Additional error logging when using setErrorHandler

[koanplaned]

Hi all,

I recently upgraded from fastify version 3.26.0 to 4.0.1 and now have an issue with the setErrorHandler method.

My setErrorHandler is implemented like this

fastify.setErrorHandler(function (error, request, reply) {

  // some custom logging operations

  reply.send(error);
}

In the previous version 3.26.0, the application did just my custom-defined logging and then returned a well-formed json like this:

{
  "message": "Error intentionally triggered",
  "error": "Internal Server Error",
  "statusCode": 500
}

Since the 4.0.1 version, there is now an additional error logging done by fastify, which I can not turn off. I narrowed the issue down to following statement:

// Problematic part in the code
reply.send(error);

# Excerpt of the additional logging
 err: {
      "type": "Error",
      "message": "The error message",
      "stack":
          Error: The error message
...

My observation: If I send the "complete" error object, the additional logging occurs, but when I just send parts of the error (e.g. the error message) the additional logging is gone:

reply.send(error.message);

Is there a way to turn off the additional logging, when returning the "full" error object?

I want to use the "full" object, since I also need the statusCode and the error in the reply message like this:

{
  "message": "Error intentionally triggered",
  "error": "Internal Server Error",
  "statusCode": 500
}

As a workaround, I tried to construct this object myself, but I don't understand, how I can access the actual error statusCode to send it back, for example like this:

reply.send({
  "message": error.message
  "error": error.error
  "statusCode": error.statusCode
})

The property "error.statusCode" is undefined on the error object, so I guess there must be another way to access it somehow.

Help would be really appreciated here :-)

[mcollina]

The error system in Fastify v4 has been refactored.

If you want to customize things, you must do:

fastify.setErrorHandler(function (error, request, reply) {

  // some custom logging operations

  const toSend = {
    "message": error.message
    "error": error.error
    "statusCode": error.statusCode || 500 // default status code
  }

  reply.core(toSend.statusCode).send(toSend);
}

Fastify will only set the http status code in the default error handler, which is not customizable.

[mcollina]

We should include this in the https://github.com/fastify/fastify/blob/main/docs/Guides/Migration-Guide-V4.md

[koanplaned]

Hi @mcollina,

thanks for the feedback. I tried the described approach, but still have some troubles.

But first the good news: the additional logging is gone, as soon as I switched to the reply.code().send() statement 👍

fastify.setErrorHandler(function (error, request, reply) {
    
  // some custom logging operations

  const errorResponse = {
    message: error.message,
    error: error.error, // still missing in response json
    statusCode: error.statusCode || 500
  };

  reply.code(errorResponse.statusCode).send(errorResponse);
});

I tested the setErrorHandler by setting up a route, that validates an incoming request header with the help of a JSON schema. The JSON schema defines a mandatory header property "Client-Locale".

// JSON Schema for mandatory header property 'Client-Locale'
{
    type: 'object',
    required: ['Client-Locale'],
    properties: {
      'Client-Locale': {type: 'string'}
    }
}

In the prior fastify version, the setErrorHandler returned with a status code 400 as well as the error.error property "Bad Request", when the Client-Locale property was not present in the request header.

Now the setErrorHandler returns the status code 500 and the "error"-property with the value "Bad Request" is missing.

// Error object with fastify 3.26.0
// HTTP status code in header: 400
{
    "message": "headers should have required property 'client-locale'",
    "error": "Bad Request"
    "statusCode": 400
}

// Error object with fastify 4.0.1
// HTTP status code in header: 500
{
  "message": "headers must have required property 'client-locale'",
  "statusCode": 500
}

Is there anything else I have to consider for validation errors?
Thanks again for your help!

[mcollina]

Can you provide steps to reproduce? We often need a reproducible example, e.g. some code that allows someone else to recreate your problem by just copying and pasting it. If it involves more than a couple of different file, create a new repository on GitHub and add a link to that.

[koanplaned]

Hi @mcollina,

sure, I prepared a GitHub repo to demonstrate the issue: https://github.com/koanplaned/fastify-error-handler-issue

It includes two fastify sample applications, which are the same except for the setErrorHandler implementation.

Hope you find it useful to reproduce the issue.

[mcollina]

In the v4 example, you have to change

handler: async function (request, reply) {
            reply.send({ hello: 'world' });
        }

into

handler: async function (request, reply) {
            reply.send({ hello: 'world' });
     return reply
        }

or

handler: function (request, reply) {
            reply.send({ hello: 'world' });
        }

[koanplaned]

Hi @mcollina,

I tried both options, but this didn't change the result. The error handler still returns with error code 500 instead of 400 and with missing error.error property for both options.

// Result with updated route handler implementation

HTTP/1.1 500 Internal Server Error
content-type: application/json; charset=utf-8
content-length: 82
Date: Sat, 18 Jun 2022 12:36:20 GMT
Connection: close

{
  "message": "headers must have required property 'client-locale'",
  "statusCode": 500
}

I also played around with the setErrorHandler function a little bit more, e.g.

fastify.setErrorHandler(function (error, request, reply) {
    reply.send(error);
}

// or 

fastify.setErrorHandler(async function (error, request, reply) {
    reply.send(error);
    return reply;
}

With these tweaks, the error response was correct - but the additional error logging happened again.

I added a new folder v4-2 and updated the code with the route handler adjustments: https://github.com/koanplaned/fastify-error-handler-issue/tree/master/v4-2

Please note:
The previous /test-route is now split up into two routes in the v4-2 implementation:

GET /tests/option-1 -> async route handler with additional return statement
GET /tests/option-2 -> route handler without async option and without return statement

[mcollina]

I finally understood the problem you are facing.

we are not setting error.statuCode to 400 on errors when failing headers validation.
This actually a bug.

[koanplaned]

Yes, and it seems the same applies for error.error

[mcollina]

Would you like to send a Pull Request to address this issue? Remember to add unit tests.

The function to update is likely

fastify/lib/validation.js

Line 107 in 2324549

function wrapValidationError (result, dataVar, schemaErrorFormatter) {

.