Issue #2928256 by marcoscano, seanB: Users shouldn't be able to change the...

Issue #2928256 by marcoscano, seanB: Users shouldn't be able to change the media source plugin after the media type is created

core/modules/media/src/MediaTypeForm.php

@@ -118,14 +118,23 @@ public function form(array $form, FormStateInterface $form_state) {
       '#attributes' => ['id' => 'source-dependent'],
     ];
 
+    if ($source) {
+      $source_description = $this->t('<em>The media source cannot be changed after the media type is created.</em>');
+    }
+    else {
+      $source_description = $this->t('Media source that is responsible for additional logic related to this media type.');
+    }
     $form['source_dependent']['source'] = [
       '#type' => 'select',
       '#title' => $this->t('Media source'),
       '#default_value' => $source ? $source->getPluginId() : NULL,
       '#options' => $options,
-      '#description' => $this->t('Media source that is responsible for additional logic related to this media type.'),
+      '#description' => $source_description,
       '#ajax' => ['callback' => '::ajaxHandlerData'],
       '#required' => TRUE,
+      // Once the media type is created, its source plugin cannot be changed
+      // anymore.
+      '#disabled' => !empty($source),
     ];
 
     if (!$source) {

core/modules/media/tests/src/FunctionalJavascript/MediaTypeCreationTest.php

@@ -49,6 +49,10 @@ public function testMediaTypeCreationFormWithDefaultField() {
     $this->drupalGet("admin/structure/media/manage/{$mediaTypeMachineName}");
 
     $assert_session->pageTextContains('Test source field is used to store the essential information about the media item.');
+
+    // Check that the plugin cannot be changed after it is set on type creation.
+    $assert_session->fieldDisabled('Media source');
+    $assert_session->pageTextContains('The media source cannot be changed after the media type is created.');
   }
 
   /**