Add sync helper to give plugins utilities to work with async stuff more easily when one does not care about async

src/Composer/Factory.php

@@ -591,6 +591,8 @@ class Factory
     }
 
     /**
+     * If you are calling this in a plugin, you probably should instead use $composer->getLoop()->getHttpDownloader()
+     *
      * @param  IOInterface      $io      IO instance
      * @param  Config           $config  Config instance
      * @param  array            $options Array of options passed directly to HttpDownloader constructor

src/Composer/Util/SyncHelper.php

@@ -0,0 +1,54 @@
+<?php
+
+namespace Composer\Util;
+
+use Composer\Downloader\DownloaderInterface;
+use Composer\Package\PackageInterface;
+use React\Promise\PromiseInterface;
+
+class SyncHelper
+{
+    /**
+     * Helps you download + install a single package in a synchronous way
+     *
+     * This executes all the required steps and waits for promises to complete
+     *
+     * @param Loop                  $loop        Loop instance which you can get from $composer->getLoop()
+     * @param string                $path        the installation path
+     * @param PackageInterface|null $prevPackage the previous package if this is an update and not an initial installation
+     */
+    public static function downloadAndInstallPackageSync(Loop $loop, DownloaderInterface $downloader, $path, PackageInterface $package, PackageInterface $prevPackage = null)
+    {
+        $type = $prevPackage ? 'update' : 'install';
+
+        try {
+            self::await($loop, $downloader->download($package, $path, $prevPackage));
+
+            self::await($loop, $downloader->prepare($type, $package, $path, $prevPackage));
+
+            if ($type === 'update') {
+                self::await($loop, $downloader->update($package, $path, $prevPackage));
+            } else {
+                self::await($loop, $downloader->install($package, $path, $prevPackage));
+            }
+        } catch (\Exception $e) {
+            self::await($loop, $downloader->cleanup($type, $package, $path, $prevPackage));
+            throw $e;
+        }
+
+        self::await($loop, $downloader->cleanup($type, $package, $path, $prevPackage));
+    }
+
+    /**
+     * Waits for a promise to resolve
+     *
+     * @param Loop                  $loop    Loop instance which you can get from $composer->getLoop()
+     * @param PromiseInterface|null $promise
+     */
+    public static function await(Loop $loop, PromiseInterface $promise = null)
+    {
+        if ($promise) {
+            $loop->wait(array($promise));
+        }
+    }
+}