Incorrect source map when "to" option passed

[htanjo]

I'm using gulp-sass, gulp-postcss and gulp-sourcemaps in my project.
When I set to option in the postcss(), the source map content is incorrect.

gulp.src('app/styles/*.scss')
  .pipe($.sourcemaps.init({loadMaps: true}))
  .pipe($.sass().on('error', $.sass.logError))
  .pipe($.postcss(processors, {to: '.tmp/styles/main.css'}))
  .pipe($.sourcemaps.write('.'))
  .pipe(gulp.dest('.tmp/styles'))

Without to option, I could get proper source map, but some postcss plugins need this option.
Am I overlooking something, or is this bug?

I created simple gulp tasks to reproduce this issue. I hope this helps.
https://github.com/htanjo/postcss-sourcemaps-test

[TrySound]

@htanjo Which plugins need this option?

[htanjo]

In my project, postcss-copy-assets requires it.
And I tried postcss-copy previously, it also depends on to option.

[w0rm]

@htanjo sorry for the delay, I'm extremely busy this week, will be able to check it over the Weekend.

[htanjo]

@w0rm That's ok! Thanks.

[w0rm]

@htanjo I sent you a temporary fix that should work.

Sourcemaps are really complicated in gulp. The source map from the previous operation has to be combined with the sourcemap from the next operation. This only works when file from the previous map matches element from sources of the current map.

This is what was happening when the to='dist/main.css' option was set:

previousMap = {sources: [ 'main.scss' ], file: 'main.css'}
currentMap = {sources: [ '../src/main.css' ], file: 'main.css'}

Because main.css and ../src/main.css do not match, only the currentMap was output.

The additional complication comes from the base option of the gulp file, gulp source maps work relatively to the base directory, but postcss doesn't have this separation.

[w0rm]

It seems that I can only properly fix this issue after this is implemented in postcss: postcss/postcss#240

[htanjo]

@w0rm I see...! I understood the situation.
Your workaround is very helpful and I'll use it at the moment.
Thanks!

[alvint]

I get the same issue regardless of the to option. In fact, even if I run postcss with an empty processor list and no options I have this problem. The issue is also affecting plugins which rely on postcss, such as gulp-autoprefixer.

[w0rm]

@alvint hi, if it happens regardless of the to option then it's not related to this issue. Please open a separate issue and add a link to a minimal project that reproduces it. This way I will be able to help you.

Sometimes gulp-postcss is not even an issue. A previous gulp plugin may generate a wrong source map that cannot be merged, so I really need to see the code to figure out what went wrong!

[alexander-akait]

Some issue, I try to understand in detail what the problem is

[timkraut]

Any updates on that?

[w0rm]

@timkraut nope and postcss/postcss#240 wasn't implemented.

[w0rm]

Closing as wontfix because postcss/postcss#240 is closed.