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https://github.com/thomasballinger/rlundo

undo in any program that uses readline
https://github.com/thomasballinger/rlundo

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undo in any program that uses readline

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README 

        
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# rlundo



rlundo grants interactive interpreters magical undo powers!



![rlundo preview example](http://ballingt.com/assets/rlundopreview.gif)



For a long read about the motivation for such a tool, see [this blog

post](http://ballingt.com/interactive-interpreter-undo)



A patched version of readline is used to fork an interpreter

at each prompt. If the user enters `undo` then that child process dies

and execution is resume.

rlundo also removes the terminal output that occurred in the recently-deceased

child process, restoring the terminal to its previous state.



The goal is for this to work with any interpreter:



    $ python rlundo irb



The name rlundo is modeled off of

[rlwrap](https://github.com/hanslub42/rlwrap), which wraps interactive

command line interfaces with the readline editing interface. Like that

command, rlundo wraps other interactive interfaces.

To make the analogy work better it probably should have been called undowrap, or

rlundowrap to suggest that the way undo is implemented uses readline.



---



Using a patched version of the readline library only works for interactive

interpreters that dynamically link readline. To address this, this project

includes shims for various interpreters that implement undo via fork in a

less general way in /rlundo/interps/. Compiling the patched readline library

is not required for interpreters implemented this way. Add your favorite!



    $ python rlundo python



(python seems to usually statically link readline)



## Modified Readline library



rlundoable is a patched version of the gnu readline library with the following

modifications:



* calling readline causes the process to fork

* the user entering "undo" causes the process to die

* tcp socket connections are made when the process forks or dies to notify

  a listener that might be recording terminal state



To build this patched readline library:



    cd rlundoable

    make -f Makefileosx



Read more about the patched readline library in that [readme](rlundoable/readme.md).



The library substitution works more reliably for me on linux right now. Maybe

this is because homebrew more often links readline statically? That's just

speculation. Writing workarounds for common interpreters or digging into how

to make readline hijacking more reliable would both be really helpful!



## Rewriting terminal state



In order to restore prior terminal state on undo, interpreters are run

in a pseudo terminal that takes snapshots of terminal state when the

interpreter forks and restores previous terminal state when an interpreter

process dies.



try it with



    $ python rewrite.py



and then in another terminal run



    nc -U path/to/tmp/unix/socket/*save



to save terminal states, and



    nc -U path/tmp/tmp/unix/socket/*save; nc -U path/to/tmp/unix/socket/*restore;



to restore previous terminal states. Restore always goes back two state, so it

is necessary to call save before restore as shown above to restore the previous

save. Ordinarily these signals are sent by the modified interpreter (or the

patched readline it calls) after printing the prompt but before the user types

anything. Since you'll be sending the commands manually in the above demo, the

`>` prompt will not reappear after undo.



# Running the tests



* clone the repo, create a virtual environment

* pip install nose

* install [tmux](https://github.com/tmux/tmux) 1.9a or 2.1 or later. 1.8 is

  too early, 2.0 has a regression.

* `nosetests test` in the root directory

* or try `RLUNDO_USE_EXISTING_TMUX_SESSION=1 nosetests test` while you have a tmux

  session open to watch the tests which use tmux run



---



Thanks to



* John Hergenroeder for help with fixing race conditions with terminal

  rewriting

* John Connor for discussion and Python 3 fixes

* Agustín Benassi for ipython support, improved terminal rewriting, memory

  monitoring work and much more

* Joe Jean for work on Travis tests

* Madelyn Freed for work on the executable rlundo script



---



## License



Copyright 2015 Thomas Ballinger



Released under GPL3, like GNU readline.



This program is free software: you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation, either version 3 of the License, or

(at your option) any later version.



This program is distributed in the hope that it will be useful,

but WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

GNU General Public License for more details.



You should have received a copy of the GNU General Public License

along with this program.  If not, see .



---



    $ python rlundo ipython



![undo with ctrl+c](http://ballingt.com/assets/undoable_ipython.gif)