SelectableChannels and Process API

Peter Levart peter.levart at
Wed Apr 15 06:31:47 UTC 2015

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:
> 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." 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:
>> 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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