[lwip-devel] [bug #47485] lwip_netconn_do_close_internal blocks caller during memory error

[Joel Cunningham]

URL:
  <http://savannah.nongnu.org/bugs/?47485>

                 Summary: lwip_netconn_do_close_internal blocks caller during
memory error
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: jcunningham
            Submitted on: Mon 21 Mar 2016 02:37:30 PM GMT
                Category: sockets/netconn
                Severity: 3 - Normal
              Item Group: Change Request
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: 
            lwIP version: git head

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Details:

lwip_netconn_do_close_internal() will keep the calling thread (which issued
close()) blocked if a memory error is encountered.

This has been observed in the case where an application tries to close a TCP
socket when the send buffer is full.  When a socket's TCP buffer is full, we
can't send a FIN packet (due to logic inside the TCP core).

The calling thread will stay blocked until LWIP_TCP_CLOSE_TIMEOUT_MS_DEFAULT
expires or we receive an ACK and space opens up in the send buffer, allowing
use to send a FIN.  

Here's the description of default close behavior in BSD sockets.  Taken from
Steven's, located under the section on SO_LINGER (7.5 Generic Socket
Options):

"This option specifies how the close function operates for a
connection-oriented protocol (e.g., for TCP and SCTP, but not for UDP). By
default, close returns immediately, but if there is any data still remaining
in the socket send buffer, the system will try to deliver the data to the
peer."

In order to conform to the BSD behavior, we need a mechanism in LwIP to
asynchronously initiate the closure again after encountering a memory failure
in lwip_netconn_do_close_internal().  The calling thread would be unblocked in
all execution paths and we could trigger the closure again in sent_tcp upon
receiving an ACK.  Other memory failures may need to be addressed
appropriately.  The LWIP_TCP_CLOSE_TIMEOUT_MS_DEFAULT timeout would also apply
if the memory condition never resolves itself




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