lib: Add a helper function to dump Lua stack

Very handy for debugging.

In Lua script just use "log.trace(table)":

```
function on_rib_process_dplane_results(ctx)
	log.trace(ctx.rinfo.zd_ng)
end
```

You will get something like:

```
Aug 28 17:04:36 donatas-laptop zebra[3782199]: [GCZ7N-MM9D9] {
                                                 1: {
                                                   type: 2
                                                   weight: 1
                                                   flags: 5
                                                   backup_idx: 0
                                                   vrf_id: 0
                                                   nh_encap_type: 0
                                                   gate: {
                                                     value: 5.87967e+08
                                                     string: "192.168.11.35"
                                                   }
                                                   nh_label_type: 0
                                                   srte_color: 0
                                                   ifindex: 0
                                                   backup_num: 0
                                                 }
                                                 2: {
                                                   type: 3
                                                   weight: 1
                                                   flags: 3
                                                   backup_idx: 0
                                                   vrf_id: 0
                                                   nh_encap_type: 0
                                                   nh_label_type: 0
                                                   srte_color: 0
                                                   ifindex: 4
                                                   backup_num: 0
                                                 }
                                               }
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

doc/developer/scripting.rst

@@ -523,6 +523,7 @@ object which contains methods corresponding to each of the ``zlog`` levels:
    log.error("error")
    log.notice("notice")
    log.debug("debug")
+   log.trace("trace")
 
 The log messages will show up in the daemon's log output.
 

lib/frrlua.c

@@ -382,6 +382,12 @@ static const char *frrlua_log_thunk(lua_State *L)
 	return lua_tostring(L, 1);
 }
 
+static int frrlua_log_trace(lua_State *L)
+{
+	zlog_debug("%s", frrlua_stackdump(L));
+	return 0;
+}
+
 static int frrlua_log_debug(lua_State *L)
 {
 	zlog_debug("%s", frrlua_log_thunk(L));
@@ -413,6 +419,7 @@ static int frrlua_log_error(lua_State *L)
 }
 
 static const luaL_Reg log_funcs[] = {
+	{ "trace", frrlua_log_trace },
 	{ "debug", frrlua_log_debug },
 	{ "info", frrlua_log_info },
 	{ "notice", frrlua_log_notice },
@@ -432,6 +439,67 @@ void frrlua_export_logging(lua_State *L)
  * Debugging.
  */
 
+void lua_table_dump(lua_State *L, int index, struct buffer *buf, int level)
+{
+	char tmpbuf[64] = {};
+
+	lua_pushnil(L);
+
+	while (lua_next(L, index) != 0) {
+		int key_type;
+		int value_type;
+
+		for (int i = 0; i < level; i++)
+			buffer_putstr(buf, "  ");
+
+		key_type = lua_type(L, -2);
+		if (key_type == LUA_TSTRING) {
+			const char *key = lua_tostring(L, -2);
+
+			buffer_putstr(buf, key);
+			buffer_putstr(buf, ": ");
+		} else if (key_type == LUA_TNUMBER) {
+			snprintf(tmpbuf, sizeof(tmpbuf), "%g",
+				 lua_tonumber(L, -2));
+			buffer_putstr(buf, tmpbuf);
+			buffer_putstr(buf, ": ");
+		}
+
+		value_type = lua_type(L, -1);
+		switch (value_type) {
+		case LUA_TSTRING:
+			snprintf(tmpbuf, sizeof(tmpbuf), "\"%s\"\n",
+				 lua_tostring(L, -1));
+			buffer_putstr(buf, tmpbuf);
+			break;
+		case LUA_TBOOLEAN:
+			snprintf(tmpbuf, sizeof(tmpbuf), "%s\n",
+				 lua_toboolean(L, -1) ? "true" : "false");
+			buffer_putstr(buf, tmpbuf);
+			break;
+		case LUA_TNUMBER:
+			snprintf(tmpbuf, sizeof(tmpbuf), "%g\n",
+				 lua_tonumber(L, -1));
+			buffer_putstr(buf, tmpbuf);
+			break;
+		case LUA_TTABLE:
+			buffer_putstr(buf, "{\n");
+			lua_table_dump(L, lua_gettop(L), buf, level + 1);
+			for (int i = 0; i < level; i++)
+				buffer_putstr(buf, "  ");
+			buffer_putstr(buf, "}\n");
+			break;
+		default:
+			snprintf(tmpbuf, sizeof(tmpbuf), "%s\n",
+				 lua_typename(L, value_type));
+			buffer_putstr(buf, tmpbuf);
+			break;
+		}
+
+		lua_pop(L, 1);
+	}
+}
+
 char *frrlua_stackdump(lua_State *L)
 {
 	int top = lua_gettop(L);
@@ -458,6 +526,11 @@ char *frrlua_stackdump(lua_State *L)
 				 lua_tonumber(L, i));
 			buffer_putstr(buf, tmpbuf);
 			break;
+		case LUA_TTABLE: /* tables */
+			buffer_putstr(buf, "{\n");
+			lua_table_dump(L, i, buf, 1);
+			buffer_putstr(buf, "}\n");
+			break;
 		default: /* other values */
 			snprintf(tmpbuf, sizeof(tmpbuf), "%s\n",
 				 lua_typename(L, t));

lib/frrlua.h

@@ -181,6 +181,9 @@ int frrlua_table_get_integer(lua_State *L, const char *key);
  */
 void frrlua_export_logging(lua_State *L);
 
+/* A helper fuction that dumps the Lua stack */
+void lua_table_dump(lua_State *L, int index, struct buffer *buf, int level);
+
 /*
  * Dump Lua stack to a string.
  *