internal/resolver: introduce a new delegating resolver to handle both target URI and proxy address resolution

internal/proxyattributes/proxyattributes.go

@@ -31,15 +31,13 @@ type keyType string
 const proxyOptionsKey = keyType("grpc.resolver.delegatingresolver.proxyOptions")
 
 // Options holds the proxy connection details needed during the CONNECT
-// handshake. It includes the user information and the connect address.
+// handshake.
 type Options struct {
 	User        url.Userinfo
 	ConnectAddr string
 }
 
-// Set returns a copy of addr with attributes containing the provided user
-// and connect address, which are needed during the CONNECT handshake for a
-// proxy connection.
+// Set returns a copy of addr with opts set in its attributes.
 func Set(addr resolver.Address, opts Options) resolver.Address {
 	addr.Attributes = addr.Attributes.WithValue(proxyOptionsKey, opts)
 	return addr

internal/resolver/delegatingresolver/delegatingresolver.go

@@ -188,12 +188,6 @@ func (r *delegatingResolver) updateClientConnStateLocked() error {
 		return nil
 	}
 
-	// Update curState to include target resolver's state information, such as
-	// the service config, provided by the target resolver. This ensures
-	// curState contains all necessary information when passed to UpdateState.
-	// The state update is only sent after both the target and proxy resolvers
-	// have sent their updates, and curState has been updated with the combined
-	// addresses.
 	curState := *r.targetResolverState
 	// If multiple resolved proxy addresses are present, we send only the
 	// unresolved proxy host and let net.Dial handle the proxy host name
@@ -242,6 +236,7 @@ func (r *delegatingResolver) updateClientConnStateLocked() error {
 		}
 		endpoints = append(endpoints, resolver.Endpoint{Addresses: addrs})
 	}
+	// Use the targetResolverState for its service config and attributes contents.
 	curState.Addresses = addresses
 	curState.Endpoints = endpoints
 	return r.cc.UpdateState(curState)
@@ -259,12 +254,17 @@ func (r *delegatingResolver) updateProxyResolverState(state resolver.State) erro
 		logger.Infof("Addresses received from proxy resolver: %s", state.Addresses)
 	}
 	if len(state.Endpoints) > 0 {
+
+		// We expect exactly one address per endpoint because the proxy
+		// resolver uses "dns" resolution.
 		r.proxyAddrs = make([]resolver.Address, 0, len(state.Endpoints))
 		for _, endpoint := range state.Endpoints {
 			r.proxyAddrs = append(r.proxyAddrs, endpoint.Addresses...)
 		}
-	} else {
+	} else if state.Addresses != nil {
 		r.proxyAddrs = state.Addresses
+	} else {
+		r.proxyAddrs = []resolver.Address{} // ensure proxyAddrs is non-nil to indicate an update has been received
 	}
 	err := r.updateClientConnStateLocked()
 	// Another possible approach was to block until updates are received from