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Hijack: Get A Live IRB Prompt For Any Existing Ruby Process

By Ari Brown / August 21, 2009

jackSometimes taking an app down for debugging purposes is just not an option. Luckily a new tool called Hijack can provide a live IRB prompt for an existing Ruby process in the same way that Erlang provides hot swapping of code (changing the definition of a system while the system is still up and running).

Hijack (it's still in a beta state, so be careful and don't use it in production yet!) lets you to pry your way into a running Ruby process, where it drops you into a live IRB session running over DRB. Gone are the days of stopping live applications just to make a minor update!

Using the GDB (GNU DeBugger), Hijack connects to the running Ruby process, injecting a small payload to start a DRB session which provides an IRB session:

$ ruby hijack 16451
        => Hijacking...
        => Mirroring: 100%
        => Hijacked 16451 (my_script.rb) (ruby 1.8.7 [i686-darwin9])

You may already be familiar with live-console which provides a similar functionality. The key difference, however, is that Hijack can "inject" itself into an existing Ruby process without needing the code to have included it explicitly. Developer Ian Leitch explains:

I've just changed Hijack so that you can hijack any Ruby process - no need for your target process to require any code before it can be hijacked. It does this by first injecting a payload using gdb, then it signals the process to start up a DRb server which the hijack client then connects to.

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  1. Chris says:

    I don't understand the Erlang analogy, but there's a similar thing available for Cocoa by way of NuAnywhere:

  2. Bradly Feeley says:

    this is some slick willy!

  3. Sho says:

    Looks cool, but might help if the lib directory was included in the gem .. ; )

  4. Ian Leitch says:

    Sorry about that, gem should be fixed now :)

  5. Senthil says:

    That would be real cool, will check it out.

    can somebody post some tricks which they have tried with it

  6. Guoliang Cao says:

    I was trying to hijack our rails process and it got stuck after 'Hijacking...'

    Any idea?

    OSX 10.5.7
    ruby 1.8.7 (2009-06-12 patchlevel 174) [i686-darwin9]
    Rails 2.3.2

  7. Ian Leitch says:

    Was the process started with 'script/server'? If so it's a known bug. Use mongrel_rails start or thin start instead.

  8. awwaiid says:

    Fancy! We got that in perl doing similar things,

  9. Josh Jore says:

    I wrote a similar thing for Perl awhile ago (and recently blogged The thing I never finished off was dealing with the fact the gdb has attached at an arbitrary instruction and not at a safe time. I spent a short bit of time trying to inform perl that a "signal" had arrived and to wait for the sighandler to run and use that as a break point. That'd get me back to safe signal handling and therefore a perfectly safe time to go launch the debugger.

    I'd set the global PL_sig_pending to true, then break on Perl_despatch_signals and try to convince Perl it was ok that there was no actual signal to handle. Didn't quite work.

    I suspect you've got the same problem here in Ruby. You should probably "set variable rb_trap_pending = 1" and break on rb_trap_exec or something like that.

  10. Guoliang Cao says:

    Thanks for looking.

    Yes, it was started with script/server.

    Now I can get it hijacked after running as 'mongrel_rails start'

  11. j_king says:

    Yet another Lisp feature re-implemented in a young programming language.

    They grow up so fast! ;)

    Really cool stuff though. The next step is to provide a SWANK-like server for running processes. Then you can connect irb to remote processes! Yet another thing Lisp already has.

    Great work though. Really pushing the boundaries.

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