Standard IO streams in Ruby

In my rails controller, I am doing the following :

@output = g++ j.cpp -o "prog" && ./prog

This gives the output in the @output variable which i can display in my
view. But the above works only if the j.cpp is correct and doesn’t
expect any user input. How can I use the stdin/stderr and stdout streams
here so that :

If the user has to give input, I open a dialog box on the view with a
textfield where he/she can enter the input, and the program continues to
execute.
If there are any errors in the file, then I should be able to get the
errors and display them to the user.
I tried doing this :

@output = g++ j.cpp -o "prog" && ./prog| tee prog

This allows me to enter the user input at the server command prompt (the
server log…i don’t know what do we call it), but I want it to be
entered in a textfield in the view.

In short, how can I get control of stdin, so that whenever the stdin
waits for some user input, i get to know that it is waiting for the user
input and then i can show the user a text field and get the user input
to be passed in the stdin stream.
Let me know if there’s a doubt in the question.

On Sunday, August 01, 2010 01:06:59 pm Jatin K. wrote:

In my rails controller, I am doing the following :

@output = g++ j.cpp -o "prog" && ./prog

Why are you compiling it every time?

From some of your questions, it seems like you’re going to let the user
enter
source code, then compile it and run it. Is that true? If so, do you
realize
how insanely dangerous that is?

If the user has to give input, I open a dialog box on the view with a
textfield where he/she can enter the input, and the program continues to
execute.

You can’t do that from inside a Rails controller. You can capture IO
with
popen or open3:

http://www.ensta.fr/~diam/ruby/online/ruby-doc-
stdlib/libdoc/open3/rdoc/classes/Open3.html

But that doesn’t solve your problem by itself – you need to send data
to and
from the browser interactively. That means you at least need AJAX, and
possibly Comet or Web Sockets, combined with some sort of background
processing.

I tried doing this :

@output = g++ j.cpp -o "prog" && ./prog| tee prog

That would overwrite your program with the output of your program. Is
that
really what you want to do?

man tee

In short, how can I get control of stdin, so that whenever the stdin
waits for some user input, i get to know that it is waiting for the user
input

You can’t do that either, not really. The best you can do is always
provide
that text field, and show the user standard output and standard error,
so they
can see the program prompting them – and detect the program exiting, so
you
can remove the text field.

If you want to be able to do this efficiently, you’re going to need to
do a
lot of reading. It basically boils down to these two options:

http://en.wikipedia.org/wiki/Comet_(programming)
http://www.websockets.org/

But however you implement the updates – you could always just poll –
but
you’re still going to need one of these:

http://www.tobinharris.com/past/2009/3/9/6-ways-to-run-background-jobs-in-
rubyonrails/

There are probably newer/better alternatives to those, too…

In my rails controller, I am doing the following :

@output = g++ j.cpp -o "prog" && ./prog

Why are you compiling it every time?

I am not compiling it every time, only when the user clicks on the
compile/execute button that i provide in the view.

From some of your questions, it seems like you’re going to let the user
enter
source code, then compile it and run it. Is that true? If so, do you
realize
how insanely dangerous that is?

Yes, I am exactly doing that. Why does it sound to you insanely
dangerous??
Could you elaborate a little on that.?

If the user has to give input, I open a dialog box on the view with a
textfield where he/she can enter the input, and the program continues to
execute.

You can’t do that from inside a Rails controller. You can capture IO
with
popen or open3:

http://www.ensta.fr/~diam/ruby/online/ruby-doc-
stdlib/libdoc/open3/rdoc/classes/Open3.html

Yes, i know that I have to capture IO for that. Thanks for the link.
Really appreciated.

But that doesn’t solve your problem by itself – you need to send data
to and
from the browser interactively. That means you at least need AJAX, and
possibly Comet or Web Sockets, combined with some sort of background
processing.

Yes, I am using AJAX. I will look into Comet or Web Sockets, but I have
no idea of a web socket and how do they work? How can I use them in
here?

I tried doing this :

@output = g++ j.cpp -o "prog" && ./prog| tee prog

That would overwrite your program with the output of your program. Is
that
really what you want to do?

man tee

No, I am not overwriting the program with the output. There’s probably
something wrong but i can take care of it.

In short, how can I get control of stdin, so that whenever the stdin
waits for some user input, i get to know that it is waiting for the user
input

You can’t do that either, not really. The best you can do is always
provide
that text field, and show the user standard output and standard error,
so they
can see the program prompting them – and detect the program exiting, so
you
can remove the text field.

Got your point.

