UnhandledPromiseRejectionWarning when sending a transaction and the error event handler throws

[gabmontes]

Expected behavior

Send a transaction that will fail i.e. because the gas amount is not specified, and use the events interface of the promi-event. If the error event handler throws, that error should be handled exclusively in "userland". If the promises interface is used as well, the no-gas-specified error should be seen.

Actual behavior

An unhandled promise rejection is thrown within web3. On top of that, the error coming into the .catch() is not the original gas error but the error within the buggy error event handler. This is misleading because the issue is not within the library but in the user code and the error obtained shadows the actual send transaction error.

Spoiler alert

The following line must be wrapped in a try-catch block to prevent the error to bubble up and wreak havoc:

https://github.com/ethereum/web3.js/blob/87e668275ac7d9b8af7c909137fc0626c3866929/packages/web3-core-method/src/index.js#L732

Steps to reproduce the behavior

const Web3 = require('web3')
const web3 = new Web3()

web3.eth.accounts.wallet.create(0).add(process.env.PRIVATE_KEY)
const from = web3.eth.accounts.wallet[0].address

const emitter = web3.eth.sendTransaction({ from, to: from, value: '1' }) // no gas was specified!
emitter.on('error', function () { throw new Error('UNHANDLED') })
emitter.catch(console.error)

Logs

$ node bug.js                                                                          
(node:21330) UnhandledPromiseRejectionWarning: Error: UNHANDLED
    at Promise.<anonymous> (./bug.js:8:41)
    at Promise.emit (./packages/web3-core-promievent/node_modules/eventemitter3/index.js:181:35)
    at ./packages/web3-core-method/src/index.js:720:60
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
(node:21330) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2)
(node:21330) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Environment

web3@1.2.11

[GregTheGreek]

Thanks for reporting this and opening a pr! I'll take a look at this later today 😊