Make sure Transaction sorts operations correctly to begin with

src/Composer/DependencyResolver/Transaction.php

@@ -248,6 +248,9 @@ class Transaction
      */
     private function movePluginsToFront(array $operations)
     {
+        $dlModyingPluginsNoDeps = array();
+        $dlModyingPluginsWithDeps = array();
+        $dlModyingPluginRequires = array();
         $pluginsNoDeps = array();
         $pluginsWithDeps = array();
         $pluginRequires = array();
@@ -261,6 +264,30 @@ class Transaction
                 continue;
             }
 
+            $isDownloadsModifyingPlugin = $package->getType() === 'composer-plugin' && ($extra = $package->getExtra()) && isset($extra['plugin-modifies-downloads']) && $extra['plugin-modifies-downloads'] === true;
+
+            // is this a downloads modifying plugin or a dependency of one?
+            if ($isDownloadsModifyingPlugin || count(array_intersect($package->getNames(), $dlModyingPluginRequires))) {
+                // get the package's requires, but filter out any platform requirements
+                $requires = array_filter(array_keys($package->getRequires()), function ($req) {
+                    return !PlatformRepository::isPlatformPackage($req);
+                });
+
+                // is this a plugin with no meaningful dependencies?
+                if ($isPlugin && !count($requires)) {
+                    // plugins with no dependencies go to the very front
+                    array_unshift($dlModyingPluginsNoDeps, $op);
+                } else {
+                    // capture the requirements for this package so those packages will be moved up as well
+                    $dlModyingPluginRequires = array_merge($dlModyingPluginRequires, $requires);
+                    // move the operation to the front
+                    array_unshift($dlModyingPluginsWithDeps, $op);
+                }
+
+                unset($operations[$idx]);
+                continue;
+            }
+
             // is this package a plugin?
             $isPlugin = $package->getType() === 'composer-plugin' || $package->getType() === 'composer-installer';
 
@@ -286,7 +313,7 @@ class Transaction
             }
         }
 
-        return array_merge($pluginsNoDeps, $pluginsWithDeps, $operations);
+        return array_merge($dlModyingPluginsNoDeps, $dlModyingPluginsWithDeps, $pluginsNoDeps, $pluginsWithDeps, $operations);
     }
 
     /**

src/Composer/Installer/InstallationManager.php

@@ -240,7 +240,7 @@ class InstallationManager
             foreach ($operations as $index => $operation) {
                 if (in_array($operation->getOperationType(), array('update', 'install'), true)) {
                     $package = $operation->getOperationType() === 'update' ? $operation->getTargetPackage() : $operation->getPackage();
-                    if ($package->getType() === 'composer-plugin' && $extra = $package->getExtra() && isset($extra['plugin-modifies-downloads']) && $extra['plugin-modifies-downloads'] === true) {
+                    if ($package->getType() === 'composer-plugin' && ($extra = $package->getExtra()) && isset($extra['plugin-modifies-downloads']) && $extra['plugin-modifies-downloads'] === true) {
                         if ($batch) {
                             $batches[] = $batch;
                         }

tests/Composer/Test/DependencyResolver/TransactionTest.php

@@ -42,8 +42,28 @@ class TransactionTest extends TestCase
             $packageG = $this->getPackage('g/g', '1.0.0'),
             $packageA0first = $this->getPackage('a0/first', '1.2.3'),
             $packageFalias2 = $this->getAliasPackage($packageF, 'dev-bar'),
+            $plugin = $this->getPackage('x/plugin', '1.0.0'),
+            $plugin2Dep = $this->getPackage('x/plugin2-dep', '1.0.0'),
+            $plugin2 = $this->getPackage('x/plugin2', '1.0.0'),
+            $dlModifyingPlugin = $this->getPackage('x/downloads-modifying', '1.0.0'),
+            $dlModifyingPlugin2Dep = $this->getPackage('x/downloads-modifying2-dep', '1.0.0'),
+            $dlModifyingPlugin2 = $this->getPackage('x/downloads-modifying2', '1.0.0'),
         );
 
+        $plugin->setType('composer-installer');
+        foreach (array($plugin2, $dlModifyingPlugin, $dlModifyingPlugin2) as $pluginPackage) {
+            $pluginPackage->setType('composer-plugin');
+        }
+
+        $plugin2->setRequires(array(
+            'x/plugin2-dep' => new Link('x/plugin2', 'x/plugin2-dep', $this->getVersionConstraint('=', '1.0.0'), Link::TYPE_REQUIRE),
+        ));
+        $dlModifyingPlugin2->setRequires(array(
+            'x/downloads-modifying2-dep' => new Link('x/downloads-modifying2', 'x/downloads-modifying2-dep', $this->getVersionConstraint('=', '1.0.0'), Link::TYPE_REQUIRE),
+        ));
+        $dlModifyingPlugin->setExtra(array('plugin-modifies-downloads' => true));
+        $dlModifyingPlugin2->setExtra(array('plugin-modifies-downloads' => true));
+
         $packageD->setRequires(array(
             'f/f' => new Link('d/d', 'f/f', $this->getVersionConstraint('>', '0.2'), Link::TYPE_REQUIRE),
             'g/provider' => new Link('d/d', 'g/provider', $this->getVersionConstraint('>', '0.2'), Link::TYPE_REQUIRE),
@@ -54,6 +74,12 @@ class TransactionTest extends TestCase
             array('job' => 'uninstall', 'package' => $packageC),
             array('job' => 'uninstall', 'package' => $packageE),
             array('job' => 'markAliasUninstalled', 'package' => $packageEalias),
+            array('job' => 'install', 'package' => $dlModifyingPlugin),
+            array('job' => 'install', 'package' => $dlModifyingPlugin2Dep),
+            array('job' => 'install', 'package' => $dlModifyingPlugin2),
+            array('job' => 'install', 'package' => $plugin),
+            array('job' => 'install', 'package' => $plugin2Dep),
+            array('job' => 'install', 'package' => $plugin2),
             array('job' => 'install', 'package' => $packageA0first),
             array('job' => 'update', 'from' => $packageB, 'to' => $packageBnew),
             array('job' => 'install', 'package' => $packageG),