The "error_marshaling_enabled" setting seems to be always enabled - tarantool

Question

When I run this code:
$client->evaluate('
box.session.settings.error_marshaling_enabled = false
box.error{code = 42, reason = "Foobar", type = "MyError"}
');
regardless of the value of error_marshaling_enabled I always get a response with a new (extended) error format:
[
49 => 'Foobar',
82 => [
0 => [
0 => [
0 => 'CustomError',
2 => 3,
1 => 'eval',
3 => 'Foobar',
4 => 0,
5 => 42,
6 => [
'custom_type' => 'MyError',
],
],
],
],
],
Why is that?

Answer 1

Short answer.
error_marshaling_enabled option affects only how error objects are encoded in response body (48, IPROTO_DATA). It does not affect how they are returned as exceptions, in the response header (82, IPROTO_ERROR).
Long answer.
In Tarantool an error object can be returned in 2 ways: as an exception and as an object. For example, this is how to throw an error as exception:
function throw_error()
box.error({code = 1000, reason = "Error message"})
-- Or
error('Some error string')
end
This is how to return it as an object:
function return_error()
return box.error.new({code = 1000, reason = "Error message"})
end
If the function was called remotely, using IPROTO protocol via a connector like netbox, or PHP connector, or any other one, the error return way affects how it is encoded into MessagePack response packet. When the function throws, and the error reaches the top stack frame without being caught, it is encoded as IPROTO_ERROR (82) and IPROTO_ERROR_24 (49).
When the error object is returned as a regular value, not as an exception, it is encoded also as a regular value, inside IPROTO_DATA (48). Just like a string, a number, a tuple, etc.
With encoding as IPROTO_ERROR/IPROTO_ERROR_24 there is no much of a configuration space. Format of these values can't be changed. IPROTO_ERROR is always returned as a MessagePack map, with a stack of errors in it. IPROTO_ERROR_24 is always an error message. The IPROTO_ERROR_24 field is kept for compatibility with connectors to Tarantool versions < 2.4.1.
With encoding as a part of IPROTO_DATA you can choose serialization way using error_marshaling_enabled option. When it is true, errors are encoded as MessagePack extension type MP_EXT, and contain the whole error stack, encoded exactly like IPROTO_ERROR value. When the option is false (default behaviour in 2.4.1), the error is encoded as a string, MP_STR, which is the error's message. If there is a stack of errors, only the newest error is encoded.
error_marshaling_enabled option exists for backward compatibility, in case your application on Tarantool wants to be compatible with old connectors, which don't support MP_EXT encoded errors.
In Tarantool < 2.4.1 errors were encoded into result MessagePack as a string with error message, and error stacks didn't exist at all. So when the new format and the error stacks feature were introduced, making the new format default would be a too radical change breaking the old connectors.
Consider these examples of how error marshaling affects results. I use Tarantool 2.4.1 console here, and built-in netbox connector. The code below can be copy pasted into the console.
First instance:
box.cfg{listen = 3313}
box.schema.user.grant('guest', 'super')
function throw_error()
box.error({code = 1000, reason = "Error message"})
end
function return_error()
return box.error.new({code = 1000, reason = "Error message"})
end
Second instance:
netbox = require('net.box')
c = netbox.connect(3313)
Now I try to call the function on the second instance:
tarantool> c:call('throw_error')
---
- error: Error message
...
The c:call('throw_error') threw an exception. If I catch it using pcall() Lua function, I will see the error object.
tarantool> ok, err = pcall(c.call, c, 'throw_error')
tarantool> err:unpack()
---
- code: 1000
base_type: ClientError
type: ClientError
message: Error message
trace:
- file: '[string "function throw_error()..."]'
line: 2
...
As you can see, I didn't set error_marshaling_enabled, but got the full error. Now I will call the other function, without exceptions. But the error object won't be full.
tarantool> err = c:call('return_error')
tarantool> err
---
- Error message
...
tarantool> err:unpack()
---
- error: '[string "return err:unpack()"]:1: attempt to call method ''unpack'' (a nil
value)'
...
The error was returned as a mere string, error message. Not as an error object. Now I will turn on the marshaling:
tarantool> c:eval('box.session.settings.error_marshaling_enabled = true')
---
...
tarantool> err = c:call('return_error')
---
...
tarantool> err:unpack()
---
- code: 1000
base_type: ClientError
type: ClientError
message: Error message
trace:
- file: '[C]'
line: 4294967295
...
Now the same function returned the error in the new format, more featured.
On the summary: error_marshaling_enabled affects only returned errors. Not thrown errors.