[sr-dev] [kamailio/kamailio] TCP : queue full, x requests queued (total handled y) (#2392)

[ThomasSevestre]

Hello,

Do you know what can cause this issue ?
Do you need more information ?


### Description

I had an issue on a kamailio instance with TLS connections.

User agents where not able to register to kamailio using TLS

With the follonwing setup : UDP => Kamailio => TLS
Kamailio was able to route from UDP to TLS but not from TLS to UDP.


### Troubleshooting

#### Log Messages

Here are some unusual logs during the issue :

```
ERROR: <core> [core/tcp_main.c:3504]: send_fd_queue_run(): send_fd failed on socket 27 , queue entry 0, retries 17, connection 0x7f913e1d7fd8, tcp socket 134, errno=11 (Resource temporarily unavailable)
ERROR: <core> [core/tcp_main.c:3504]: send_fd_queue_run(): send_fd failed on socket 23 , queue entry 1, retries 13, connection 0x7f913e1a0eb0, tcp socket 105, errno=11 (Resource temporarily unavailable)
ERROR: <core> [core/tcp_main.c:3504]: send_fd_queue_run(): send_fd failed on socket 25 , queue entry 2, retries 11, connection 0x7f913e1ada08, tcp socket 137, errno=11 (Resource temporarily unavailable)
ERROR: <core> [core/tcp_main.c:3504]: send_fd_queue_run(): send_fd failed on socket 27 , queue entry 3, retries 9, connection 0x7f913e1c2cf0, tcp socket 170, errno=11 (Resource temporarily unavailable)
ERROR: <core> [core/tcp_main.c:3504]: send_fd_queue_run(): send_fd failed on socket 23 , queue entry 0, retries 7, connection 0x7f913e1cb480, tcp socket 281, errno=11 (Resource temporarily unavailable)
ERROR: <core> [core/tcp_main.c:3504]: send_fd_queue_run(): send_fd failed on socket 25 , queue entry 1, retries 5, connection 0x7f913e1e4b30, tcp socket 398, errno=11 (Resource temporarily unavailable)
ERROR: <core> [core/tcp_main.c:3504]: send_fd_queue_run(): send_fd failed on socket 27 , queue entry 2, retries 3, connection 0x7f913e1e8ef8, tcp socket 406, errno=11 (Resource temporarily unavailable)
CRITICAL: <core> [core/tcp_main.c:4216]: send2child(): tcp child 1, socket 23: queue full, 304 requests queued (total handled 425)
CRITICAL: <core> [core/tcp_main.c:4216]: send2child(): tcp child 2, socket 25: queue full, 304 requests queued (total handled 4940)
CRITICAL: <core> [core/tcp_main.c:4216]: send2child(): tcp child 3, socket 27: queue full, 304 requests queued (total handled 310)
CRITICAL: <core> [core/tcp_main.c:4216]: send2child(): tcp child 1, socket 23: queue full, 305 requests queued (total handled 426)
CRITICAL: <core> [core/tcp_main.c:4216]: send2child(): tcp child 2, socket 25: queue full, 305 requests queued (total handled 4941)
CRITICAL: <core> [core/tcp_main.c:4216]: send2child(): tcp child 3, socket 27: queue full, 305 requests queued (total handled 311)
```
Just before the CRITICAL log, there is this comment in sources :  /* FIXME: remove after debugging */

### Additional Information

```
version: kamailio 5.3.5 (x86_64/linux) 9e70e8
flags: USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, TLSF_MALLOC, DBG_SR_MEMORY, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_DNS_CACHE, USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLACKLIST, HAVE_RESOLV_RES
ADAPTIVE_WAIT_LOOPS 1024, MAX_RECV_BUFFER_SIZE 262144, MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB
poll method support: poll, epoll_lt, epoll_et, sigio_rt, select.
id: 9e70e8
compiled on 03:49:09 Jun 24 2020 with /usr/bin/gcc 5.4.0
```


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