fix: if preprocessor is async function and doesn't return a content then await donePromise (#3387)

Author: anthony-redFox

lib/preprocessor.js

@@ -24,7 +24,15 @@ function executeProcessor (process, file, content) {
 }
 }
 })
-returnprocess(content,file,done)||donePromise
+
+return(process(content,file,done)||Promise.resolve()).then((content)=>{
+if(content){
+// async process correctly returned content
+returncontent
+}
+// process called done() (Either old sync api or an async function that did not return content)
+returndonePromise
+})
 }
 
 asyncfunctionrunProcessors(preprocessors,file,content){

test/unit/preprocessor.spec.js

@@ -80,6 +80,48 @@ describe('preprocessor', () => {
 })
 })
 
+it('should get content if preprocessor is an async function or return Promise with content',(done)=>{
+constfakePreprocessor=sinon.spy(async(content,file,done)=>{
+file.path=file.path+'-preprocessed'
+return'new-content'
+})
+
+constinjector=newdi.Injector([{
+'preprocessor:fake': ['factory',function(){returnfakePreprocessor}]
+},emitterSetting])
+pp=m.createPriorityPreprocessor({'**/*.js': ['fake']},{},null,injector)
+
+constfile={originalPath: '/some/.dir/a.js',path: 'path'}
+
+pp(file,()=>{
+expect(fakePreprocessor).to.have.been.called
+expect(file.path).to.equal('path-preprocessed')
+expect(file.content).to.equal('new-content')
+done()
+})
+})
+
+it('should get content if preprocessor is an async function still calling done()',(done)=>{
+constfakePreprocessor=sinon.spy(async(content,file,done)=>{
+file.path=file.path+'-preprocessed'
+done(null,'new-content')
+})
+
+constinjector=newdi.Injector([{
+'preprocessor:fake': ['factory',function(){returnfakePreprocessor}]
+},emitterSetting])
+pp=m.createPriorityPreprocessor({'**/*.js': ['fake']},{},null,injector)
+
+constfile={originalPath: '/some/.dir/a.js',path: 'path'}
+
+pp(file,()=>{
+expect(fakePreprocessor).to.have.been.called
+expect(file.path).to.equal('path-preprocessed')
+expect(file.content).to.equal('new-content')
+done()
+})
+})
+
 it('should check patterns after creation when invoked',(done)=>{
 constfakePreprocessor=sinon.spy((content,file,done)=>{
 file.path=file.path+'-preprocessed'