DNS can omit URIs which results in confusing errors for services created with a single node.

[felixdesouza]

What happened?

An internal service created a client for a single node via DialogueClients.create(interface, clientConfiguration) where clientConfiguration has a single configured URI. They had validation to make sure that they would only ever pass in a single node to the ClientConfiguration. However, they ended up seeing errors like:

com.palantir.logsafe.exceptions.SafeIllegalStateException: There are no URIs configured to handle requests
	at com.palantir.dialogue.core.ZeroUriNodeSelectionChannel.maybeExecute(ZeroUriNodeSelectionChannel.java:44)
...

Arguably they can probably use the perHost feature in #2192 but that's relatively new here.

Tracing through:

• DialogueClients#create(Class<T> clientInterface, ClientConfiguration clientConfiguration) -> LegacyConstruction#getNonReloading(Class<T> clientInterface, ClientConfiguration clientConfiguration)
• LegacyConstruction#getNonReloading(Class<T> clientInterface, ClientConfiguration clientConfiguration) uses ReloadingFactory configured with an empty reloadable block:

  dialogue/dialogue-clients/src/main/java/com/palantir/dialogue/clients/LegacyConstruction.java

  Lines 27 to 29 in b24d0f1

  	private static final ReloadingFactory FACTORY = DialogueClients.create(
  	Refreshable.only(ServicesConfigBlock.empty()))
  	.withHostEventsSink(NoOpHostEventsSink.INSTANCE);

• ReloadingClientFactory#getNonReloading -> ReloadingClientFactory#getNonReloadingChannel
• ReloadingClientFactory#getNonReloadingChannel underneath uses a LiveReloadingChannel that's configured to poll for DNS changes in the background via DnsSupport.
• LiveReloadingChannel takes in a Refreshable<DnsResolutionResults<ServiceConfiguration>> and instead of the URIs flowing from the clientConfiguration, they instead flow from the DnsResolutionResults via getTargetUris.
• going inside getTargetUris, the only way to get an empty set of URIs is to have DNS enabled and have no parse errors.
• DialogueChannel by default uses the uris from the clientConfiguration, but if you do specify uris it uses that. So if you resolve nothing, you will have no uris.

What did you want to happen?

If I can't connect, fail with a more familiar exception i.e. ConnectException or UnknownHostException.