Error client: return the error via Brand()

capability.go

@@ -843,6 +843,9 @@ type errorClient struct {
 // ErrorClient returns a Client that always returns error e.
 // An ErrorClient does not need to be released: it is a sentinel like a
 // nil Client.
+//
+// The returned client's State() method returns a State with its
+// Brand.Value set to e.
 func ErrorClient(e error) *Client {
 	if e == nil {
 		panic("ErrorClient(nil)")
@@ -870,7 +873,7 @@ func (ec errorClient) Recv(_ context.Context, r Recv) PipelineCaller {
 }
 
 func (ec errorClient) Brand() Brand {
-	return Brand{}
+	return Brand{Value: ec.e}
 }
 
 func (ec errorClient) Shutdown() {

rpc/rpc.go

@@ -1181,12 +1181,23 @@ func (c *Conn) isLocalClient(client *capnp.Client) bool {
 	if client == nil {
 		return false
 	}
-	if ic, ok := client.State().Brand.Value.(*importClient); ok {
+
+	bv := client.State().Brand.Value
+
+	if ic, ok := bv.(*importClient); ok {
 		// If the connections are different, we must be proxying
 		// it, so as far as this connection is concerned, it lives
 		// on our side.
 		return ic.c != c
 	}
+
+	if _, ok := bv.(error); ok {
+		// Returned by capnp.ErrorClient. No need to treat this as
+		// local; all methods will just return the error anyway,
+		// so violating E-order will have no effect on the results.
+		return false
+	}
+
 	// TODO: We should return false for results from pending remote calls
 	// as well. But that can wait until sendCap() actually emits
 	// CapDescriptors of type receiverAnswer, since until then it won't