but the
consuming end is still open. You would just have to keep the file
descriptor open and let user drain and close it (or leave it to
FileInputStream finalizer to close it). Yes, a file descriptor will be
potentially open some more time, but you wouldn't loose any data. That's
how Windows implementation works, I think. There's not reaper thread in
Windows that would trigger asynchronous actions when subprocess exits.
Regards, Peter
On Tue, Apr 14, 2015 at 11:31 PM, Peter Levart <peter.lev...@gmail.com
<mailto:peter.lev...@gmail.com>> 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/
<http://cr.openjdk.java.net/%7Eplevart/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.
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
