[sr-dev] [kamailio/kamailio] Kamailio-Master tcp request relaying problem (EAGAIN) (#1836)

[Yasin CANER]

### Description
Hello,
Kamailio -master version couldnt relay tcp request to other sides. it gives EAGAIN error. When i test scenario on kamailio-5.2 version , it can relay.

### Troubleshooting

#### Reproduction
```
route[TCPTEST]{
remove_hf("Route");
#loose_route();
if (!is_method("INVITE")) return;

$du="sip: mykamailio1.test:5060;transport=tcp";

if(!t_newtran()){
 xlog("L_WARN"," Couldnt create new transaction \r\n");
 sl_send_reply("503", "Service not available");
 exit;
}

if(!t_relay()){
 send_reply("503", "Service not available");
 exit;
}
exit;
}

```

#### Debugging Data

#### Log Messages

<!--
Check the syslog file and if there are relevant log messages printed by Kamailio, add them next, or attach to issue, or provide a link to download them (e.g., to a pastebin site).
-->

```
 7(15781) DEBUG: <core> [core/tcp_main.c:1229]: tcpconn_new(): on port 5060, type 2
 7(15781) DEBUG: <core> [core/tcp_main.c:1539]: tcpconn_add(): hashes: 507:2382:0, 2
 7(15781) DEBUG: <core> [core/tcp_main.c:2873]: tcpconn_1st_send(): pending write on new connection 0x7fd15b5e7c98 (-1/1483 bytes written) (err: 11 - Resource temporarily unavailable)
 7(15781) ERROR: <core> [core/tcp_main.c:2891]: tcpconn_1st_send(): [FD61:C23C:B3B6:0:0:0:0:5]:5060: EAGAIN and write queue full or failed for 0x7fd15b5e7c98
 7(15781) DEBUG: <core> [core/tcp_main.c:3153]: tcpconn_chld_put(): destroying connection 0x7fd15b5e7c98 (2, -1) flags 0060
 
 7(15781) ERROR: tm [../../core/forward.h:251]: msg_send_buffer(): tcp_send failed
```

#### SIP Traffic

<!--
If the issue is exposed by processing specific SIP messages, grab them with ngrep or save in a pcap file, then add them next, or attach to issue, or provide a link to download them (e.g., to a pastebin site).
-->

```
UAC: IPv6: TCP ---> Kamailio (5.2) :IPv6 :TCP --->  Client or Server IPv6:TCP 
```

### Possible Solutions

<!--
If you found a solution or workaround for the issue, describe it. Ideally, provide a pull request with a fix.
-->

### Additional Information

```
Kamailio master from git
```

* **Operating System**:

<!--
Details about the operating system, the type: Linux (e.g.,: Debian 8.4, Ubuntu 16.04, CentOS 7.1, ...), MacOS, xBSD, Solaris, ...;
Kernel details (output of `uname -a`)
-->

```
NAME="CentOS Linux"
VERSION="7 (Core)"
3.10.0-957.1.3.el7.x86_64
```


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https://github.com/kamailio/kamailio/issues/1836
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