[sr-dev] [kamailio/kamailio] app_ruby: print backtrace when ruby exception occurs (#1766)

[Fatih Orhan]

### Description
Hello,

When there is an error in my ruby script, app_ruby prints exception message without a stack trace, like this:
```
Dec 14 16:25:17 servername /usr/sbin/kamailio[24313]: ERROR: app_ruby [app_ruby_api.c:105]: app_ruby_print_last_exception(): exception: wrong number of arguments (1 for 2..3)
Dec 14 16:25:17 servername /usr/sbin/kamailio[24313]: ERROR: app_ruby [app_ruby_api.c:1078]: app_ruby_run_ex(): ruby exception (6) on callback for: ksr_request_route (res type: 17)
```

Altough this message tells what is the error, it doesn't tell **where** the error happened, so it's hard to find the source of the problem.

In ruby, exceptions have _backtrace()_ method returning stack trace as an array of strings. It would be really helpful if app_ruby also call this method and append its result to error message.

### Reproduction
in kamailio.cfg
```
loadmodule "app_ruby.so"
modparam("app_ruby", "load", "/etc/kamailio/ruby/routes.rb")
cfgengine "ruby"
```

in /etc/kamailio/ruby/routes.rb
```
def some_method(param1, param2)
end

def ksr_request_route
  some_method(1)
end
```

### Expected behavior
```
Dec 14 16:46:24 servername /usr/sbin/kamailio[24314]: ERROR: <core> [core/kemi.c:87]: sr_kemi_core_err(): wrong number of arguments (1 for 2): /etc/kamailio/ruby/routes.rb:1:in `some_method'
                                                                     /etc/kamailio/ruby/routes.rb:4:in `ksr_request_route'
```
#### Actual observed behavior
```
Dec 14 16:45:42 servername /usr/sbin/kamailio[24311]: ERROR: app_ruby [app_ruby_api.c:105]: app_ruby_print_last_exception(): exception: wrong number of arguments (1 for 2)
```
### Possible Solutions

Relevant code seem to be here: https://github.com/kamailio/kamailio/blob/master/src/modules/app_ruby/app_ruby_api.c#L98

I found example C code to print stack trace here but couldn't figure out how to integrate to app_ruby: http://zoo-project.org/svn/trunk/zoo-project/zoo-kernel/service_internal_ruby.c

Equivalent ruby code would be (this code is not idiomatic, my aim is to provide C-like ruby code):
```
def some_error_generating_method
  method_taking_two_params(1)
rescue => exception
  message = exception.to_s() + "\n"
  stack_trace = exception.backtrace()
  for line in stack_trace
    message = message + line + "\n"
  end
  print(message)
end
```

### Additional Information

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

```
version: kamailio 5.2.0 (x86_64/linux) 535e13
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_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB
poll method support: poll, epoll_lt, epoll_et, sigio_rt, select.
id: 535e13
compiled on 10:26:34 Nov 28 2018 with gcc 4.8.5
```

* **Operating System**:

Centos 7.2 64bit

```
Linux servername 3.10.0-327.28.2.el7.x86_64 #1 SMP Wed Aug 3 11:11:39 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

```


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