Allow passing a URL to EtherscanProvider and InfuraProvider (possibly others)

[gabrielKerekes]

We use HTTP proxies to hide our Etherscan/Infura API keys and so we need to set custom URLs for EtherscanProvider and InfuraProvider.

However there is currently no straightforward way to pass a URL to either of the providers. The solution we came up with is simply extending the providers and overriding some of the functions. E.g.:

class _EtherscanProvider extends ethers.providers.EtherscanProvider {
  constructor(network: string) {
    super(network, config.etherscanApiKey)
  }

  getBaseUrl(): string {
    return config.etherscanApiUrl
  }
}

and

class _InfuraProvider extends ethers.providers.JsonRpcProvider {
  constructor(network: string) {
    super(`${config.infuraApiUrl}/${config.infuraApiKey}`, network)
  }
}

The problem with _EtherscanProvider is that we can't inject the URL into the constructor because super calls the getBaseUrl function. I.e. the following doesn't work because getBaseUrl is called before this.url gets set:

class _EtherscanProvider extends ethers.providers.EtherscanProvider {
  private url: string
  constructor(url: string, apiKey: string | null, network: string) {
    super(network, apiKey || '')
    this.url = url
  }

  getBaseUrl(): string {
    return this.url
  }
}

Accessing this before the super call is not allowed.

Notice also that our _InfuraProvider is extending JsonRpcProvider not InfuraProvider since JsonRpcProvider accepts a URL in the constructor which wasn't possible with InfuraProvider. If we were to extend InfuraProvider directly we could do the following:

class _InfuraProvider extends ethers.providers.InfuraProvider {
  constructor(network: string) {
    super(network, config.infuraApiKey)
  }

  static getUrl(): ConnectionInfo {
    return {
      allowGzip: true,
      url: `${config.infuraApiUrl}/${config.infuraApiKey}`,
    }
  }
}

But again it wouldn't be possible to inject the url since the getUrl is static.

Is there a better way to allow us to proxy our requests? If not, what is the reasoning behind the current design of the providers and what do you think of updating the providers so that a URL can be passed in via their constructors?