pdo_dblib: Do not reuse dead connections

In case of persistent connection it was not checked if the
connection was still alive always assuming it was.
If the connection was broken this caused PHP to reuse the
broken connection over and over.
dbdead function is supported by all dblib implementation
(MS, Sybase, FreeTDS).
Change tested manually, see
FreeTDS/freetds#711 (comment)

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>

Author: freddy77

ext/pdo_dblib/dblib_driver.c

@@ -422,6 +422,16 @@ static int dblib_get_attribute(pdo_dbh_t *dbh, zend_long attr, zval *return_valu
 	return 1;
 }
 
+static zend_result dblib_handle_check_liveness(pdo_dbh_t *dbh)
+{
+	pdo_dblib_db_handle *H = (pdo_dblib_db_handle *)dbh->driver_data;
+
+	if (dbdead(H->link))
+		return FAILURE;
+
+	return SUCCESS;
+}
+
 static const struct pdo_dbh_methods dblib_methods = {
 	dblib_handle_closer,
 	dblib_handle_preparer,
@@ -434,7 +444,7 @@ static const struct pdo_dbh_methods dblib_methods = {
 	dblib_handle_last_id, /* last insert id */
 	dblib_fetch_error, /* fetch error */
 	dblib_get_attribute, /* get attr */
-	NULL, /* check liveness */
+	dblib_handle_check_liveness, /* check_liveness */
 	NULL, /* get driver methods */
 	NULL, /* request shutdown */
 	NULL, /* in transaction, use PDO's internal tracking mechanism */