feat: Preprocessor can return Promise (#3376)

preprocessor plugins can execute 'done()' or return a Promise

lib/preprocessor.js

@@ -8,29 +8,39 @@ const CryptoUtils = require('./utils/crypto-utils')
 
 const log = require('./logger').create('preprocess')
 
-function createNextProcessor (preprocessors, file, done) {
-  return function nextPreprocessor (error, content) {
-    // normalize B-C
-    if (arguments.length === 1 && typeof error === 'string') {
-      content = error
-      error = null
-    }
-
-    if (error) {
-      file.content = null
-      file.contentPath = null
-      return done(error)
+function executeProcessor (process, file, content) {
+  let done = null
+  const donePromise = new Promise((resolve, reject) => {
+    done = function (error, content) {
+      // normalize B-C
+      if (arguments.length === 1 && typeof error === 'string') {
+        content = error
+        error = null
+      }
+      if (error) {
+        reject(error)
+      } else {
+        resolve(content)
+      }
     }
+  })
+  return process(content, file, done) || donePromise
+}
 
-    if (!preprocessors.length) {
-      file.contentPath = null
-      file.content = content
-      file.sha = CryptoUtils.sha1(content)
-      return done()
+async function runProcessors (preprocessors, file, content) {
+  try {
+    for (let process of preprocessors) {
+      content = await executeProcessor(process, file, content)
     }
-
-    preprocessors.shift()(content, file, nextPreprocessor)
+  } catch (error) {
+    file.contentPath = null
+    file.content = null
+    throw error
   }
+
+  file.contentPath = null
+  file.content = content
+  file.sha = CryptoUtils.sha1(content)
 }
 
 function createPriorityPreprocessor (config, preprocessorPriority, basePath, injector) {
@@ -103,7 +113,6 @@ function createPriorityPreprocessor (config, preprocessorPriority, basePath, inj
           .map((duo) => duo[0])
 
         let preprocessors = []
-        const nextPreprocessor = createNextProcessor(preprocessors, file, done)
         sortedPreprocessorNames.forEach((name) => {
           const p = instances[name] || instantiatePreprocessor(name)
 
@@ -124,7 +133,7 @@ function createPriorityPreprocessor (config, preprocessorPriority, basePath, inj
           }
         })
 
-        nextPreprocessor(null, isBinary ? buffer : buffer.toString())
+        runProcessors(preprocessors, file, isBinary ? buffer : buffer.toString()).then(done, done)
       })
     }
     return fs.readFile(file.originalPath, readFileCallback)