Update var name and PHP docs

Unit tests have been updated according to feedback (using assertSame for simple comparisons), and removing filter removes.

Author: ramonjd

src/wp-includes/link-template.php

@@ -1814,7 +1814,8 @@ function get_next_post( $in_same_term = false, $excluded_terms = '', $taxonomy =
  * Can either be next or previous post.
  *
  * @since 2.5.0
- * @since 6.9.1 Adds deterministic fallback for sort clause if not modified by a filter.
+ * @since 6.9.0 Introduce deterministic fallback based in IDs to account for date collisions.
+ * @since 6.9.1 Remove deterministic fallback for sites modifying the WHERE clause via a filter. See #64390.
  *
  * @global wpdb $wpdb WordPress database abstraction object.
  *
@@ -1966,8 +1967,9 @@ function get_adjacent_post( $in_same_term = false, $excluded_terms = '', $previo
 	$join = apply_filters( "get_{$adjacent}_post_join", $join, $in_same_term, $excluded_terms, $taxonomy, $post );
 
 	// Prepare the where clause for the adjacent post query.
-	$where_prepared_with_deterministic_fallback = $wpdb->prepare( "WHERE (p.post_date $comparison_operator %s OR (p.post_date = %s AND p.ID $comparison_operator %d)) AND p.post_type = %s $where", $current_post_date, $current_post_date, $post->ID, $post->post_type ); // phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared -- $comparison_operator is a string literal, either '<' or '>'.
-	$where_prepared                             = $wpdb->prepare( "WHERE p.post_date $comparison_operator %s AND p.post_type = %s $where", $current_post_date, $post->post_type );
+	$where_prepared               = $wpdb->prepare( "WHERE p.post_date $comparison_operator %s AND p.post_type = %s $where", $current_post_date, $post->post_type ); // phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared -- $comparison_operator is a string literal, either '<' or '>'.
+	$deterministic_where_prepared = $wpdb->prepare( "WHERE (p.post_date $comparison_operator %s OR (p.post_date = %s AND p.ID $comparison_operator %d)) AND p.post_type = %s $where", $current_post_date, $current_post_date, $post->ID, $post->post_type ); // phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared -- $comparison_operator is a string literal, either '<' or '>'.
+
 	/**
 	 * Filters the WHERE clause in the SQL for an adjacent post query.
 	 *
@@ -1982,7 +1984,7 @@ function get_adjacent_post( $in_same_term = false, $excluded_terms = '', $previo
 	 * @since 2.5.0
 	 * @since 4.4.0 Added the `$taxonomy` and `$post` parameters.
 	 *
-	 * @param string       $where          The `WHERE` clause in the SQL.
+	 * @param string       $where_prepared The `WHERE` clause in the SQL.
 	 * @param bool         $in_same_term   Whether post should be in the same taxonomy term.
 	 * @param int[]|string $excluded_terms Array of excluded term IDs. Empty string if none were provided.
 	 * @param string       $taxonomy       Taxonomy. Used to identify the term used when `$in_same_term` is true.
@@ -1992,9 +1994,11 @@ function get_adjacent_post( $in_same_term = false, $excluded_terms = '', $previo
 
 	// Only force deterministic fallback if the where clause has not been modified by a filter.
 	if ( $where === $where_prepared ) {
-		$where = $where_prepared_with_deterministic_fallback;
+		$where = $deterministic_where_prepared;
 	}
 
+	$sort_prepared = "ORDER BY p.post_date $order LIMIT 1";
+
 	/**
 	 * Filters the ORDER BY clause in the SQL for an adjacent post query.
 	 *
@@ -2009,13 +2013,14 @@ function get_adjacent_post( $in_same_term = false, $excluded_terms = '', $previo
 	 * @since 2.5.0
 	 * @since 4.4.0 Added the `$post` parameter.
 	 * @since 4.9.0 Added the `$order` parameter.
+	 * @since 6.9.0 Adds ID sort to ensure deterministic ordering for posts with identical dates.
+	 * @since 6.9.1 Remove deterministic fallback for sites modifying the SORT clause via a filter. See #64390.
 	 *
 	 * @param string $order_by The `ORDER BY` clause in the SQL.
 	 * @param WP_Post $post    WP_Post object.
 	 * @param string  $order   Sort order. 'DESC' for previous post, 'ASC' for next.
 	 */
-	$sort_prepared = "ORDER BY p.post_date $order LIMIT 1";
-	$sort          = apply_filters( "get_{$adjacent}_post_sort", $sort_prepared, $post, $order );
+	$sort = apply_filters( "get_{$adjacent}_post_sort", $sort_prepared, $post, $order );
 
 	// Only force deterministic sort if the sort clause has not been modified by a filter.
 	if ( $sort === $sort_prepared ) {

tests/phpunit/tests/link/getAdjacentPost.php

@@ -657,9 +657,7 @@ static function ( $where ) {
 		// Next post should be the 4th post (higher ID, same date) - deterministic.
 		$next = get_adjacent_post( false, '', false );
 		$this->assertInstanceOf( 'WP_Post', $next );
-		$this->assertEquals( $post_ids[3], $next->ID );
-
-		remove_all_filters( 'get_next_post_where' );
+		$this->assertSame( $post_ids[3], $next->ID, 'Next post should be the 4th post (higher ID, same date).' );
 	}
 
 	/**
@@ -710,8 +708,6 @@ static function ( $where ) use ( &$filter_received, &$filter_returned ) {
 		$this->assertStringNotContainsString( 'AND p.ID', $filter_received, 'Filter should receive WHERE clause without deterministic ID fallback.' );
 		// Verify the filter's modification is preserved (proves deterministic logic wasn't applied on top).
 		$this->assertStringContainsString( 'AND 1=1', $filter_returned, 'Filter modification should be preserved.' );
-
-		remove_all_filters( 'get_next_post_where' );
 	}
 
 	/**
@@ -749,9 +745,7 @@ static function ( $sort ) {
 		// Next post should be the 4th post (higher ID, same date) - deterministic.
 		$next = get_adjacent_post( false, '', false );
 		$this->assertInstanceOf( 'WP_Post', $next );
-		$this->assertEquals( $post_ids[3], $next->ID );
-
-		remove_all_filters( 'get_next_post_sort' );
+		$this->assertSame( $post_ids[3], $next->ID, 'Next post should be the 4th post (higher ID, same date).' );
 	}
 
 	/**
@@ -801,8 +795,6 @@ static function ( $sort, $post, $order ) use ( &$filter_received, &$filter_retur
 		// Verify the filter's modification is preserved (proves deterministic logic wasn't applied on top).
 		$this->assertStringNotContainsString( 'p.ID', $filter_returned, 'Filter modification should not include ID when filter removes it.' );
 		$this->assertStringContainsString( 'ORDER BY p.post_date', $filter_returned, 'Filter modification should be preserved.' );
-
-		remove_all_filters( 'get_next_post_sort' );
 	}
 
 	/**
@@ -846,10 +838,7 @@ static function ( $sort ) {
 		// Previous post should be the 2nd post (lower ID, same date) - deterministic.
 		$previous = get_adjacent_post( false, '', true );
 		$this->assertInstanceOf( 'WP_Post', $previous );
-		$this->assertEquals( $post_ids[1], $previous->ID );
-
-		remove_all_filters( 'get_previous_post_where' );
-		remove_all_filters( 'get_previous_post_sort' );
+		$this->assertSame( $post_ids[1], $previous->ID, 'Previous post should be the 2nd post (lower ID, same date).' );
 	}
 
 	/**