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Re: [lwip-users] lwip_close() doesn't work when lwip_write() hangs

From: Alhad Palkar
Subject: Re: [lwip-users] lwip_close() doesn't work when lwip_write() hangs
Date: Mon, 12 Oct 2015 14:57:12 -0700

ah. I think I found the fix 

Commit: aecbce283db243ebfc786879e50b0da8d7006ed8 [aecbce2]
Parents: b8d798158b
Author: goldsimon <address@hidden>
Date: October 21, 2014 at 2:09:07 AM PDT

fixed bug #38219 Assert on TCP netconn_write with sndtimeout set


On Mon, Oct 12, 2015 at 2:50 PM, Alhad Palkar <address@hidden> wrote:
Figured out why I am hitting this issue. In the partial write case, the write_offset is not reset back to zero. This causes the "already writing or closing" assert in the next lwip_write() or lwip_close(). Once I set this back to zero like this

diff --git a/lib/lwip/src/api/api_msg.c b/lib/lwip/src/api/api_msg.c
index 0a7edfe..2160766 100644
--- a/lib/lwip/src/api/api_msg.c
+++ b/lib/lwip/src/api/api_msg.c
@@ -1239,6 +1239,7 @@ do_writemore(struct netconn *conn)
       /* partial write */
       err = ERR_OK;
       conn->current_msg->msg.w.len = conn->write_offset;
+      conn->write_offset = 0; /* reset this back to zero */
       km_printf("partial write\n");
   } else

things look to be working fine. Is this a known bug?


On Mon, Oct 12, 2015 at 2:02 PM, Alhad Palkar <address@hidden> wrote:

I tried using the SO_SNDTIMEO. Here is what I am seeing. 

1. i create a socket using lwip_socket(AF_INET, SOCK_STREAM, 0) and then connect using lwip_connect()
2. set the SO_SNDTIME0 to 1s using lwip_setsockopt(socket, SOL_SOCKET, SO_SNDTIMEO, &timeout_int, sizeof(timeout_int))
3. then start sending data as follows

 while(err > 0) {
     err = lwip_write(socket, buffer, size);

I see that I hit the timeout condition in the 2nd to last transfer 

lwip_send(0, data="" size=86264, flags=0x0)  <--- this is the packet that times out
so_sndtimeo <-- added these printouts to check the condition I am hitting
partial write
lwip_send(0) err=0 written=67348

lwip_send(0, data="" size=86264, flags=0x0) <--- the last packet that causes the assert
panic: already writing or closing <--- we hit this in do_write() ( api_msg.c:1360)

Some questions;
1. Does that mean I cannot call lwip_write() after timing out on a previous call to lwip_write()? 
2. Can I use the fact that the size returned by lwip_write() < total buffer size as an indication that our transfer timed out?

On Mon, Oct 12, 2015 at 1:16 PM, Joel Cunningham <address@hidden> wrote:
It's a great trick, hopefully others can leverage it as well :)

I'm not sure what I'd do without it.  Having select() and non-blocking sockets operate as the blocking construct of a server's event loop is essential for managing multiple connections in a high performance manner.


On Oct 12, 2015, at 02:19 PM, Sylvain Rochet <address@hidden> wrote:

Hi Joel,

On Mon, Oct 12, 2015 at 07:10:39PM +0000, Joel Cunningham wrote:
You can use SO_SNDTIMEOUT, which should work on LwIP 1.4.1. I have used it in my port with LwIP 1.4.1, so possibly there's a problem with your port?

I've also written applications that used non-blocking sockets and
select to achieve a similar behavior of having blocking I/O that can
be canceled. The trick here is adding a second socket to the read FD
set in select and then set select to block until your write or read is
ready. This second socket is bound to the loopback address. When you
want to cancel the blocking select from another thread, simply send a
datagram to the additional socket, which will return the select call.
Then you can detect that a cancel/wakeup happened because the second
socket is marked as ready.

I really like this trick. It remembers myself of the well known wake up
pipe I explained here[1], but using the loopback to do so in lwIP is
very very clever :-)


[1] http://lists.gnu.org/archive/html/lwip-devel/2015-09/msg00028.html
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