WebSocketClient.setClient can't borrow a client created without a Context #1599

Problem

With #1568, WebSocketClient.setClient(Client) lets a WebSocket client share a borrowed client's event loop. It finds the client's NettyClientHelper through the org.restlet.engine.helper context attribute, so it only accepts a client that has a Context.

The most common way to create a client is new Client(Protocol.HTTP), and that client has no context, so it can't be borrowed. start() throws:

The borrowed client must have a context and use the Netty HTTP connector

The workaround is new Client(new Context(), Protocol.HTTP). It's shown in the #1568 description, but it's easy to miss, and the error doesn't say what to change.

Why it matters

setClient is meant to accept the client an application already has. The GraphQL WebSocket client (#1594) borrows the application's client the same way and inherits this restriction.

Proposed direction

1. Find the helper without going through the context, for example with an engine-level accessor on Client. That way any started client that uses the Netty connector can be borrowed.
2. Otherwise, keep the context lookup, document the requirement on setClient, and name the fix in the error message.

Option 1 is preferred.

Related

• WebSocketClient should reuse the application's Netty event loop instead of creating its own #1567, fix: let WebSocketClient share a borrowed client's event loop #1568: shared event loop for WebSocketClient (comment)
• feat: add the GraphQL WebSocket client (G4, graphql-transport-ws) #1594: GraphQL WebSocket client