Fix category does not exist error

If we get into a state where a category doesn't exist then we end up in a denial of service state.
This impacts pages like My View, View Issues, and others.

We can get into this state due to several cases that we should fix.  However, there is no reason why we won't handle them like we handle unknown enumerations.

This change adds a category_exists check in category_full_name() api, and changes category_exists to populate the cache to not incur extra overhead.

Fixes #15420

core/category_api.php

@@ -56,19 +56,8 @@
  * @access public
  */
 function category_exists( $p_category_id ) {
-	global $g_category_cache;
-	if( isset( $g_category_cache[(int)$p_category_id] ) ) {
-		return true;
-	}
-
-	$t_query = 'SELECT COUNT(*) FROM {category} WHERE id=' . db_param();
-	$t_count = db_result( db_query( $t_query, array( $p_category_id ) ) );
-
-	if( 0 < $t_count ) {
-		return true;
-	} else {
-		return false;
-	}
+	$t_category_row = category_get_row( $p_category_id, /* error_if_not_exists */ false );
+	return $t_category_row !== false;
 }
 
 /**
@@ -256,10 +245,11 @@ function category_remove_all( $p_project_id, $p_new_category_id = 0 ) {
 /**
  * Return the definition row for the category
  * @param integer $p_category_id Category identifier.
+ * @param boolean $p_error_if_not_exists true: error if not exists, otherwise return false.
  * @return array An array containing category details.
  * @access public
  */
-function category_get_row( $p_category_id ) {
+function category_get_row( $p_category_id, $p_error_if_not_exists = true ) {
 	global $g_category_cache;
 
 	$p_category_id = (int)$p_category_id;
@@ -272,7 +262,11 @@ function category_get_row( $p_category_id ) {
 	$t_result = db_query( $t_query, array( $p_category_id ) );
 	$t_row = db_fetch_array( $t_result );
 	if( !$t_row ) {
-		trigger_error( ERROR_CATEGORY_NOT_FOUND, ERROR );
+		if( $p_error_if_not_exists ) {
+			trigger_error( ERROR_CATEGORY_NOT_FOUND, ERROR );
+		} else {
+			return false;
+		}
 	}
 
 	$g_category_cache[$p_category_id] = $t_row;
@@ -562,6 +556,8 @@ function category_full_name( $p_category_id, $p_show_project = true, $p_current_
 	if( 0 == $p_category_id ) {
 		# No Category
 		return lang_get( 'no_category' );
+	} else if( !category_exists( $p_category_id ) ) {
+		return '@' . $p_category_id . '@';
 	} else {
 		$t_row = category_get_row( $p_category_id );
 		$t_project_id = $t_row['project_id'];