fastify-http-proxy: Failure with "Promise may not be fulfilled with 'undefined'

Question

I am using the fastify with fastify-http-proxy with the following configuration:

const proxy = require('fastify-http-proxy');
fastify.register(proxy, {
    upstream: "https://myendpoint/occm/api/azure",
    prefix: '/azure',
    onResponse: (request, reply, res) => {
        console.log('Inside onResponse')
        reply.send('New body of different length');
    },
    onError: (reply, error) => {
        console.log('Inside onError')
        reply.send(`My Error: ${error}`);
    },
    replyOptions: {
        rewriteRequestHeaders: (originalRequest, headers) => {
            console.log('Inside rewriteRequestHeaders')
            return {
                ...headers,
            };
        },
    },
});

I have configured a fastify route as this:

 fastify.route({
        method: 'GET',
        url: `/azure/vsa/working-environments`,
        schema: {
            description: 'Trying out',
            tags: ['notifications'],
            security: [{
                authorization: []
            }],
            params:{
                title: "Working Env GET",
                description:'Trying out...',
                type: 'object',
                example: 'No input',
                properties: {
                    'accountId': {
                        description: "Account ID",
                        type: 'string'
                    },
                },
            }
        },
        handler: async (request, reply) => {
        }

I am calling it as:

GET http://localhost:xxxx/azure/vsa/working-environments

Basically I would like to proxy the request to this endpoint:

https://myendpoint/occm/api/azure/vsa/working-environments

Now whenever I am running it, getting this error:

{"level":30,"time":1644249539394,"pid":12100,"hostname":"pradipm02-PC","requestId":"Q3T9RjI11N","req":{"method":"GET","url":"/azure/vsa/working-environments","hostname":"localhost:8092","remoteAddress":"127.0.0.1","remotePort":63308},"msg":"incoming request"}
{"level":50,"time":1644249540657,"pid":12100,"hostname":"pradipm02-PC","requestId":"Q3T9RjI11N","err":{"type":"FastifyError","message":"Promise may not be fulfilled with 'undefined' when statusCode is not 204","stack":"FastifyError: Promise may not be fulfilled with 'undefined' when statusCode is not 204\n    at C:\\Users\\pradipm\\clients\\CloudManager\\cm_2\\occm\\service-infra\\notifications\\node_modules\\fastify\\lib\\wrapThenable.js:30:30\n    at processTicksAndRejections (node:internal/process/task_queues:96:5)","name":"FastifyError","code":"FST_ERR_PROMISE_NOT_FULFILLED","statusCode":500},"msg":"Promise may not be fulfilled with 'undefined' when statusCode is not 204"}

Any of the console.log in the rewriteRequestHeaders, onReponse, onError are not showing up.

Any help is appreciated.

Answer 1

This is discussed in the following thread in fastify-http-proxy forum. It is suggested to use fastify-reply-from instead. That solves my issue.