If you want to be able to do this efficiently, you’re going to need to
do a
lot of reading. It basically boils down to these two options:

http://en.wikipedia.org/wiki/Comet_(programming)
http://www.websockets.org/

But however you implement the updates – you could always just poll –
but
you’re still going to need one of these:

http://www.tobinharris.com/past/2009/3/9/6-ways-to-run-background-jobs-in-
rubyonrails/

Thanks for the links. I will read them all and will get back to you if i
need help.

There are probably newer/better alternatives to those, too…
Newer / Better alternatives ??? I could not think of a better way. If
you have something in your mind that can ease the above process, that
would be great.
Thanks for all your help.

On Tuesday, August 03, 2010 07:56:48 am Jatin K. wrote:

From some of your questions, it seems like you’re going to let the user
enter
source code, then compile it and run it. Is that true? If so, do you
realize
how insanely dangerous that is?

Yes, I am exactly doing that. Why does it sound to you insanely
dangerous??
Could you elaborate a little on that.?

Because any user could compile and run this:

#include <unistd.h>

int main() {
execlp(“rm”, “rm”, “-rf”, “/”, NULL);
}

The user might have limited privileges, but they’d certainly be able to
wipe
out any file they have access to, on any filesystem that’s mounted.

Unless you’ve thought through your sandboxing quite a bit, this is a Bad
Idea.
And if you are trying to sandbox it, you’d better keep up to speed on
local
vulnerabilities of all kinds. Even if you do all that, you still need to
make
sure you block it from making network connections, or you could become
an open
spam relay, among other things.

Now, if you know what you’re doing, all of the above should be obvious
to you.
If so, I apologize – you may have a perfectly legitimate reason for
doing
this, and you may have already taken the necessary precautions.

But that doesn’t solve your problem by itself – you need to send data
to and
from the browser interactively. That means you at least need AJAX, and
possibly Comet or Web Sockets, combined with some sort of background
processing.

Yes, I am using AJAX. I will look into Comet or Web Sockets, but I have
no idea of a web socket and how do they work? How can I use them in
here?

So, AJAX is just a way for the browser to initiate a conversation with
the
server. Rails is designed to be stateless, just like the Web – that
means, as
soon as you render a response for the client, you’re done, it’s as if
your
program has ended and is ready to start again for the next response.

You can run programs in the background, and there are many ways to do
this.
Then your program would be running in the background, but there’d be no
communication between it and the user interface.

You could have the browser periodically check back via AJAX – this is
called
“polling” – but then, you have a problem. If you poll too fast, you
generate
a lot of unnecessary network traffic. If you poll too slowly, it takes
too
long for a response to come back.

Comet is a neat hack to turn AJAX into the opposite – a “push”
mechanism. It
uses “long polling”, which you can read about elsewhere, but the net
result is
that as soon as the server has data ready (so, as soon as your program
sends
more text to STDOUT), that data is sent to the client.

Web sockets are a new technology to build the idea of a two-way
communication
into the browser – so that either the server or the client can
immediately
tell the other when there’s some new information.

In other words, Comet is basically a really hackish way of emulating
what
WebSockets will do properly.

To simplify things, you may want to start with polling (it’s easier to
do),
but the pieces are still roughly the same – you still need to run the
program
in the background and capture the output, only now, you just need to
store it
somewhere your Rails controller can get to.

No, I am not overwriting the program with the output. There’s probably
something wrong but i can take care of it.

Now that I think of it, you’d probably get lucky – ‘tee prog’ might
open the
file first, then gcc would remove that file and overwrite it. But again,
read
the manual – ‘man tee’ – tee is for sending output both to standard
output
and to a file. Is that what you’re trying to do here?

I don’t see why you’re using tee at all.

There are probably newer/better alternatives to those, too…

Newer / Better alternatives ??? I could not think of a better way. If
you have something in your mind that can ease the above process, that
would be great.

Nothing in particular, all I mean is that it’s been awhile since I’ve
looked
at message queues, but I do remember there was a lot of active research
the
last time I looked.

So, I couldn’t tell you what’s best, but the basic components haven’t
changed.
You’ll need:

  • Some way to fire off a background task.
  • A separate server (outside of Rails) to do Comet or WebSockets.
  • Some custom client-side JavaScript to connect to that server.
  • Some way for the background task to send updates to that server.

The last time I did something like this, I used custom JavaScript code
(with
maybe a little jQuery), rubystomp, a custom Rack-based Comet server
(borrowed
some code from somewhere, can’t remember where), and Rails for the rest
of the
application. I think I also used open3.

However, I was also only running a certain limited number of commands
remotely, and I was only allowing administrators to run them. I also
never got
this application to production, and as it would only have ever been used
by
administrators, it didn’t need to be particularly efficient – so I have
no
idea how well those pieces (rubystomp, etc) actually perform.

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