core/modules/search/search.module                         | 8 +++++++-
 core/modules/search/src/Controller/SearchController.php   | 3 ++-
 core/modules/search/src/Plugin/SearchInterface.php        | 7 ++++---
 core/modules/search/src/Tests/SearchPageCacheTagsTest.php | 5 +++++
 core/modules/simpletest/src/WebTestBase.php               | 8 ++++----
 5 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/core/modules/search/search.module b/core/modules/search/search.module
index 3818879..4c3336d 100644
--- a/core/modules/search/search.module
+++ b/core/modules/search/search.module
@@ -154,10 +154,14 @@ function search_index_clear($type = NULL, $sid = NULL, $langcode = NULL) {
   $query_index->execute();
   $query_dataset->execute();
 
-  if ($type !== NULL) {
+  if ($type) {
     // Invalidate all render cache items that contain data from this index.
     Cache::invalidateTags(['search_index:' . $type]);
   }
+  else {
+    // Invalidate all render cache items that contain data from any index.
+    Cache::invalidateTags(['search_index']);
+  }
 }
 
 /**
@@ -505,6 +509,8 @@ function search_index($type, $sid, $langcode, $text) {
     $tag = !$tag;
   }
 
+  // Remove the item $sid from the search index, and invalidate the relevant
+  // cache tags.
   search_index_clear($type, $sid, $langcode);
 
   // Insert cleaned up data into dataset
diff --git a/core/modules/search/src/Controller/SearchController.php b/core/modules/search/src/Controller/SearchController.php
index 3a14a7b..f51466a 100644
--- a/core/modules/search/src/Controller/SearchController.php
+++ b/core/modules/search/src/Controller/SearchController.php
@@ -131,7 +131,8 @@ public function view(Request $request, SearchPageInterface $entity) {
     // If this plugin uses a search index, then also add the cache tag tracking
     // that search index, so that cached search result pages are invalidated
     // when necessary.
-    if ($plugin->getType() !== NULL) {
+    if ($plugin->getType()) {
+      $build['search_results']['#cache']['tags'][] = 'search_index';
       $build['search_results']['#cache']['tags'][] = 'search_index:' . $plugin->getType();
     }
 
diff --git a/core/modules/search/src/Plugin/SearchInterface.php b/core/modules/search/src/Plugin/SearchInterface.php
index d43e74a..72007e6 100644
--- a/core/modules/search/src/Plugin/SearchInterface.php
+++ b/core/modules/search/src/Plugin/SearchInterface.php
@@ -65,12 +65,13 @@ public function getAttributes();
   public function isSearchExecutable();
 
   /**
-   * Returns the used search index type.
+   * Returns the search index type this plugin uses.
    *
    * @return string|null
-   *   The plugin ID or other machine-readable type for the search index this
-   *   search uses. NULL if no search index is used.
+   *   The type used by this search plugin in the search index, or NULL if this
+   *   plugin does not use the search index.
    *
+   * @see search_index()
    * @see search_index_clear()
    */
   public function getType();
diff --git a/core/modules/search/src/Tests/SearchPageCacheTagsTest.php b/core/modules/search/src/Tests/SearchPageCacheTagsTest.php
index 8f2e509..f54662f 100644
--- a/core/modules/search/src/Tests/SearchPageCacheTagsTest.php
+++ b/core/modules/search/src/Tests/SearchPageCacheTagsTest.php
@@ -72,6 +72,7 @@ function testSearchText() {
     $this->drupalPostForm('search/node', $edit, t('Search'));
     $this->assertText('bike shed shop');
     $this->assertCacheTag('config:search.page.node_search');
+    $this->assertCacheTag('search_index');
     $this->assertCacheTag('search_index:node_search');
 
     // Updating a node should invalidate the search plugin's index cache tag.
@@ -80,6 +81,7 @@ function testSearchText() {
     $this->drupalPostForm('search/node', $edit, t('Search'));
     $this->assertText('bike shop');
     $this->assertCacheTag('config:search.page.node_search');
+    $this->assertCacheTag('search_index');
     $this->assertCacheTag('search_index:node_search');
 
     // Deleting a node should invalidate the search plugin's index cache tag.
@@ -87,17 +89,20 @@ function testSearchText() {
     $this->drupalPostForm('search/node', $edit, t('Search'));
     $this->assertText('Your search yielded no results.');
     $this->assertCacheTag('config:search.page.node_search');
+    $this->assertCacheTag('search_index');
     $this->assertCacheTag('search_index:node_search');
 
     // Initial page for searching users.
     $this->drupalGet('search/user');
     $this->assertCacheTag('config:search.page.user_search');
+    $this->assertNoCacheTag('search_index');
     $this->assertNoCacheTag('search_index:user_search');
 
     // User search results.
     $edit['keys'] = $this->searchingUser->getUsername();
     $this->drupalPostForm('search/user', $edit, t('Search'));
     $this->assertCacheTag('config:search.page.user_search');
+    $this->assertNoCacheTag('search_index');
     $this->assertNoCacheTag('search_index:user_search');
   }
 
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 3bb404d..c4f5cee 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -2765,12 +2765,12 @@ protected function assertCacheTag($expected_cache_tag) {
   /**
    * Asserts whether an expected cache tag was absent in the last response.
    *
-   * @param string $expected_cache_tag
-   *   The expected cache tag.
+   * @param string $cache_tag
+   *   The cache tag to check.
    */
-  protected function assertNoCacheTag($expected_cache_tag) {
+  protected function assertNoCacheTag($cache_tag) {
     $cache_tags = explode(' ', $this->drupalGetHeader('X-Drupal-Cache-Tags'));
-    $this->assertFalse(in_array($expected_cache_tag, $cache_tags), "'" . $expected_cache_tag . "' is absent in the X-Drupal-Cache-Tags header.");
+    $this->assertFalse(in_array($cache_tag, $cache_tags), "'" . $cache_tag . "' is absent in the X-Drupal-Cache-Tags header.");
   }
 
 }