[RFC PATCH v2] imapd.c: imapoptions: implement idle timeout

[Robert Mueller]

> If the client's connection has dropped out, no data will ever appear on
> the socket, so select will never flag it as readable, so we will never
> try to read from it, so we will never receive the read error even though
> tcp_keepalive detected the dropout.  And if this client was idling with
> a low-activity mailbox selected (such as Drafts or Sent), it might be a
> very long time before any activity prompts us to write to the socket, so
> we also don't receive the write error.  And so even though the socket
> itself knows there's no connection anymore thanks to tcp_keepalive, we
> don't know that, because we haven't tried to interact with it.  And so
> the connection/process doesn't get cleaned up.

This is actually one of the main points of tcp_keepalive. If you're
trying to read() from a socket where the other end has "died", the read
blocks *forever*.

However with tcp_keepalive enabled, the server eventually pings the
other side, and after it doesn't get a response (after the configured
backoff and retry times), it flags the connection as EOF, causing the
read() to return with 0 bytes as per standard EOF/shutdown semantics.

Anyway I'm not sure why this is required now. I'm pretty sure (from
memory) in older versions of cyrus, the IDLE timeout was the standard
inactivity timeout (30 minutes default), and after that time we'd always
drop the connection. IDLE RFC even mentions that clients should DONE and
re-IDLE every 29 minutes.

https://tools.ietf.org/html/rfc3501

5.4.    Autologout Timer

   If a server has an inactivity autologout timer, the duration of that
   timer MUST be at least 30 minutes.  The receipt of ANY command from
   the client during that interval SHOULD suffice to reset the
   autologout timer.


https://tools.ietf.org/html/rfc2177

   The server MAY consider a client inactive if it has an IDLE command
   running, and if such a server has an inactivity timeout it MAY log
   the client off implicitly at the end of its timeout period.  Because
   of that, clients using IDLE are advised to terminate the IDLE and
   re-issue it at least every 29 minutes to avoid being logged off.
   This still allows a client to receive immediate mailbox updates even
   though it need only "poll" at half hour intervals.


Is there a reason to have a separate idle timeout separate to the
standard inactivity timeout?

          timeout: 32
              The length of the IMAP server's inactivity autologout
              timer, in minutes.  The minimum value is 30, the default. 
              The default is 32 to allow a bit of leeway for clients
              that try to NOOP  every
              30 minutes.

-- 
Rob Mueller
robm at fastmail.fm