From: Ehsanul Hoque <ehsanul_g3@...>
Date: 2009-09-11T11:27:18+09:00
Subject: Re: Can not catch PTY::ChildExited exception

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Well=2C I'm sure I have nowhere as good an idea of PTY.spawn() as Matsumoto=
 does=2C but let me try to explain what your code is doing=2C and how to ge=
t your desired result.

First=2C you might want to consider using other methods for what you're doi=
ng. You probably don't really need a pseudo terminal=2C which is what PTY g=
ives you=2C and you can use more well documented methods. These two links g=
ive a good overview of some of the other ways you can start subprocesses in=
 ruby:
http://devver.net/blog/2009/06/a-dozen-or-so-ways-to-start-sub-processes-in=
-ruby-part-1/
http://devver.net/blog/2009/07/a-dozen-or-so-ways-to-start-sub-processes-in=
-ruby-part-2/

If you still think you really need to use a pseudo terminal=2C then read on=
.

PTY.spawn() doesn't block your ruby program like a call to system() would. =
It simply starts the process given the command=2C and lets you continue. In=
 your first email=2C you mentioned that when you took out the "sleep 10" li=
ne in your code=2C no exception took place at all. The reason for this is t=
hat your script ended before the child did=2C since the ten seconds of slee=
p were taken out. And so ruby did not have a chance to raise an exception. =
Similarly=2C in your latest code example=2C "TEST END" was never printed be=
cause the
begin block had an exception before you even reached the 'printf "TEST END\=
n"' line. The exception occurred while
it was in "sleep 10".  I hope that's clear=2C it messed me up too in the pa=
st.

To get what you want=2C you might want to use the block form for PTY.spawn(=
) instead. I just find it cleaner=2C and using the limited scope for the va=
riables pipe_read etc makes sense. The variables are gone when the pseudo t=
erminal is gone.

What I've done is put a loop inside the PTY.spawn() block=2C which can be u=
sed to handle the output from the subprocess=2C or feed it input. If you do=
n't need to do that=2C you probably don't need to use PTY anyways=2C but yo=
u can be the judge of that. Now=2C this loop stops when the subprocess exit=
s=2C and then the rest of the ruby script continues.

It seems like you also want to run some code in parallel to the subprocess=
=2C but not in the loop I mentioned. For this case=2C the code below also h=
as a thread. Only when the child exits does the thread stop=2C and "t.join"=
 finally allows the code to continue. Before "t.join" and after the thread =
block though=2C you can run code in parallel to the subprocess. And then=2C=
 "Test End" is printed after the subprocess has exited. Here's the code:

require 'pty'

cmd =3D "/home3/cman/work/product/integrator/test/syb2/OCS-15_0/bin/isql -U=
user2 -Puser2 -Scman2"

t =3D Thread.new do
  begin
    PTY.spawn(cmd) do |pipe_read=2C pipe_write=2C pid|
      loop do
        # Code here can handle output given by your command=2C and feed it =
input=2C etc
        # But the infinite loop will be broken by the exception caused when=
 the child process exits=2C and your script continues
        # Or you could just terminate this loop in advance=2C but remember=
=2C you might reach the end of your ruby script
        # before the child exits=2C in case that's a problem.
      end
    end
  rescue PTY::ChildExited =3D> msg
    puts "The child has exited"
  rescue =3D> msg
    puts "A different exception:\n" + msg
  end
end

puts "Doing stuff in parallel to the subprocess here"
t.join
puts "Test End"


I hope that was useful/clear. It was certainly too long.

- Ehsan


> Date: Fri=2C 11 Sep 2009 09:56:15 +0900
> From: denis.berezhnoy@gmail.com
> Subject: Re: Can not catch PTY::ChildExited exception
> To: ruby-talk@ruby-lang.org
>=20
> Yukihiro Matsumoto wrote:
> > Hi=2C
> >=20
> > In message "Re: Can not catch PTY::ChildExited exception"
> >     on Wed=2C 9 Sep 2009 15:14:09 +0900=2C Denis Berezhnoy=20
> > <denis.berezhnoy@gmail.com> writes:
> >=20
> > |I have problem that I can not catch PTY::ChildExited exception. Here i=
s
> > |the code:
> >=20
> > The exception was raised when the child process terminated
> > (asynchronously)=2C so you need to wrap not only spawn but everything.
> >=20
> >               matz.
>=20
> Hello Matsumoto-san=2C
>=20
> Thank you very much for clarifications! I tried to wrap around whole=20
> code with begin rescue block and I can catch ChildExited exception.  So=20
> now I have code like this:
>=20
> require 'pty'
>=20
> cmd =3D "/home3/cman/work/product/integrator/test/syb2/OCS-15_0/bin/isql=
=20
> -Uuser2 -Puser2 -Scman2"
>=20
> begin
>=20
> pipe_read=2C pipe_write=2C pid =3D PTY.spawn(cmd)
>=20
> sleep 10
>=20
> printf "TEST END\n"
>=20
> rescue PTY::ChildExited =3D> msg
>     puts "Caught child exited exception!"
> rescue =3D> msg
>     puts "Caught exception!"
> rescue =3D> e
>     puts e.msg
>     puts e.type
> end
>=20
>=20
> But now the problem is that ChildExited exception can occur at any=20
> moment after spawn and interrupt program in random place. In my test=20
> above I never get TEST END printed.
>=20
> How can I handle this in right way?
>=20
> Best regards=2C
> Denis
> --=20
> Posted via http://www.ruby-forum.com/.
>=20

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