Dutch T. Meyer created ZOOKEEPER-1856:
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Summary: 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.
Key: ZOOKEEPER-1856
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1856
Project: ZooKeeper
Issue Type: Bug
Components: c client
Reporter: Dutch T. Meyer
Priority: Minor
If a client has a 2 server list, and is currently connected to the last server
in that list, and that server then goes offline, the addrvec_next() call
handle_error() will push the client to the start of the list and terminate the
connection.
Then, the zoo_cycle_next_server() call in zookeeper_interest will be called in
response to the connection failure, and the client will cycle back to the
failed server.
In this way, a client who has a list of only 2 servers can get stuck on the one
failed server. This would only be an issue in an ensemble larger than 2 of
course, because failing 1 out of 2 would lead to quorum loss anyway.
There are other harmonics possible if every other server in the list is failed,
but this is simplest to reproduce in a 3 server ensemble where the client only
knows about 2 servers, one of which then fails. There are probably some
elegant fixes here, but I think the simplest is to add a flag to track whether
a server has been accessed before, and if it hasn't, don't call
zoo_cycle_next_server() at the top of the zookeeper_interest() function.
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