Switch to PostCSS Async API

[josh]

According to the latest PostCSS plugin guidelines, plugins should "use asynchronous methods when it is possible". Theres a number of places postcss-url makes blocking FS calls.

• postcss-url/index.js

  Line 239 in 130b43c

  if (!fs.existsSync(file)) {

• postcss-url/index.js

  Line 244 in 130b43c

  var stats = fs.statSync(file)

• postcss-url/index.js

  Line 263 in 130b43c

  file = fs.readFileSync(file)

• postcss-url/index.js

  Line 306 in 130b43c

  var contents = fs.readFileSync(filePath)

• postcss-url/index.js

  Line 318 in 130b43c

  mkdirp.sync(absoluteAssetsPath)

• postcss-url/index.js

  Line 339 in 130b43c

  mkdirp.sync(absoluteAssetsPath)

In order to clean this up, the postcss plugin API will need to return a Promise which will mark it as async forcing consumers to use process(css).then(cb). This would probably be considered a major backwards incompatible change.

But I think it would be worth while to align to the plugin best practices as this plugin is listed under the official @postcss org.

There could be other benefits besides this cleanup. For one, I think it would be great to support async logic in the custom url function.

"postcss-url": {
  url: function(url) {
    return asyncCallToComputeAssetMD5(url).then(function(md5) {
      return url + "?" + md5
    })
  }
}

I'd be happy to help work on this. But I just wanted to get an idea if this would be a welcome direction.

Thanks!

CC: @MoOx @ai

[simplesmiler]

+1 for this one.

[MoOx]

I think it's time to explode this plugin that is becoming too big...

[MoOx]

Feel free to let me know if you publish something so I can deprecated things over here.

[tinchoz49]

Hi @MoOx about what you are saying that the plugin is becoming too big...
I made postcss-copy if the community think is ok, we can remove the copy method from postcss-url and reduce the size.

[MoOx]

I prefer to deprecate the plugin directly when all features have been exported elsewhere.

[SleepWalker]

Hello, @MoOx

if I need Promises support for an options.url function may I create a pull request or should I create a separate plugin?

[MoOx]

Depends of the size of the change

[SleepWalker]

The actual change won't be big, but I will need to rewrite tests to use async api. Is it ok?

[SleepWalker]

@MoOx is it a big change 123b311?
The next commit will add promise support for DeclProcessor. It should be not so big as 123b311. And, probably, the whole code will be in fixUrl function.