Fix RPC loopback bootstrap().

When VatNetwork::connect() returns nullptr, it means that the caller is trying to connect to itself.

rpc-test.c++ failed to test this in two ways:
- The test VatNetwork's connect() never returned null.
- There was no test case for loopback connect.

As a result, the code to handle loopback in rpc.c++ had bitrotted. It failed to handle the new bootstrap mechanism introduced in v0.5, and instead only implemented the restorer mechanism from 0.4.

c++/src/capnp/rpc-test.c++

@@ -198,7 +198,7 @@ typedef VatNetwork<
 
 class TestNetworkAdapter final: public TestNetworkAdapterBase {
 public:
-  TestNetworkAdapter(TestNetwork& network): network(network) {}
+  TestNetworkAdapter(TestNetwork& network, kj::StringPtr self): network(network), self(self) {}
 
   ~TestNetworkAdapter() {
     kj::Exception exception = KJ_EXCEPTION(FAILED, "Network was destroyed.");
@@ -362,6 +362,10 @@ public:
   };
 
   kj::Maybe<kj::Own<Connection>> connect(test::TestSturdyRefHostId::Reader hostId) override {
+    if (hostId.getHost() == self) {
+      return nullptr;
+    }
+
     TestNetworkAdapter& dst = KJ_REQUIRE_NONNULL(network.find(hostId.getHost()));
 
     auto iter = connections.find(&dst);
@@ -400,6 +404,7 @@ public:
 
 private:
   TestNetwork& network;
+  kj::StringPtr self;
   uint sent = 0;
   uint received = 0;
 
@@ -411,7 +416,7 @@ private:
 TestNetwork::~TestNetwork() noexcept(false) {}
 
 TestNetworkAdapter& TestNetwork::add(kj::StringPtr name) {
-  return *(map[name] = kj::heap<TestNetworkAdapter>(*this));
+  return *(map[name] = kj::heap<TestNetworkAdapter>(*this, name));
 }
 
 // =======================================================================================
@@ -456,6 +461,12 @@ struct TestContext {
         serverNetwork(network.add("server")),
         rpcClient(makeRpcClient(clientNetwork)),
         rpcServer(makeRpcServer(serverNetwork, restorer)) {}
+  TestContext(Capability::Client bootstrap)
+      : waitScope(loop),
+        clientNetwork(network.add("client")),
+        serverNetwork(network.add("server")),
+        rpcClient(makeRpcClient(clientNetwork)),
+        rpcServer(makeRpcServer(serverNetwork, bootstrap)) {}
   TestContext(Capability::Client bootstrap,
               RealmGateway<test::TestSturdyRef, Text>::Client gateway)
       : waitScope(loop),
@@ -1264,6 +1275,26 @@ TEST(Rpc, RealmGatewayImportExport) {
   EXPECT_EQ("foo", response.getSturdyRef());
 }
 
+KJ_TEST("loopback bootstrap()") {
+  int callCount = 0;
+  test::TestInterface::Client bootstrap = kj::heap<TestInterfaceImpl>(callCount);
+
+  MallocMessageBuilder hostIdBuilder;
+  auto hostId = hostIdBuilder.getRoot<test::TestSturdyRefHostId>();
+  hostId.setHost("server");
+
+  TestContext context(bootstrap);
+  auto client = context.rpcServer.bootstrap(hostId).castAs<test::TestInterface>();
+
+  auto request = client.fooRequest();
+  request.setI(123);
+  request.setJ(true);
+  auto response = request.send().wait(context.waitScope);
+
+  KJ_EXPECT(response.getX() == "foo");
+  KJ_EXPECT(callCount == 1);
+}
+
 }  // namespace
 }  // namespace _ (private)
 }  // namespace capnp

c++/src/capnp/rpc.c++

@@ -2999,11 +2999,16 @@ public:
     KJ_IF_MAYBE(connection, network.baseConnect(vatId)) {
       auto& state = getConnectionState(kj::mv(*connection));
       return Capability::Client(state.restore(objectId));
+    } else if (objectId.isNull()) {
+      // Turns out `vatId` refers to ourselves, so we can also pass it as the client ID for
+      // baseCreateFor().
+      return bootstrapFactory.baseCreateFor(vatId);
     } else KJ_IF_MAYBE(r, restorer) {
       return r->baseRestore(objectId);
     } else {
       return Capability::Client(newBrokenCap(
-          "SturdyRef referred to a local object but there is no local SturdyRef restorer."));
+          "This vat only supports a bootstrap interface, not the old Cap'n-Proto-0.4-style "
+          "named exports."));
     }
   }