我正在从gulp@3.9.1迁移到gulp@4.0.2，并在此过程中升级我的gulp依赖项。我的gulpfile中有以下任务，您可以假设directories只是我要执行此操作的目录数组：

var gulp = require('gulp');
var ngAnnotate = require('gulp-ng-annotate'); //annotates dependencies in Angular components
var rev = require('gulp-rev'); //appends a hash to the end of file names to eliminate stale cached files
var revReplace = require('gulp-rev-replace');
var uglify = require('gulp-uglify'); // minimizes javascript files
var compressCss = require('gulp-minify-css');
var useref = require('gulp-useref'); // replaces style and script blocks in HTML files
var filter = require('gulp-filter');
var merge = require('merge-stream');
var sourcemaps = require('gulp-sourcemaps');

function minify() {
    var tasks = directories.map(function (directory) {
        var cssFilter = filter("**/all.min.css", {restore:true});
        var jsAppFilter = filter("**/app.min.js", {restore:true});
        var jsFilter = filter("**/*.js", {restore:true});

        return gulp.src(dstBasePath + directory + "index.html", {allowEmpty: true})
            .pipe(useref())
            .pipe(cssFilter)
            .pipe(compressCss({keepSpecialComments:false}))
            .pipe(rev())
            .pipe(cssFilter.restore)
            .pipe(jsAppFilter)
            .pipe(sourcemaps.init())
            .pipe(ngAnnotate({add:true, single_quotes:true}))
            .pipe(jsAppFilter.restore)
            .pipe(jsFilter)
            .pipe(uglify())
            .pipe(rev())
            .pipe(jsFilter.restore)
            .pipe(revReplace())
            .pipe(sourcemaps.write('.'))    // sourcemaps need to be written to same folder for Datadog upload to work
            .pipe(gulp.dest(dstBasePath + directory))
    });

    return merge(tasks);
}

为什么在运行任务时会导致Gulp错误“Did you forget to signal async completion？”请注意，我使用的是Gulp 4。我已经尝试将回调done传递给此任务，并将.addListener('end', done)添加到最后一个管道，但这会导致合并流过早结束（大概是在第一个插件结束的时候）。所以也许其中一个插件在完成的时候没有发出信号，但是你怎么才能让它工作呢？谢谢你提供的任何见解。