[ZOOKEEPER-2466] Client skips servers when trying to connect

• Type: Bug
• Status: Patch Available
• Priority: Major
• Resolution: Unresolved
• Component/s: c client
• Assignee: Michael Han
• Reporter: Flavio Paiva Junqueira
• Created: 05/Jul/16 21:04
• Updated: 15/Jun/23 21:09

Description

I've been looking at Zookeeper_simpleSystem::testFirstServerDown and I observed the following behavior. The list of servers to connect contains two servers, let's call them S1 and S2. The client never connects, but the odd bit is the sequence of servers that the client tries to connect to:

S1
S2
S1
S1
S1
<keeps repeating S1>

It intrigued me that S2 is only tried once and never again. Checking the code, here is what happens. Initially, zh->reconfig is 1, so in zoo_cycle_next_server we return an address from get_next_server_in_reconfig, which is taken from zh->addrs_new in this test case. The attempt to connect fails, and handle_error is invoked in the error handling path. handle_error actually invokes addrvec_next which changes the address pointer to the next server on the list.

After two attempts, it decides that it has tried all servers in zoo_cycle_next_server and sets zh->reconfig to zero. Once zh->reconfig == 0, we have that each call to zoo_cycle_next_server moves the address pointer to the next server in zh->addrs. But, given that handle_error also moves the pointer to the next server, we end up moving the pointer ahead twice upon every failed attempt to connect, which is wrong.

Issue Links

• is related to: ZOOKEEPER-1856 zookeeper C-client can fail to switch from a dead server in a 3+ server ensemble if the client only has a 2 server list.