[Bug] Fix Flex not installing recipes while it is being upgraded

src/Flex.php

@@ -60,6 +60,8 @@
  */
 class Flex implements PluginInterface, EventSubscriberInterface
 {
+    public static $storedOperations = [];
+
     /**
      * @var Composer
      */
@@ -126,6 +128,13 @@ class_exists(__NAMESPACE__.str_replace('/', '\\', substr($file, \strlen(__DIR__)
         $this->config = $composer->getConfig();
         $this->options = $this->initOptions();
 
+        // if Flex is being upgraded, the original operations from the original Flex
+        // instance are stored in the static property, so we can reuse them now.
+        if (property_exists(self::class, 'storedOperations') && self::$storedOperations) {
+            $this->operations = self::$storedOperations;
+            self::$storedOperations = [];
+        }
+
         $symfonyRequire = preg_replace('/\.x$/', '.x-dev', getenv('SYMFONY_REQUIRE') ?: ($composer->getPackage()->getExtra()['symfony']['require'] ?? ''));
 
         if ($composer2 = version_compare('2.0.0', PluginInterface::PLUGIN_API_VERSION, '<=')) {
@@ -293,6 +302,8 @@ class_exists(__NAMESPACE__.str_replace('/', '\\', substr($file, \strlen(__DIR__)
 
     public function deactivate(Composer $composer, IOInterface $io)
     {
+        // store operations in case Flex is being upgraded
+        self::$storedOperations = $this->operations;
         self::$activated = false;
     }
 
@@ -544,10 +555,12 @@ function ($value) {
             }
         }
 
-        foreach ($postInstallRecipes as $recipe) {
-            $this->configurator->postInstall($recipe, $this->lock, [
-                'force' => $event instanceof UpdateEvent && $event->force(),
-            ]);
+        if (method_exists($this->configurator, 'postInstall')) {
+            foreach ($postInstallRecipes as $recipe) {
+                $this->configurator->postInstall($recipe, $this->lock, [
+                    'force' => $event instanceof UpdateEvent && $event->force(),
+                ]);
+            }
         }
 
         if (null !== $manifest) {