[lwip-devel] [task #16124] [LINGER] lwip_close() function never returns

[Ilgiz]

URL:
  <https://savannah.nongnu.org/task/?16124>

                 Summary: [LINGER] lwip_close() function never returns
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: wanderer
            Submitted on: Ср 16 фев 2022 09:01:43
                Category: TCP
         Should Start On: Ср 16 фев 2022 00:00:00
   Should be Finished on: Ср 16 фев 2022 00:00:00
                Priority: 5 - Normal
                  Status: None
                 Privacy: Public
        Percent Complete: 0%
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
                  Effort: 0.00

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Details:

Hi,
I tried to use the linger option in my application, but encountered random
freezes. After some investigation, it was found that the lwip_close() function
never returns if linger is enabled with a non-zero timeout. But this does not
happen regularly, the stars must converge in the sky. It was found that the
hang occurs in two scenarious. 

First scenario (Active close):
TCP connection is established(pcb->state==ESTABLISHED) and no unsent and
unacked data (conn->pcb.tcp->unsent==NULL&&
conn->pcb.tcp->unacked==NULL)->call lwip_shutdown(socket,SHUT_WR)->enable
linger with timeout by calling lwip_setsockopt()->without any delay call
lwip_close()->the other side normally closed the connection before the linger
timeout occurs; In this case, tcp will be deallocated (moved to tcp_tw_pcbs
list and after deallocated in tcp_slowtmr()) so that the api_msg.c layer will
never know about it.

Second scenario (Passive close):
TCP connection is established(pcb->state==ESTABLISHED),no unsent and unacked
data (conn->pcb.tcp->unsent==NULL&& conn->pcb.tcp->unacked==NULL)->FIN
received from remote(remote side started connection shutdown)->call
lwip_shutdown(socket,SHUT_WR)->enable linger with timeout by calling
lwip_setsockopt()->without any delay call lwip_close(); Important there was no
call to the lwip_recv() function between getting FIN and calling
lwip_setsockopt, lwip_close. Calling lwip_recv() changes the behavior due to
the TF_RXCLOSED flag set.

Also in both scenarios, the unsent and unpacked lists must be empty before the
shutdown operation begins. Otherwise everything works fine (lwip_close() does
not freeze).

To reproduce and understand, I have prepared code snippets for the device
(lwIP) and PC (Windows).

To reproduce the first scenario:
target_1.c - snippet code for targe with LwIP;
main_1.c - full application(server) for PC (Windows);

To reproduce the second scenario:
target_2.c - snippet code for targe with LwIP;
main_2.c - full application(server) for PC (Windows);

The application on the PC side must be started before the target.







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File Attachments:


-------------------------------------------------------
Date: Ср 16 фев 2022 09:01:43  Name: targe_1.c  Size: 1КиБ   By:
wanderer

<http://savannah.nongnu.org/task/download.php?file_id=52859>
-------------------------------------------------------
Date: Ср 16 фев 2022 09:01:43  Name: targe_2.c  Size: 1КиБ   By:
wanderer

<http://savannah.nongnu.org/task/download.php?file_id=52860>
-------------------------------------------------------
Date: Ср 16 фев 2022 09:01:43  Name: main_1.c  Size: 2КиБ   By:
wanderer

<http://savannah.nongnu.org/task/download.php?file_id=52861>
-------------------------------------------------------
Date: Ср 16 фев 2022 09:01:43  Name: main_2.c  Size: 2КиБ   By:
wanderer

<http://savannah.nongnu.org/task/download.php?file_id=52862>

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