iproto: add an empty body to the unprepare response

Absence of the body in the unprepare response forces users to perform
additional checks to avoid errors. Adding an empty body fixes this problem.

Closes #4769

Reviewed-by: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Reviewed-by: Nikita Pettik <korablev@tarantool.org>

src/box/iproto.cc

@@ -1780,21 +1780,24 @@ tx_process_sql(struct cmsg *m)
 	 * become out of date during yield.
 	 */
 	out = msg->connection->tx.p_obuf;
+	if (is_unprepare) {
+		if (iproto_reply_ok(out, msg->header.sync, schema_version) != 0)
+			goto error;
+		iproto_wpos_create(&msg->wpos, out);
+		return;
+	}
 	struct obuf_svp header_svp;
 	/* Prepare memory for the iproto header. */
 	if (iproto_prepare_header(out, &header_svp, IPROTO_HEADER_LEN) != 0) {
 		port_destroy(&port);
 		goto error;
 	}
-	/* Nothing to dump in case of UNPREPARE request. */
-	if (!is_unprepare) {
-		if (port_dump_msgpack(&port, out) != 0) {
-			port_destroy(&port);
-			obuf_rollback_to_svp(out, &header_svp);
-			goto error;
-		}
+	if (port_dump_msgpack(&port, out) != 0) {
 		port_destroy(&port);
+		obuf_rollback_to_svp(out, &header_svp);
+		goto error;
 	}
+	port_destroy(&port);
 	iproto_reply_sql(out, &header_svp, msg->header.sync, schema_version);
 	iproto_wpos_create(&msg->wpos, out);
 	return;

test/box/gh-4769-unprepare-response-body.result

+-- test-run result file version 2
+net_box = require('net.box')
+ | ---
+ | ...
+msgpack = require('msgpack')
+ | ---
+ | ...
+urilib = require('uri')
+ | ---
+ | ...
+
+IPROTO_REQUEST_TYPE = 0x00
+ | ---
+ | ...
+IPROTO_PREPARE      = 13
+ | ---
+ | ...
+IPROTO_SQL_TEXT     = 0x40
+ | ---
+ | ...
+IPROTO_STMT_ID      = 0x43
+ | ---
+ | ...
+
+box.schema.user.grant('guest', 'read, write, execute', 'universe')
+ | ---
+ | ...
+uri = urilib.parse(box.cfg.listen)
+ | ---
+ | ...
+socket = net_box.establish_connection(uri.host, uri.service)
+ | ---
+ | ...
+
+header = { [IPROTO_REQUEST_TYPE] = IPROTO_PREPARE }
+ | ---
+ | ...
+body = { [IPROTO_SQL_TEXT] = 'SELECT 1' }
+ | ---
+ | ...
+response = iproto_request(socket, header, body)
+ | ---
+ | ...
+
+body = { [IPROTO_STMT_ID] = response['body'][IPROTO_STMT_ID] }
+ | ---
+ | ...
+-- Decoding of the response will fail if there's no body.
+response = iproto_request(socket, header, body)
+ | ---
+ | ...
+response.body
+ | ---
+ | - {}
+ | ...
+
+box.schema.user.revoke('guest', 'read, write, execute', 'universe')
+ | ---
+ | ...
+socket:close()
+ | ---
+ | - true
+ | ...
+

test/box/gh-4769-unprepare-response-body.test.lua

+net_box = require('net.box')
+msgpack = require('msgpack')
+urilib = require('uri')
+
+IPROTO_REQUEST_TYPE = 0x00
+IPROTO_PREPARE      = 13
+IPROTO_SQL_TEXT     = 0x40
+IPROTO_STMT_ID      = 0x43
+
+box.schema.user.grant('guest', 'read, write, execute', 'universe')
+uri = urilib.parse(box.cfg.listen)
+socket = net_box.establish_connection(uri.host, uri.service)
+
+header = { [IPROTO_REQUEST_TYPE] = IPROTO_PREPARE }
+body = { [IPROTO_SQL_TEXT] = 'SELECT 1' }
+response = iproto_request(socket, header, body)
+
+body = { [IPROTO_STMT_ID] = response['body'][IPROTO_STMT_ID] }
+-- Decoding of the response will fail if there's no body.
+response = iproto_request(socket, header, body)
+response.body
+
+box.schema.user.revoke('guest', 'read, write, execute', 'universe')
+socket:close()
+