Output more warnings about plugins being disabled to hint that it may cause problems, fixes #11839 (#11842)

src/Composer/Console/Application.php

@@ -220,7 +220,7 @@ class Application extends BaseApplication
 
         // Clobber sudo credentials if COMPOSER_ALLOW_SUPERUSER is not set before loading plugins
         if ($needsSudoCheck) {
-            $isNonAllowedRoot = function_exists('posix_getuid') && posix_getuid() === 0;
+            $isNonAllowedRoot = $this->isRunningAsRoot();
 
             if ($isNonAllowedRoot) {
                 if ($uid = (int) Platform::getEnv('SUDO_UID')) {
@@ -476,6 +476,12 @@ class Application extends BaseApplication
             $io->writeError('<error>Check https://getcomposer.org/doc/06-config.md#process-timeout for details</error>', true, IOInterface::QUIET);
         }
 
+        if ($this->getDisablePluginsByDefault() && $this->isRunningAsRoot() && !$this->io->isInteractive()) {
+            $io->writeError('<error>Plugins have been disabled automatically as you are running as root, this may be the cause of the following exception. See also https://getcomposer.org/root</error>', true, IOInterface::QUIET);
+        } elseif ($exception instanceof CommandNotFoundException && $this->getDisablePluginsByDefault()) {
+            $io->writeError('<error>Plugins have been disabled, which may be why some commands are missing, unless you made a typo</error>', true, IOInterface::QUIET);
+        }
+
         $hints = HttpDownloader::getExceptionHints($exception);
         if (null !== $hints && count($hints) > 0) {
             foreach ($hints as $hint) {
@@ -678,4 +684,9 @@ class Application extends BaseApplication
 
         return $config->get('use-parent-dir');
     }
+
+    private function isRunningAsRoot(): bool
+    {
+        return function_exists('posix_getuid') && posix_getuid() === 0;
+    }
 }

src/Composer/Installer/InstallationManager.php

@@ -94,8 +94,8 @@ class InstallationManager
     /**
      * Disables plugins.
      *
-     * We prevent any plugins from being instantiated by simply
+     * We prevent any plugins from being instantiated by
-     * deactivating the installer for them. This ensure that no third-party
+     * disabling the PluginManager. This ensures that no third-party
      * code is ever executed.
      */
     public function disablePlugins(): void
@@ -105,7 +105,7 @@ class InstallationManager
                 continue;
             }
 
-            unset($this->installers[$i]);
+            $installer->disablePlugins();
         }
     }
 

src/Composer/Installer/PluginInstaller.php

@@ -43,6 +43,11 @@ class PluginInstaller extends LibraryInstaller
         return $packageType === 'composer-plugin' || $packageType === 'composer-installer';
     }
 
+    public function disablePlugins(): void
+    {
+        $this->getPluginManager()->disablePlugins();
+    }
+
     /**
      * @inheritDoc
      */

src/Composer/Plugin/PluginManager.php

@@ -153,6 +153,7 @@ class PluginManager
     public function registerPackage(PackageInterface $package, bool $failOnMissingClasses = false, bool $isGlobalPlugin = false): void
     {
         if ($this->arePluginsDisabled($isGlobalPlugin ? 'global' : 'local')) {
+            $this->io->writeError('<warning>The "'.$package->getName().'" plugin was not loaded as plugins are disabled.</warning>');
             return;
         }
 
@@ -656,6 +657,14 @@ class PluginManager
         return $this->disablePlugins === true || $this->disablePlugins === $type;
     }
 
+    /**
+     * @internal
+     */
+    public function disablePlugins(): void
+    {
+        $this->disablePlugins = true;
+    }
+
     /**
      * @internal
      */