core/modules/path/src/Plugin/Field/FieldWidget/PathWidget.php

@@ -3,6 +3,7 @@
 namespace Drupal\path\Plugin\Field\FieldWidget;
 
 use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\path_alias\Entity\PathAlias;
 use Drupal\Core\Field\WidgetBase;
 use Drupal\Core\Form\FormStateInterface;
 use Symfony\Component\Validator\ConstraintViolationInterface;
@@ -83,7 +84,7 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
    */
   public static function validateFormElement(array &$element, FormStateInterface $form_state) {
     // Trim the submitted value of whitespace and slashes.
-    $alias = rtrim(trim($element['alias']['#value']), " \\/");
+    $alias = PathAlias::trimAlias($element['alias']['#value']);
     if ($alias !== '') {
       $form_state->setValueForElement($element['alias'], $alias);
 

core/modules/path/tests/src/Functional/PathAliasTest.php

@@ -247,6 +247,22 @@ public function testAdminAlias() {
     $this->assertSession()->addressEquals('admin/config/search/path/add');
     $this->assertSession()->statusMessageContains('The source path has to start with a slash.', 'error');
     $this->assertSession()->statusMessageContains('The alias path has to start with a slash.', 'error');
+
+    // Create an alias to /.
+    $node6 = $this->drupalCreateNode();
+
+    $edit = [];
+    $edit['path[0][value]'] = '/node/' . $node6->id();
+    $edit['alias[0][value]'] = '/';
+    $this->drupalGet('admin/config/search/path/add');
+    $this->submitForm($edit, 'Save');
+    $this->assertSession()->statusMessageContains('The alias has been saved.', 'status');
+    $this->config('system.site')->set('page.front', '/')->save();
+    $this->drupalGet('<front>');
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertSession()->pageTextContains($node6->label());
+    $this->drupalGet($node6->toUrl('edit-form'));
+    $this->assertSession()->fieldValueEquals('path[0][alias]', '/');
   }
 
   /**
@@ -394,6 +410,20 @@ public function testNodeAlias() {
     $this->assertSession()->elementAttributeContains('xpath', "//a[normalize-space(text())='{$node6->getTitle()}']", 'href', base_path() . $alias);
     $this->clickLink($node6->getTitle());
     $this->assertSession()->statusCodeEquals(404);
+
+    // Create an alias to /.
+    $node7 = $this->drupalCreateNode();
+
+    $edit = [];
+    $edit['path[0][alias]'] = '/';
+    $this->drupalGet($node7->toUrl('edit-form'));
+    $this->submitForm($edit, 'Save');
+    $this->config('system.site')->set('page.front', '/')->save();
+    $this->drupalGet('<front>');
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertSession()->pageTextContains($node7->label());
+    $this->drupalGet($node7->toUrl('edit-form'));
+    $this->assertSession()->fieldValueEquals('path[0][alias]', '/');
   }
 
   /**

core/modules/path_alias/src/Entity/PathAlias.php

@@ -48,6 +48,36 @@ class PathAlias extends ContentEntityBase implements PathAliasInterface {
 
   use EntityPublishedTrait;
 
+  /**
+   * Trims a path alias.
+   *
+   * Trim whitespaces and slashes from a path alias.
+   * Ensures the leading slash is retained.
+   *
+   * @param string $alias
+   *   The path alias to trim.
+   *
+   * @throws Exception
+   *
+   * @return string
+   *   The trimmed alias.
+   */
+  public static function trimAlias(string $alias): string {
+    $filteredAlias = preg_replace('%
+      (         # At start
+        [^\/]   # never trim slash.
+      )
+      (         # Then
+        [\\\/]* # trim slashes
+        \s*     # and whitespaces
+      )*        # in zero or more repetitions at the end.
+      $%x', '\1', $alias);
+    if ($filteredAlias === NULL) {
+      throw new \Exception('A problem occurred while trimming the path alias.');
+    }
+    return trim($filteredAlias);
+  }
+
   /**
    * {@inheritdoc}
    */
@@ -94,9 +124,9 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
   public function preSave(EntityStorageInterface $storage) {
     parent::preSave($storage);
 
-    // Trim the alias value of whitespace and slashes. Ensure to not trim the
+    // Trim the alias value of whitespaces and slashes. Ensure to not trim the
     // slash on the left side.
-    $alias = rtrim(trim($this->getAlias()), "\\/");
+    $alias = static::trimAlias($this->getAlias());
     $this->setAlias($alias);
   }
 

core/modules/path_alias/tests/src/Kernel/AliasTest.php

@@ -47,6 +47,9 @@ public function testLookupBySystemPath() {
     $path_alias_repository = $this->container->get('path_alias.repository');
     $this->assertEquals('/test-alias', $path_alias_repository->lookupBySystemPath('/test-source-Case', LanguageInterface::LANGCODE_NOT_SPECIFIED)['alias']);
     $this->assertEquals('/test-alias', $path_alias_repository->lookupBySystemPath('/test-source-case', LanguageInterface::LANGCODE_NOT_SPECIFIED)['alias']);
+
+    $this->createPathAlias('/test-root-page-alias', '/');
+    $this->assertEquals('/', $path_alias_repository->lookupBySystemPath('/test-root-page-alias', LanguageInterface::LANGCODE_NOT_SPECIFIED)['alias']);
   }
 
   /**
@@ -58,6 +61,9 @@ public function testLookupByAlias() {
     $path_alias_repository = $this->container->get('path_alias.repository');
     $this->assertEquals('/test-source', $path_alias_repository->lookupByAlias('/test-alias-Case', LanguageInterface::LANGCODE_NOT_SPECIFIED)['path']);
     $this->assertEquals('/test-source', $path_alias_repository->lookupByAlias('/test-alias-case', LanguageInterface::LANGCODE_NOT_SPECIFIED)['path']);
+
+    $this->createPathAlias('/test-root-page-alias', '/');
+    $this->assertEquals('/test-root-page-alias', $path_alias_repository->lookupByAlias('/', LanguageInterface::LANGCODE_NOT_SPECIFIED)['path']);
   }
 
   /**

core/modules/path_alias/tests/src/Unit/Entity/PathAliasTest.php

+<?php
+
+namespace Drupal\Tests\path_alias\Unit\Entity;
+
+use Drupal\path_alias\Entity\PathAlias;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\path_alias\Entity\PathAlias
+ * @group path_alias
+ */
+class PathAliasTest extends UnitTestCase {
+
+  /**
+   * @covers ::trimAlias
+   * @dataProvider provideTrimAliases
+   */
+  public function testTrimAlias($alias, $expected) {
+    $this->assertEquals($expected, PathAlias::trimAlias($alias));
+  }
+
+  /**
+   * Data provider method to provide trim aliases actual / expected values.
+   */
+  public function provideTrimAliases() {
+    return [
+      ['/', '/'],
+      ['/ ', '/'],
+      ['//alias', '//alias'],
+      ['/alias ', '/alias'],
+      ['/first/second/', '/first/second'],
+      ['/first/second\\', '/first/second'],
+      ['/first/second//', '/first/second'],
+      ['/first/second// / ', '/first/second'],
+      ['/first/second//  ', '/first/second'],
+      ['/first/second // / ', '/first/second'],
+      ['/first/second\\\\', '/first/second'],
+      ['/first/second\\\\ / \\ ', '/first/second'],
+      // These values shouldn't be changed, as it is not in scope of the
+      // function:
+      ['/first/second /third', '/first/second /third'],
+      ['/first//second', '/first//second'],
+    ];
+  }
+
+}