[sr-dev] [kamailio/kamailio] cfg_get variables broken in async workers. (#1362)

[malcolmohare]

### Description

When using cfg_get in async_workers, the values cannot be retrieved.  You either get a 'soft' fail with kamailio failing to assign the value, or you get a 'hard' fail and kamailio core dumps.  The difference in behaviour appears to be determined by where the cfg variable is defined.  If its defined in kamailio.cfg, you get a soft failure.  If its defined in a cfg file which you included, you get a hard failure.

### Troubleshooting

Tried MANY combinations of setting the variable in different places, different length of variable names, including different characters.

#### Reproduction

test_crash.cfg

```
#!KAMAILIO

debug=8
fork=yes
children=1
async_workers=1

listen=udp:127.0.0.1:12345

loadmodule "kex.so"
loadmodule "mi_fifo.so"
loadmodule "tm.so"
loadmodule "tmx.so"
loadmodule "sl.so"
loadmodule "uac.so"
loadmodule "rr.so"
loadmodule "pv.so"
loadmodule "async.so"
loadmodule "xlog.so"

modparam("mi_fifo", "fifo_name", "./kamailio.fifo")

include_file "test_crash_included.cfg"

info.var = 1 desc "Info Variable"

request_route {
    if(@cfg_get.info.var == 1) {
        # This gets printed
        xlog("Variable is 1 in request route\n");
    }
    if(@cfg_get.info.var_from_other_file == 1) {
        # This gets printed
        xlog("Variable from other file is 1 in request route\n");
    }

    if(t_newtran()) {
        async_task_route("ASYNC_REQUEST_ROUTE");
    }
    exit;
}

route[ASYNC_REQUEST_ROUTE] {
    if(@cfg_get.info.var == 1) {
        # This doesn't get printed
        xlog("Variable is 1 in async route\n");
    } else {
        # This assignment fails with error:
        # 5(19705) ERROR: <core> [lvalue.c:405]: lval_assign(): assignment failed at pos: (48,39-48,39)
        $var(test) = @cfg_get.info.var;
        xlog("Variable is $var(test)\n");
    }
    # Crash occurs on this line
    if(@cfg_get.info.var_from_other_file == 1) {
        xlog("Variable from other file is 1 in async route\n");
    }

    t_reply("200", "OK");
}
```

test_crash_included.cfg

```
#!KAMAILIO

info.var_from_other_file = 1 desc "Var from other file"

```

#### Debugging Data

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  bt full
  info locals
  list

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```
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```

#### Log Messages

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#### SIP Traffic

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```

### Possible Solutions

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### Additional Information

  * **Kamailio Version** - output of `kamailio -v`

```
version: kamailio 4.4.2 (x86_64/linux) 892ad6
flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MEM, 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_LISTEN 16, MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB
poll method support: poll, epoll_lt, epoll_et, sigio_rt, select.
id: 892ad6
compiled on 18:41:47 Dec  4 2017 with x86_64-unknown-linux-gnu-gcc 4.9.4
```

* **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`)
-->

```
Linux ip-172-31-126-249 4.4.23-31.54.amzn1.x86_64 #1 SMP Tue Oct 18 22:02:09 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
```


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