Avoid duplicate loading of configuration files

Author: ajardin

app/code/Magento/Theme/Model/PageLayout/Config/Builder.php

@@ -27,6 +27,11 @@ class Builder implements \Magento\Framework\View\Model\PageLayout\Config\Builder
      */
     protected $themeCollection;
 
+    /**
+     * @var array
+     */
+    private $configFiles = [];
+
     /**
      * @param \Magento\Framework\View\PageLayout\ConfigFactory $configFactory
      * @param \Magento\Framework\View\PageLayout\File\Collector\Aggregated $fileCollector
@@ -56,11 +61,14 @@ public function getPageLayoutsConfig()
      */
     protected function getConfigFiles()
     {
-        $configFiles = [];
-        foreach ($this->themeCollection->loadRegisteredThemes() as $theme) {
-            $configFiles = array_merge($configFiles, $this->fileCollector->getFilesContent($theme, 'layouts.xml'));
+        if (empty($this->configFiles)) {
+            $configFiles = [];
+            foreach ($this->themeCollection->loadRegisteredThemes() as $theme) {
+                $configFiles[] = $this->fileCollector->getFilesContent($theme, 'layouts.xml');
+            }
+            $this->configFiles = array_merge(...$configFiles);
         }
 
-        return $configFiles;
+        return $this->configFiles;
     }
 }

app/code/Magento/Theme/Test/Unit/Model/PageLayout/Config/BuilderTest.php

@@ -83,7 +83,7 @@ public function testGetPageLayoutsConfig()
             ->disableOriginalConstructor()
             ->getMock();
 
-        $this->themeCollection->expects($this->any())
+        $this->themeCollection->expects($this->once())
             ->method('loadRegisteredThemes')
             ->willReturn([$theme1, $theme2]);