SelectableChannels and Process API
Roger Riggs
Roger.Riggs at Oracle.com
Wed Apr 15 18:14:58 UTC 2015
Hi Martin,
That sounds like finalization of the FileDescriptor, but perhaps a
mechanism built on PhantomRef
would be easier enough to maintain and can leave the data in the pipe.
Roger
On 4/15/2015 1:59 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.
>
> On Tue, Apr 14, 2015 at 11:31 PM, Peter Levart <peter.levart at gmail.com
> <mailto:peter.levart at 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
>
>
>
>
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