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https://issues.apache.org/jira/browse/NET-258?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12993951#comment-12993951
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Bogdan Drozdowski commented on NET-258:
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Yes, the keepalive thread could see the end-of-transfer message, but then
completePendingCommand() would see the reply to the NOOP, which is always
positive (as the NOOP is mandatory to implement), so it probably wouldn't choke
on this. Now, if the code would be re-written to expect exactly 226, we have a
problem.
Is there something particular in the thread-safety we need to worry about?
We're assuming that the thread is launched only when the user has no commands
to send. So, the input and output buffers (on the control channel) won't have
any interleaved data from different threads. If we assume that the thread is
used as it was meant to be used (which means not sending any commands while
it's running), then I don't know if anything could happen. The server is
"passive" - it's the client who sends commands and since the client isn't
sending any (the assumption when running the thread), nothing will appear on
the control channel. Furthermore, no other methods or fields are used. Perhaps
the only thing that could happen is a disconnection warning (code 421), but the
thread would catch it and terminate.
If the user sends other commands while the thread is running - then yes, we
could get a mess like assigning wrong replies to commands sent. But it wouldn't
be our problem, because the mechanism wasn't assumed to work like this (it's
the user that is using the client incorrectly) and this won't work.
I don't know what will happen when different threads access the client and send
commands even with the current code, not to mention the patch. If we want to be
safe under all circumstances, work every time (even if the user wants to break
the program), correctly queue the replies and commands, we need a very
different solution - this patch won't make this happen. This is a simple
example solution (perhaps it's what the users are doing already) and it's
manually-started, so it won't harm existing code. To make the whole thing
perfect, perhaps a bigger refactoring would be required, but right now I
haven't got an idea how to do it well (both queues of the commands and replies,
with the replies returned immediately to the caller).
> Implement A Keepalive Mechanism
> -------------------------------
>
> Key: NET-258
> URL: https://issues.apache.org/jira/browse/NET-258
> Project: Commons Net
> Issue Type: Improvement
> Affects Versions: 2.0
> Reporter: Rory Winston
> Assignee: Rory Winston
> Attachments: ftp-keepalive.diff
>
>
> For routers/firewalls that terminate idle connections, a separate heartbeat
> mechanism may need to be implemented to keep the control connection active.
> Some potential issues:
> * Synchronization between a heartbeat write and a __getReply() on an active
> control connection command;
> * Should use the NOOP command as a heartbeat signal;
> * Make the timeout configurable;
> * Default SocketImpl::setKeepAlive() wont do here.
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