your-diary / netrw_avoid_discarding_modified_buffer

About

About

This project supplies a patch for a bug of netrw. The bug is explained in Problem section.

Current Status

The patch was partially adopted in netrw 170e and fully adopted in netrw 170f on April 30, 2020. Now the documentation pi_netrw.txt says

v170:   Mar 11, 2020   ...
        Apr 13, 2020   ...
        Apr 30, 2020   * (reported by Manatsu Takahashi) while
                         using Lexplore, a modified file could
                         be overwritten.  Sol'n: will not overwrite,
                         but will emit an |E37| (although one cannot
                         add an ! to override)

However, I suspect it will take long for the version of netrw to be included in the upstream of vim. Till the day, if you'd like to remove the bug, manually download the plugin from the official and install it.

For Users

Problem

Say you have a window W associated with a modified buffer A. When you try to load a file B using netrw into the window W, the buffer A is discarded without any prompt, which is extremely disastrous. This bug was first discussed in this StackExchange question.

How to Reproduce

1. Execute vim A command from shell.

2. Edit the buffer associated with A in some way. Now &modified == 1.

3. Execute :Lexplore^†1 command to enable the side pane of netrw.

4. In the side pane, press Enter on an arbitrary file B.

_{†1: The problem is not specific to :Lexplore command. Netrw opened via :Explore or :e also shows the behavior.}

Cause

The cause is that :e! is called instead of :e inside s:NetrwBrowseChgDir(), which is defined in autoload/netrw.vim. Note, contrary to the name of the function, it is used not only to change the directory but also to edit a file, according to the comment in the file.

If you are a developer, see Algorithm for the detailed explanation.

Fix

To fix the bug, just apply the patch incorporated in this project by, for example, the command below.

$ patch netrw.vim patch

Environments

The problem was reproduced under Arch Linux, Arch Linux ARM, Raspbian Buster and FreeBSD. Even in the latest vim 8.2 and netrw 170c Mar 30, 2020, we can observe it.

The patch is tested under Arch Linux with the following commands though recompiling vim is essentially not needed.

$ git clone "https://github.com/vim/vim"
$ cd vim
$ ./configure --prefix=/usr/local --with-features=huge --enable-python3interp=dynamic\
  && make\
  && patch runtime/autoload/netrw.vim <path to patch>\
  && sudo make install

Known Bugs

The patch is written for us and not tested very well.

For Developers

Algorithm

The patch follows the algorithm below.

1. Trying to load B calls three functions: s:NetrwGetWord(), s:NetrwBrowseChgDir() and netrw#LocalBrowseCheck(). The patch keeps the first and the third functions untouched since they do nothing which will be broken if the second function is modified.

2. Inside s:NetrwBrowseChgDir(), the patch replaces inappropriate e! with :e.

3. The replacement causes a side effect. If the flag dolockout is set, :setl ma noro nomod is called before exiting the function. And nomod is evil; setting nomod[ified] in a modified buffer is to say "I'm modified but please treat me as an unmodified buffer." As a result, the modified A is discarded if you try to open B twice. The first trial fails since the buffer is modified but the second trial succeeds since the buffer disguises itself to be unmodified. Thus, the patch additionally unsets the flag.

How to Debug Netrw

The way of debugging netrw is described in :help netrw but it is far from complete. Here is the more detailed instructions.

1. Install the latest Netrw and Decho, where the latter is a debugging plugin. To install them, download .vba.gz files from the links and follow either the following ways.

   • Open each file in vim, and execute :source % command or :UseVimball ~/.vim/ command. By this, the plugins are installed under ~/.vim/. To uninstall them, just delete appropriate files under the directory. In this case, :RmVimball command doesn't work unless you manually remove the part .gz from the contents of ~/.vim/.VimballRecord file.

   • (Recommended) Execute gunzip --keep <vba.gz file> command to get *.vba files, open each in vim, and execute :source % command or :UseVimball ~/.vim/ command. By this, the plugins are installed under ~/.vim/. To uninstall them, execute :RmVimball <plugin> ~/.vim/ command, where <plugin> is the name of the file used to install the plugin minus the extension .vba (e.g. netrw_170e for netrw_170e.vba).

2. Create a minimal configuration file minimal_vimrc whose contents are shown below. This file is included in this project.

set nocp
so $HOME/.vim/plugin/Decho.vim
so $HOME/.vim/plugin/netrwPlugin.vim

3. Open ~/.vim/plugin/netrwPlugin.vim and ~/.vim/autoload/netrw.vim in vim and execute :DechoOn, then save and exit. This turns on debugging outputs.

4. Execute the following command to start debugging. We also provide the script start_debugging.sh, where ./start_debugging.sh [<file>] is essentially the same as the command below.

$ vim -u minimal_vimrc --noplugins -i NONE [<file>]