rlundo

rlundo grants interactive interpreters magical undo powers!

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

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, 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.

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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.

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 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

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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

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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 http://www.gnu.org/licenses/.

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$ python rlundo ipython