Hooks: Fix resort_active_iterations() skipping next priority on self-removal.

When a callback removes itself during `do_action()`/`apply_filters()` and is the
sole callback at its priority, `resort_active_iterations()` repositions the
internal array pointer one position too far. The main loop's `next()` call then
skips the following priority entirely.

The fix detects when the current priority was removed (the pointer advanced
past it) and calls `prev()` so that `next()` in the calling loop lands on the
correct priority.

Fixes #64653.

Author: mrcasual

src/wp-includes/class-wp-hook.php

@@ -150,6 +150,11 @@ private function resort_active_iterations( $new_priority = false, $priority_exis
 				}
 			}
 
+			// If the current priority was removed, step back so the next() call in the main loop lands correctly.
+			if ( false !== current( $iteration ) && current( $iteration ) !== $current ) {
+				prev( $iteration );
+			}
+
 			// If we have a new priority that didn't exist, but ::apply_filters() or ::do_action() thinks it's the current priority...
 			if ( $new_priority === $this->current_priority[ $index ] && ! $priority_existed ) {
 				/*

tests/phpunit/tests/hooks/doAction.php

@@ -291,6 +291,71 @@ public function _filter_do_action_doesnt_change_value3( $value ) {
 		return 'x3';
 	}
 
+	/**
+	 * Verify that a callback removing itself during execution does not cause
+	 * the next priority to be skipped.
+	 *
+	 * When a callback is the sole entry at its priority and removes itself
+	 * mid-iteration, resort_active_iterations() repositions the internal
+	 * array pointer. Before the fix, the pointer ended up one position too
+	 * far, causing apply_filters()'s next() call to skip the following
+	 * priority entirely.
+	 *
+	 * @ticket 64653
+	 */
+	public function test_self_removing_callback_does_not_skip_next_priority() {
+		$hook      = new WP_Hook();
+		$hook_name = __FUNCTION__;
+		$log       = array();
+
+		$hook->add_filter( $hook_name, function () use ( &$log ) {
+			$log[] = 10;
+		}, 10, 0 );
+
+		// Callback that removes itself -- the only callback at priority 50.
+		$self_removing = function () use ( &$log, &$self_removing, $hook, $hook_name ) {
+			$hook->remove_filter( $hook_name, $self_removing, 50 );
+
+			$log[] = 50;
+		};
+
+		$hook->add_filter( $hook_name, $self_removing, 50, 0 );
+
+		$hook->add_filter( $hook_name, function () use ( &$log ) {
+			$log[] = 100;
+		}, 100, 0 );
+
+		$hook->do_action( array() );
+
+		$this->assertSame( array( 10, 50, 100 ), $log, 'Priority 100 should not be skipped when priority 50 removes itself during iteration.' );
+	}
+
+	/**
+	 * Verify the fix when the self-removing callback is at the first priority.
+	 *
+	 * @ticket 64653
+	 */
+	public function test_self_removing_callback_at_lowest_priority() {
+		$hook      = new WP_Hook();
+		$hook_name = __FUNCTION__;
+		$log       = array();
+
+		$self_removing = function () use ( &$log, &$self_removing, $hook, $hook_name ) {
+			$hook->remove_filter( $hook_name, $self_removing, 10 );
+			$log[] = 10;
+		};
+
+		$hook->add_filter( $hook_name, $self_removing, 10, 0 );
+
+		$hook->add_filter( $hook_name, function () use ( &$log ) {
+			$log[] = 50;
+		}, 50, 0 );
+
+		$hook->do_action( array() );
+
+		$this->assertSame( array( 10, 50 ), $log, 'Priority 50 should execute when priority 10 removes itself.' );
+	}
+
 	/**
 	 * Use this rather than MockAction so we can test callbacks with no args
 	 *