Unable to catch error thrown in WebSocketProvider

[niklasnatter]

Hey,
I am trying to use the WebSocketProvider of ethers. Unfortunately, the websocket RPC of the Fantom Opera network is wonky at the moment and causes errors in my application. I would like to catch these errors and fallback fallback to another RPC if an error with the WebSocketProvider occurs. Unfortunately, I am not able to catch the error that is thrown when using the provider. Instead of being able to catch and handle the error, my whole application crashes.

As far as i see, the behaviour is not specific to the Fantom Opera network, but happens in other error cases too. For example, I am not able to catch the error that happens when specifying an invalid websocket URL:

try {
    const provider = new ethers.providers.WebSocketProvider('wss://example.com');
    console.log(await provider.getBlockNumber());
} catch (e) {
    console.log('catched error', e);
}

console.log('next command');

Instead of printing next command and exiting gracefully, the code snippet above exits with an error code and leads to the following output:

/Users/nn/Development/evm/playground/node_modules/ws/lib/websocket.js:604
      abortHandshake(
      ^
Error: Unexpected server response: 200
    at ClientRequest.<anonymous> (/Users/nn/Development/evm/playground/node_modules/ws/lib/websocket.js:604:7)
    at ClientRequest.emit (events.js:376:20)
    at ClientRequest.emit (domain.js:470:12)
    at HTTPParser.parserOnIncomingClient [as onIncoming] (_http_client.js:647:27)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:126:17)
    at TLSSocket.socketOnData (_http_client.js:515:22)
    at TLSSocket.emit (events.js:376:20)
    at TLSSocket.emit (domain.js:470:12)
    at addChunk (internal/streams/readable.js:309:12)
    at readableAddChunk (internal/streams/readable.js:284:9)

I am not sure if this is something that should be solved inside of ethers or if there are other preferred solution. But I would be thankful for any hint how to handle such errors gracefully 🙂

Thanks a lot in advance!

[czzhengkw]

You can't catch the exception, the only thing that you can do, is listen to debug event and filter out the data whose error is set

const provider = new ethers.providers.WebSocketProvider('wss://example.com');
provider.on('debug', (data)=> {
    if(data.error) {
        //do something....
    }
});

[m1cm1c]

As stated in #2921, this problem occurs (potentially among other reasons) when the RPC provider returns code 503 or 504.

[niklasnatter]

I encountered this problem again today. As a workaround, I extracted the initialization of the provider into a method that resolves only if the websocket of the WebSocketProvider is opened successfully. This allows to catch the error while waiting for the initialized provider.

This is probably not a perfect solution, but maybe it is helpful for somebody else too:

const getInitializedWebsocketProvider = (rpcUrl: string) => {
    // listen for errors during handshake and wait until websocket is opened to prevent uncatched errors:
    // https://github.com/ethers-io/ethers.js/discussions/2896
    return new Promise<ethers.providers.WebSocketProvider>((resolve, reject) => {
        const provider = new ethers.providers.WebSocketProvider(rpcUrl);

        (provider.websocket as unknown as EventEmitter)
            .once('open', () => {
                resolve(provider);
            })
            .once('error', (error) => {
                reject(error);
            });
    });
};

[m1cm1c]

Thank you!

To anyone else who wants to use this code: It might be required to adapt it a bit. I'm using Ethers 5.5.4. In this version, instances of ethers.providers.WebSocketProvider do not have a property websocket. However, _websocket does exist.

import EventEmitter from 'events';

[...]

function getInitializedWebsocketProvider(rpcUrl: string): Promise<ethers.providers.WebSocketProvider> {
  // Listen for errors during handshake and wait until websocket is opened to prevent
  // uncaught errors: https://github.com/ethers-io/ethers.js/discussions/2896
  return new Promise<ethers.providers.WebSocketProvider>((resolve, reject) => {
    const provider = new ethers.providers.WebSocketProvider(rpcUrl);
    (provider._websocket as EventEmitter)
      .once('open', () => resolve(provider))
      .once('error', (error) => reject(error));
  });
}

[sebastiendan]

Hi there, I'm having a similar problem with the JsonRpcProvider which throws an "uncaughtable" exception if the passed url is incorrect. Using the debug event from the EventEmitter is fine in some cases, but it seems there's no open event for the JsonRpcProvider, so I have no way to asynchronously know when or if the provider is correctly instantiated.

More practically, in my context, I'm instantiating a provider, a wallet, and sending a transaction, all within a single http request to my web server. This is because my web server is agnostic as to which network the user wants to send a transaction to. In that case, I'm unable to wait for a potential debug event before returning a response to the client.

Shouldn't the provider also emit an event when instantiated with success? (similarly to the open event from the WebSocketProvider)