MXS-1220: Add JSON Pointer support for json_t objects

Using the JSON Pointer syntax specified in RFC 6901
(https://tools.ietf.org/html/rfc6901) allows for a convenient way to
access values deep in a JSON object.

server/core/json_api.cc

@@ -61,3 +61,73 @@ void mxs_json_add_relation(json_t* rel, const char* id, const char* type)
     json_object_set_new(obj, CN_TYPE, json_string(type));
     json_array_append(data, obj);
 }
+
+static string grab_next_component(string* s)
+{
+    std::string& str = *s;
+
+    while (str.length() > 0 && str[0] == '/')
+    {
+        str.erase(str.begin());
+    }
+
+    size_t pos = str.find("/");
+    string rval;
+
+    if (pos != string::npos)
+    {
+        rval = str.substr(0, pos);
+        str.erase(0, pos);
+        return rval;
+    }
+    else
+    {
+        rval = str;
+        str.erase(0);
+    }
+
+    return rval;
+}
+
+static bool is_integer(const string& str)
+{
+    char* end;
+    return strtol(str.c_str(), &end, 10) >= 0 && *end == '\0';
+}
+
+static json_t* mxs_json_pointer_internal(json_t* json, string str)
+{
+    json_t* rval = NULL;
+    string comp = grab_next_component(&str);
+
+    if (comp.length() == 0)
+    {
+        return json;
+    }
+
+    if (json_is_array(json) && is_integer(comp))
+    {
+        size_t idx = strtol(comp.c_str(), NULL, 10);
+
+        if (idx < json_array_size(json))
+        {
+            rval = mxs_json_pointer_internal(json_array_get(json, idx), str);
+        }
+    }
+    else if (json_is_object(json))
+    {
+        json_t* obj = json_object_get(json, comp.c_str());
+
+        if (obj)
+        {
+            rval = mxs_json_pointer_internal(obj, str);
+        }
+    }
+
+    return rval;
+}
+
+json_t* mxs_json_pointer(json_t* json, const char* json_ptr)
+{
+    return mxs_json_pointer_internal(json, json_ptr);
+}