Issue #2898274 by aleevas, piotrkonefal, borisson_, GaëlG, ankithashetty,...

Issue #2898274 by aleevas, piotrkonefal, borisson_, GaëlG, ankithashetty, dietr_ch, robpowell, alexpott: Uninstalled custom theme stuck in config blocking config import

core/lib/Drupal/Core/Extension/ThemeInstaller.php

@@ -266,7 +266,7 @@ public function uninstall(array $theme_list) {
     $theme_config = $this->configFactory->getEditable('system.theme');
     $list = $this->themeHandler->listInfo();
     foreach ($theme_list as $key) {
-      if (!isset($list[$key])) {
+      if ($extension_config->get("theme.$key") === NULL) {
         throw new UnknownExtensionException("Unknown theme: $key.");
       }
       if ($key === $theme_config->get('default')) {
@@ -277,7 +277,7 @@ public function uninstall(array $theme_list) {
       }
       // Base themes cannot be uninstalled if sub themes are installed, and if
       // they are not uninstalled at the same time.
-      if (!empty($list[$key]->sub_themes)) {
+      if (isset($list[$key]) && !empty($list[$key]->sub_themes)) {
         foreach ($list[$key]->sub_themes as $sub_key => $sub_label) {
           if (isset($list[$sub_key]) && !in_array($sub_key, $theme_list, TRUE)) {
             throw new \InvalidArgumentException("The base theme $key cannot be uninstalled, because theme $sub_key depends on it.");

core/lib/Drupal/Core/Extension/ThemeInstallerInterface.php

@@ -41,12 +41,13 @@ public function install(array $theme_list, $install_dependencies = TRUE);
    * Uninstalling a theme removes all related configuration (like blocks) and
    * invokes the 'themes_uninstalled' hook.
    *
+   * Themes are allowed to be uninstalled even when their code has been removed
+   * from the filesystem, this is because themes do not allow uninstall hooks to
+   * be defined.
+   *
    * @param array $theme_list
    *   The themes to uninstall.
    *
-   * @throws \Drupal\Core\Extension\Exception\UnknownExtensionException
-   *   Thrown when trying to uninstall a theme that was not installed.
-   *
    * @throws \InvalidArgumentException
    *   Thrown when trying to uninstall the admin theme, the default theme or
    *   a theme that another theme depends on.

core/tests/Drupal/KernelTests/Core/Theme/ThemeInstallerTest.php

@@ -355,17 +355,8 @@ public function testUninstallNonExisting() {
     $themes = $this->themeHandler()->listInfo();
     $this->assertEmpty(array_keys($themes));
 
-    try {
-      $message = 'ThemeInstaller::uninstall() throws UnknownExtensionException upon uninstalling a non-existing theme.';
+    $this->expectException(UnknownExtensionException::class);
     $this->themeInstaller()->uninstall([$name]);
-      $this->fail($message);
-    }
-    catch (\Exception $e) {
-      $this->assertInstanceOf(UnknownExtensionException::class, $e);
-    }
-
-    $themes = $this->themeHandler()->listInfo();
-    $this->assertEmpty(array_keys($themes));
   }
 
   /**
@@ -397,14 +388,10 @@ public function testUninstall() {
   public function testUninstallNotInstalled() {
     $name = 'test_basetheme';
 
-    try {
-      $message = 'ThemeInstaller::uninstall() throws UnknownExtensionException upon uninstalling a theme that is not installed.';
+    $themes = $this->themeHandler()->listInfo();
+    $this->assertEmpty(array_keys($themes));
+    $this->expectException(UnknownExtensionException::class);
     $this->themeInstaller()->uninstall([$name]);
-      $this->fail($message);
-    }
-    catch (\Exception $e) {
-      $this->assertInstanceOf(UnknownExtensionException::class, $e);
-    }
   }
 
   /**