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https://issues.apache.org/jira/browse/ZOOKEEPER-2466?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15375293#comment-15375293
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Flavio Junqueira commented on ZOOKEEPER-2466:
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{quote}
zoo_cycle_next_server does more than addrvec_next - and in some cases the
addrvec_next might not get called if zoo_cycle_next_server returns earlier
within the reconfig logic and in this case the current server addr is reset to
NULL. So it seems to me that addrvec_next and zoo_cycle_next_server is not
semantically equivalent.
{quote}
True, they aren't semantically equivalent, but that actually could be a bug.
What if the client is in RO mode and reconfig is enabled? Shouldn't it try
follow the same process in zoo_cycle_next_server to find an RW server?
{quote}
Yeah, I think the addrvec_next(&zh->addrs, NULL) just change the state of the
zh-addrs vector without changing the state of the current server in zh handle -
zoo_cycle_next_server will change the state of the current server, so both are
not semantically equivalent.
{quote}
We could either leave as is or have a separate function to advance that only
takes one parameter. The latter sounds a bit unnecessary. Another option is to
have a second parameter for zoo_cycle_next_server so that we update according
to the second parameter rather than always updating zh->addr_cur. Does it work?
> Client skips servers when trying to connect
> -------------------------------------------
>
> Key: ZOOKEEPER-2466
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2466
> Project: ZooKeeper
> Issue Type: Bug
> Components: c client
> Reporter: Flavio Junqueira
> Assignee: Flavio Junqueira
> Priority: Critical
> Fix For: 3.5.3, 3.6.0
>
>
> 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:
> {noformat}
> S1
> S2
> S1
> S1
> S1
> <keeps repeating S1>
> {noformat}
> 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.
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