Taxonomy: Better error handling when fetching object terms from cache.

Since [37573], `get_object_term_cache()` has expected term IDs to be
stored in the taxonomy relationship cache. The function would then
reach directly into the 'terms' cache to fetch the data corresponding
to a given term, before returning a `WP_Term` object. This caused
problems when, for one reason or another, term data was cached
inconsistently:

* If the 'terms' cache is empty for a given term ID, despite the earlier call to `_prime_term_caches()`, `get_term()` would return an error object.
* If the array of cached term IDs contains an invalid ID, `get_term()` would return an error object.

We avoid these errors by no longer touching the 'terms' cache directly,
but running term IDs through `get_term()` and allowing that function to
reference the cache (and database, as needed). If `get_term()` returns
an error object for any of the cached term IDs, `get_object_term_cache()`
will return that error object alone. This change ensures that upstream
functions, like `get_the_terms()`, return `WP_Error` objects in a
predictable fashion.

Props dd32, michalzuber.
Fixes #37291.

git-svn-id: https://develop.svn.wordpress.org/trunk@38776 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/taxonomy.php

@@ -3010,10 +3010,14 @@ function clean_term_cache($ids, $taxonomy = '', $clean_taxonomy = true) {
  * function only fetches relationship data that is already in the cache.
  *
  * @since 2.3.0
+ * @since 4.6.2 Returns a WP_Error object if get_term() returns an error for
+ *              any of the matched terms.
  *
  * @param int    $id       Term object ID.
  * @param string $taxonomy Taxonomy name.
- * @return bool|array Array of `WP_Term` objects, if cached False if cache is empty for `$taxonomy` and `$id`.
+ * @return bool|array|WP_Error Array of `WP_Term` objects, if cached.
+ *                             False if cache is empty for `$taxonomy` and `$id`.
+ *                             WP_Error if get_term() returns an error object for any term.
  */
 function get_object_term_cache( $id, $taxonomy ) {
 	$_term_ids = wp_cache_get( $id, "{$taxonomy}_relationships" );
@@ -3038,10 +3042,15 @@ function get_object_term_cache( $id, $taxonomy ) {
 
 	$terms = array();
 	foreach ( $term_ids as $term_id ) {
-		$terms[] = wp_cache_get( $term_id, 'terms' );
+		$term = get_term( $term_id );
+		if ( is_wp_error( $term ) ) {
+			return $term;
+		}
+
+		$terms[] = $term;
 	}
 
-	return array_map( 'get_term', $terms );
+	return $terms;
 }
 
 /**

tests/phpunit/tests/term/cache.php

@@ -390,4 +390,30 @@ class Tests_Term_Cache extends WP_UnitTestCase {
 		$this->assertSame( $term_meta, 'bar' );
 		$this->assertEquals( $num_queries, $wpdb->num_queries );
 	}
+
+	/**
+	 * @ticket 37291
+	 */
+	public function test_get_object_term_cache_should_return_error_if_any_term_is_an_error() {
+		register_taxonomy( 'wptests_tax', 'post' );
+
+		$t = self::factory()->term->create( array( 'taxonomy' => 'wptests_tax' ) );
+		$p = self::factory()->post->create();
+		wp_set_object_terms( $p, $t, 'wptests_tax' );
+
+		// Prime cache.
+		$terms = get_the_terms( $p, 'wptests_tax' );
+		$this->assertEqualSets( array( $t ), wp_list_pluck( $terms, 'term_id' ) );
+
+		/*
+		 * Modify cached array to insert an empty term ID,
+		 * which will trigger an error in get_term().
+		 */
+		$cached_ids = wp_cache_get( $p, 'wptests_tax_relationships' );
+		$cached_ids[] = 0;
+		wp_cache_set( $p, $cached_ids, 'wptests_tax_relationships' );
+
+		$terms = get_the_terms( $p, 'wptests_tax' );
+		$this->assertWPError( $terms );
+	}
 }