Use extensions field for custom error fields

It is needed to avoid possible name clash with future versions of
GraphQL standard. June, 2018 version of the standard strongly recommends
to use `extensions` field for custom fields of an error.

[1]: https://github.com/facebook/graphql/pull/407/files
[2]: https://github.com/facebook/graphql/releases/tag/June2018

Author: Totktonada

graphql/server/server.lua

@@ -1,4 +1,5 @@
 local fio = require('fio')
+local graphql_utils = require('graphql.utils')
 local utils = require('graphql.server.utils')
 local json = require('json')
 
@@ -100,11 +101,18 @@ function server.init(graphql, host, port)
 
         local query = parsed.query
 
-        local ok, compiled_query = pcall(graphql.compile, graphql, query)
+        local traceback
+        local ok, compiled_query = xpcall(function()
+            return graphql:compile(query)
+        end, function(err)
+            traceback = debug.traceback()
+            return err
+        end)
         if not ok then
+            local err = graphql_utils.serialize_error(compiled_query, traceback)
             return {
                 status = 200,
-                body = json.encode({errors = {{message = compiled_query}}})
+                body = json.encode({errors = {err}})
             }
         end
 

graphql/utils.lua

@@ -226,20 +226,21 @@ function utils.optional_require_rex()
 end
 
 function utils.serialize_error(err, traceback)
+    local extensions = {traceback = traceback}
     if type(err) == 'string' then
         return {
             message = utils.strip_error(err),
-            traceback = traceback,
+            extensions = extensions,
         }
     elseif type(err) == 'cdata' and
             tostring(ffi.typeof(err)) == 'ctype<const struct error &>' then
         return {
             message = tostring(err),
-            traceback = traceback,
+            extensions = extensions,
         }
     elseif type(err) == 'table' and type(err.message) == 'string' then
         local err = table.copy(err)
-        err.traceback = traceback
+        err.extensions = extensions
         return err
     end
 
@@ -248,10 +249,10 @@ function utils.serialize_error(err, traceback)
     json.cfg({encode_use_tostring = true})
     local orig_error = json.encode(err)
     json.cfg({encode_use_tostring = encode_use_tostring_orig})
+    extensions.orig_error = orig_error
     return {
         message = message,
-        orig_error = orig_error,
-        traceback = traceback,
+        extensions = extensions,
     }
 end