Currently, if a previous source map's "sources" array contains relative URLs, those URLs are interpreted as relative to the current working directory rather than the location of the source map. This produces incorrect original file paths, which is particularly problematic in error messages. In conjunction with #1347, this makes it essentially impossible to correctly handle source map paths for CSS files outside the current working directory.

// test.js
const postcss = require('postcss');
const fs = require('fs');

const root = postcss.parse(
  fs.readFileSync('subdir/test.css', 'utf8'), {from: 'subdir/test.css'});
console.log(root.source.input.origin(1, 1));

$ echo 'a {b: c}' > test.scss
$ mkdir subdir
$ sass test.scss subdir/test.css
$ pwd
/home/nweiz/goog/postcss
$ node test.js
{ file: '/home/nweiz/goog/test.scss', line: 1, column: 0 }

Note that the file path is /home/nweiz/goog/test.scss, while it should be /home/nweiz/goog/postcss/test.scss.