PHPC-1359: Prohibit startTransaction() on sharded clusters

In versions of the driver that do not support mongos pinning, we should prohibit starting a transaction on a sharded cluster (even if the server version might actually support transactions).

Author: jmikola

src/MongoDB/Session.c

@@ -35,6 +35,13 @@ zend_class_entry* php_phongo_session_ce;
 		return; \
 	}
 
+static bool php_phongo_topology_is_sharded_cluster(mongoc_client_t* client)
+{
+	mongoc_server_description_t* sd = mongoc_client_select_server(client, true, NULL, NULL);
+
+	return (sd && !strcmp(mongoc_server_description_type(sd), php_phongo_server_description_type_map[PHONGO_SERVER_MONGOS].name));
+}
+
 static bool php_phongo_session_get_timestamp_parts(zval* obj, uint32_t* timestamp, uint32_t* increment TSRMLS_DC)
 {
 	bool retval = false;
@@ -334,6 +341,11 @@ static PHP_METHOD(Session, startTransaction)
 		return;
 	}
 
+	if (php_phongo_topology_is_sharded_cluster(mongoc_client_session_get_client(intern->client_session))) {
+		phongo_throw_exception(PHONGO_ERROR_RUNTIME TSRMLS_CC, "PHP MongoDB driver %s does not support running multi-document transactions on sharded clusters", PHP_MONGODB_VERSION);
+		return;
+	}
+
 	if (!mongoc_client_session_start_transaction(intern->client_session, txn_options, &error)) {
 		phongo_throw_exception_from_bson_error_t(&error TSRMLS_CC);
 	}

tests/session/session-startTransaction_error-004.phpt

@@ -0,0 +1,25 @@
+--TEST--
+MongoDB\Driver\Session::startTransaction() does not support sharded clusters
+--SKIPIF--
+<?php require __DIR__ . "/../utils/basic-skipif.inc"; ?>
+<?php skip_if_not_libmongoc_crypto() ?>
+<?php skip_if_not_mongos(); ?>
+<?php skip_if_server_version('<', '4.0'); ?>
+--FILE--
+<?php
+require_once __DIR__ . "/../utils/basic.inc";
+
+$manager = new MongoDB\Driver\Manager(URI);
+$session = $manager->startSession();
+
+echo throws(function() use ($session) {
+    $session->startTransaction();
+}, 'MongoDB\Driver\Exception\RuntimeException'), "\n";
+
+?>
+===DONE===
+<?php exit(0); ?>
+--EXPECTF--
+OK: Got MongoDB\Driver\Exception\RuntimeException
+PHP MongoDB driver %s does not support running multi-document transactions on sharded clusters
+===DONE===