Make get_term() behave more consistently in the context of shared terms.

When `WP_Term` was introduced in [34997], the `$taxonomy` parameter for
`get_term()` was made optional. This meant that, when the optional param was
omitted, `get_term()` had no way of determining which term was intended when
the term_id was shared between multiple taxonomies. As a (somewhat sneaky) way
of fixing things, `get_term()` split any shared terms it found. But this could
cause problems with developer expectations: it's not clear why requesting a
term should result in a database update, much less a potential change in the
ID of a term.

In place of this technique, this changeset introduces a number of changes that
make the handling of shared terms a bit less insane:

* When a taxonomy is provided to `get_term()`, and a cached term is found matching the term_id, make sure the taxonomy also matches before returning it.
* When a taxonomy is not provided, ensure that the term is not shared before adding it to the cache.
* When a term is shared between taxonomies and no taxonomy is provided, return a `WP_Error` rather than splitting the term.
* When a term is shared between taxonomies, only one of which is valid, return the term from that taxonomy.

Props boonebgorges, dlh.
Fixes #34533.

git-svn-id: https://develop.svn.wordpress.org/trunk@35537 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -115,10 +115,14 @@ final class WP_Term {
 	 *
 	 * @global wpdb $wpdb WordPress database abstraction object.
 	 *
-	 * @param int $term_id Term ID.
-	 * @return WP_Term|false Term object, false otherwise.
+	 * @param int    $term_id  Term ID.
+	 * @param string $taxonomy Optional. Limit matched terms to those matching `$taxonomy`. Only used for
+	 *                         disambiguating potentially shared terms.
+	 * @return WP_Term|WP_Error|false Term object, if found. WP_Error if `$term_id` is shared between taxonomies and
+	 *                                there's insufficient data to distinguish which term is intended.
+	 *                                False for other failures.
 	 */
-	public static function get_instance( $term_id ) {
+	public static function get_instance( $term_id, $taxonomy = null ) {
 		global $wpdb;
 
 		$term_id = (int) $term_id;
@@ -129,14 +133,58 @@ final class WP_Term {
 		$_term = wp_cache_get( $term_id, 'terms' );
 
 		// If there isn't a cached version, hit the database.
-		if ( ! $_term ) {
-			$_term = $wpdb->get_row( $wpdb->prepare( "SELECT t.*, tt.* FROM $wpdb->terms AS t INNER JOIN $wpdb->term_taxonomy AS tt ON t.term_id = tt.term_id WHERE t.term_id = %d LIMIT 1", $term_id ) );
+		if ( ! $_term || ( $taxonomy && $taxonomy !== $_term->taxonomy ) ) {
+			// Grab all matching terms, in case any are shared between taxonomies.
+			$terms = $wpdb->get_results( $wpdb->prepare( "SELECT t.*, tt.* FROM $wpdb->terms AS t INNER JOIN $wpdb->term_taxonomy AS tt ON t.term_id = tt.term_id WHERE t.term_id = %d", $term_id ) );
+			if ( ! $terms ) {
+				return false;
+			}
+
+			// If a taxonomy was specified, find a match.
+			if ( $taxonomy ) {
+				foreach ( $terms as $match ) {
+					if ( $taxonomy === $match->taxonomy ) {
+						$_term = $match;
+						break;
+					}
+				}
+
+			// If only one match was found, it's the one we want.
+			} elseif ( 1 === count( $terms ) ) {
+				$_term = reset( $terms );
+
+			// Otherwise, the term must be shared between taxonomies.
+			} else {
+				// If the term is shared only with invalid taxonomies, return the one valid term.
+				foreach ( $terms as $t ) {
+					if ( ! taxonomy_exists( $t->taxonomy ) ) {
+						continue;
+					}
+
+					// Only hit if we've already identified a term in a valid taxonomy.
+					if ( $_term ) {
+						return new WP_Error( 'ambiguous_term_id', __( 'Term ID is shared between multiple taxonomies' ), $term_id );
+					}
+
+					$_term = $t;
+				}
+			}
+
 			if ( ! $_term ) {
 				return false;
 			}
 
+			// Don't return terms from invalid taxonomies.
+			if ( ! taxonomy_exists( $_term->taxonomy ) ) {
+				return new WP_Error( 'invalid_taxonomy', __( 'Invalid taxonomy' ) );
+			}
+
 			$_term = sanitize_term( $_term, $_term->taxonomy, 'raw' );
-			wp_cache_add( $term_id, $_term, 'terms' );
+
+			// Don't cache terms that are shared between taxonomies.
+			if ( 1 === count( $terms ) ) {
+				wp_cache_add( $term_id, $_term, 'terms' );
+			}
 		}
 
 		$term_obj = new WP_Term( $_term );