Hi Peter,
Sorry to have taken so long to get back to this thread on
SelectableChannels for process I/O.
Working on the pipelines idea got me back to thinking about this proposal.
One bit of spec/implementation to address is the state of the In/Out/Err
streams
of the process when using Redirect.PIPE_CHANNEL. It may be as simple as
specifying, as is done for the other Redirect cases, that they have
'null' implementations and
setting the respective elements of fds[0..2] to -1 before the call to
initStreams (line 383//401).
If this moves forward, there should be more tests especially of the
Selectable functions
and error cases. Note that 'assert' is not always checked, it depends
on how the test is invoked.
I've got a busy August, but could spend time on this later (between all
the other priorities).
Thanks, Roger
On 4/15/2015 2:31 AM, Peter Levart wrote:
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.
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
