[Dovecot] [PATCH] net_connect_*(): Wait for fd to complete connect(2) when fd is non-blocking

[SATOH Fumiyasu]

At Wed, 10 Aug 2011 20:37:46 +0300,
Timo Sirainen wrote:
> On 2.8.2011, at 5.25, SATOH Fumiyasu wrote:
> 
> >>> Dovecot ignores EINPROGRESS on connect(2) for non-blocking fd.
> >>> This is wrong. After that, read(2) to fd (or write(2) to fd) fails
> >>> with ENOTCONN if the connection of fd is not completed.
> >>> 
> >>> The attached patch fixes this problem.
> 
> If you do that, then there's no point in making the socket
> non-blocking before connect().

Linux connect(2) manpage said:

       EINPROGRESS
              The socket is nonblocking and the connection
              cannot be completed immediately.  It is pos-
              sible to select(2) or poll(2) for completion
              by  selecting the socket for writing.  After
              select(2) indicates  writability,  use  get-
              sockopt(2)  to  read  the SO_ERROR option at
              level SOL_SOCKET to determine  whether  con-
              nect()  completed  successfully (SO_ERROR is
              zero) or unsuccessfully (SO_ERROR is one  of
              the  usual error codes listed here, explain-
              ing the reason for the failure).

Solaris 10 connect(3SOCKET) manpage said:

       EINPROGRESS             The socket is non-blocking,
                               and  the  connection cannot
                               be  completed  immediately.
                               You  can use  select(3C) to
                               complete the connection  by
                               selecting  the  socket  for
                               writing.

Windows connect function document said
(http://msdn.microsoft.com/en-us/library/ms737625%28v=vs.85%29.aspx):

  With a nonblocking socket, the connection attempt cannot be completed
  immediately. In this case, connect will return SOCKET_ERROR, and
  WSAGetLastError will return WSAEWOULDBLOCK. In this case, there are
  three possible scenarios:

    * Use the select function to determine the completion of the
      connection request by checking to see if the socket is writeable.
    * If the application is using WSAAsyncSelect to indicate interest
      in connection events, then the application will receive an
      FD_CONNECT notification indicating that the connect operation is
      complete (successfully or not).
    * If the application is using WSAEventSelect to indicate interest
      in connection events, then the associated event object will be
      signaled indicating that the connect operation is complete
      (successfully or not).

> > On a high-load Solaris 10 box, dovecot-lda fails to query (I/O) to
> > dovecot dict socket with ENOTCONN. My patch fixes this problem.
> 
> I think Linux/etc returns EAGAIN in such situation. Maybe the right
> fix is to just add EINPROGRESS check for net_connect_unix_with_retries()?
> (With some extra changes so that it actually sees that errno from
> net_connect_unix())

I think you MUST wait for the fd to complete connect() before
read() from / write() to the fd in such situation.

-- 
-- Name: SATOH Fumiyasu (fumiyas @ osstech co jp)
-- Business Home: http://www.OSSTech.co.jp/
-- Personal Home: http://www.SFO.jp/blog/