Report an error when wp theme mod set cannot save the value

features/theme-mod.feature

@@ -159,6 +159,40 @@ Feature: Manage WordPress theme mods
       Success: Theme mod background_color set to 123456.
       """
 
+  Scenario: Setting a theme mod that does not get saved
+    Given a WP install
+    And a wp-content/mu-plugins/override-theme-mod.php file:
+      """
+      <?php
+      add_filter( 'pre_set_theme_mod_background_color', function () {
+        return 'ffffff';
+      } );
+      """
+
+    When I try `wp theme mod set background_color 123456`
+    Then STDERR should be:
+      """
+      Error: Could not update theme mod background_color.
+      """
+    And STDOUT should be empty
+    And the return code should be 1
+
+  Scenario: Setting a theme mod whose value is changed when it is read
+    Given a WP install
+    And a wp-content/mu-plugins/filter-theme-mod.php file:
+      """
+      <?php
+      add_filter( 'theme_mod_background_color', function () {
+        return 'ffffff';
+      } );
+      """
+
+    When I run `wp theme mod set background_color 123456`
+    Then STDOUT should be:
+      """
+      Success: Theme mod background_color set to 123456.
+      """
+
   Scenario: Removing theme mods
     Given a WP install
 

src/Theme_Mod_Command.php

@@ -338,10 +338,12 @@ public function set( $args, $assoc_args ) {
 
 		set_theme_mod( $mod, $value );
 
-		if ( get_theme_mod( $mod ) === $value ) {
+		// Read the stored mods, as a theme_mod_{$mod} filter can change get_theme_mod() without changing what was saved.
+		$mods = get_theme_mods();
+		if ( isset( $mods[ $mod ] ) && $mods[ $mod ] === $value ) {
 			WP_CLI::success( "Theme mod {$mod} set to {$value}." );
 		} else {
-			WP_CLI::success( "Could not update theme mod {$mod}." );
+			WP_CLI::error( "Could not update theme mod {$mod}." );
 		}
 	}
 }