Expose more system functionalities to Lua

[tarleb]

The system Lua module provides utility functions to interact with the
operating- and file system. Like the text module, the system module
must be loaded before use. E.g.

local system = require 'system'
print(system.currentdir())

[tarleb]

This adds about 40kB to the executable.

[jgm]

What happens if you cd to change directory in a lua filter?
Does this run in a subprocess, affecting only the working directory for the subprocess?
Or does this mean that when pandoc exits, you'll be in a different directory (which would be bad to allow, I think).

[tarleb]

Indeed. The directory doesn't change after pandoc exits, but with chdir.lua containing (require 'system').chdir '/tmp' and filter.sh being any filter in the current directory we get

$ echo test | pandoc --lua-filter=chdir.lua --filter=filter.sh
Error running filter filter.sh:
Could not find executable filter.sh

I'll have to add withCurrentDirectory somewhere in the Lua filter code.

[tarleb]

The current working directory and environment will now be restored after each Lua invocation.

[tarleb]

A bit more context for this PR.

• The main motivation was to get access to Haskell's functions for temporary files and directories. These would be of great help with tempfile-heavy filters like diagram-generator.
• Environment modifying functions where added to allow configuration of external programs. Filter authors may want to set or override environment variables before calling out to another executable. Example: temporarily overriding TEXMF* variables before calling pdflatex.
• The rest is merely for completeness.

Other features that I think could be helpful, but aren't implemented (yet):

• Platform-dependent creation and canonicalization of file paths;
• turning relative paths into absolute paths and vice versa.

[jgm]

I am still a bit worried about these environment-polluting things; would it make more sense to add functions like with_environment(env, function)and with_working_directory(dir, function) instead of setenv and cd?

[tarleb]

That makes sense and seems like the cleaner solution.

My approach will then be to remove the system module and replace it with a pandoc.sys module which contains just the three functions with_environment, with_working_directory, and with_temp_directory. That should strike a good balance between useful features and unnecessary bloat.

[jgm]

I think it should be harmless to keep the "getter" functions exposed: e.g. getenv and get_current_directory.

[tarleb]

Should be ok now. I left out getenv, as Lua already comes with os.getenv().

[jgm]

Should be ok now. I left out getenv, as Lua already comes with os.getenv().

Confused about this, since there is still environment -- is that different from os.getenv?

[jkr]

Confused about this, since there is still environment -- is that different from os.getenv?

os.getenv takes an argument (os.getenv("HOME")) , while according to the docs, environment returns a table of all env vars.