How to handle pre-handler exceptions

[bamoo456]

Hi,

I'm a novice in Hapi.js, does anyone knows how to handle unexpected exceptions in request lifecycle?

In other frameworks(e.g. Koa.js), that will be a default error handler for unhandled exceptions and return 500 errors back to clients.
Just wondering to know if hapi.js support this kind of mechanism ?

[Marsup]

hapi already handles that for you through domains, you shouldn't have to do anything, unless you want to change the 500 reply.

[bamoo456]

Hi Marsup,

Thanks for the explanation.
After several tests on pre-handler, I got weird results.

• With Promise code inside pre-handler, an uncaught exception cause server crashed

{
    method: 'GET',
    path: 'test,
    config: {
        pre: [{
            assign: '1st',
            method: function (request, reply) {
                new Promise(function (resolve) {
                    console.log('Start pre-handle');
                    resolve();
                }).then(function () {
                    reply('pre-handler done');
                });
            },
        }, {
            assign: '2nd',
            method: function (request, reply) {
                console.log('Ready to throw error');
                if (reply) {
                    throw new Error('test error');
                }
                return reply('Done 2nd handler');
            },
        }],
        handler: function (request, reply) {
            console.log('Processed done', request.pre);
            reply(request.pre);
        },
    },
}

• Without Promise inside pre-handler, an exception can be caught and reply with 500 error.

{
    method: 'GET',
    path: 'test',
    config: {
        pre: [{
            assign: '1st',
            method: function (request, reply) {
                reply('Done 1st handler');
            },
        }, {
            assign: '2nd',
            method: function (request, reply) {
                console.log('Ready to throw error');
                if (reply) {
                    throw new Error('test error');
                }
                return reply('Done 2nd handler');
            },
        }],
        handler: function (request, reply) {
            console.log('Processed done', request.pre);
            reply(request.pre);
        },
    },
}

Did I use pre-handler in wrong way?

[johnbrett]

Are you on the latest version of hapi @bamoo456 ? I remember a similar issue to this came up before, but I thought it had been fixed.

[Marsup]

I tried on hapi@16.1, it leaks, and I can't even explain it, but I only spent 5 minutes looking.

[kanongil]

This sounds like a bug in the way hapi handles pre-handlers. Calling reply inside a promise loses the domain binding for the second handler. Hapi assumes that all calls to reply() are called from the correct domain.

See nodejs/node-v0.x-archive#8648 for some context on why the domain binding is lost inside promises.

[kanongil]

I have proposed a fix for hapi in hapijs/hapi#3448.

[johnbrett]

@Marsup I thought this had been fixed!!

global.Promise = require('bluebird') - everything works as expected.

Maybe we need to look at using bluebird in core again? It seems there's constant issues with native promises and domains, I believe bluebird has much more built in to deal with all these edge cases around promise errors with domains...

Edit: Nice work @kanongil, maybe I jumped the gun a little 😆

[bamoo456]

Hi @johnbrett

Actually, I'm using bluebird as my global promise library, and I encountered that bug.
The project environment is node 6.9.5 and hapi 16.1.0

[acmoune]

I had a more beginner issue. I was thinking that when you do reply(err) from a pre, then the err object will be assigned to the pre assign variable, and you will have to check on your own from the handler if the pre returned an error or not. I thought I had to throw an error from the pre to stop that.

But returning an error from a pre seems to be a particular case, hapi just stop the request there and reply with the error. I think it is cool.

[anuraggupta816]

Hi
I am getting 500 error instead of 400 error. I am using pre method in hapi js with promise whenever I error occur pre menthod throw 500 error instead of custom error.

[anuraggupta816]

@Marsup @johnbrett @kanongil @acmoune

[anuraggupta816]

hapi version 17.8.5