Implement handling of Resolve messages. Though currently they are never sent.

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

@@ -278,10 +278,9 @@ TEST_F(RpcTest, Basic) {
   request1.setJ(true);
   auto promise1 = request1.send();
 
-  auto request2 = client.bazRequest();
-  initTestMessage(request2.initS());
-  auto promise2 = request2.send();
-
+  // We used to call bar() after baz(), hence the numbering, but this masked the case where the
+  // RPC system actually disconnected on bar() (thus returning an exception, which we decided
+  // was expected).
   bool barFailed = false;
   auto request3 = client.barRequest();
   auto promise3 = loop.there(request3.send(),
@@ -291,6 +290,10 @@ TEST_F(RpcTest, Basic) {
         barFailed = true;
       });
 
+  auto request2 = client.bazRequest();
+  initTestMessage(request2.initS());
+  auto promise2 = request2.send();
+
   EXPECT_EQ(0, restorer.callCount);
 
   auto response1 = loop.wait(kj::mv(promise1));

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

@@ -241,6 +241,7 @@ public:
         disconnectFulfiller(kj::mv(disconnectFulfiller)),
         tasks(eventLoop, *this) {
     tasks.add(messageLoop());
+    tables.getWithoutLock().resolutionChainTail = kj::refcounted<ResolutionChain>();
   }
 
   kj::Own<const ClientHook> restore(ObjectPointer::Reader objectId) {
@@ -255,7 +256,8 @@ public:
       // We need a dummy paramCaps since null normally indicates that the question has completed.
       question.paramCaps = kj::heap<CapInjectorImpl>(*this);
 
-      questionRef = kj::refcounted<QuestionRef>(*this, questionId, kj::mv(paf.fulfiller));
+      questionRef = kj::refcounted<QuestionRef>(*this, questionId, kj::mv(paf.fulfiller),
+                                                kj::addRef(*lock->resolutionChainTail));
       question.selfRef = *questionRef;
 
       paf.promise.attach(kj::addRef(*questionRef));
@@ -347,6 +349,8 @@ public:
   }
 
 private:
+  class ResolutionChain;
+  class RpcClient;
   class ImportClient;
   class PromiseClient;
   class CapInjectorImpl;
@@ -413,9 +417,14 @@ private:
     Import& operator=(Import&&) = default;
     // If we don't explicitly write all this, we get some stupid error deep in STL.
 
-    kj::Maybe<ImportClient&> client;
+    kj::Maybe<ImportClient&> importClient;
     // Becomes null when the import is destroyed.
 
+    kj::Maybe<RpcClient&> appClient;
+    // Either a copy of importClient, or, in the case of promises, the wrapping PromiseClient.
+    // Becomes null when it is discarded *or* when the import is destroyed (e.g. the promise is
+    // resolved and the import is no longer needed).
+
     kj::Maybe<kj::Own<kj::PromiseFulfiller<kj::Own<const ClientHook>>>> promiseFulfiller;
     // If non-null, the import is a promise.
   };
@@ -438,6 +447,12 @@ private:
     std::unordered_map<const ClientHook*, ExportId> exportsByCap;
     // Maps already-exported ClientHook objects to their ID in the export table.
 
+    kj::Own<ResolutionChain> resolutionChainTail;
+    // The end of the resolution chain.  This node actually isn't filled in yet, but it will be
+    // filled in and the chain will be extended with a new node any time a `Resolve` is received.
+    // CapExtractors need to hold a ref to resolutionChainTail to prevent resolved promises from
+    // becoming invalid while the app is still processing the message.  See `ResolutionChain`.
+
     kj::Maybe<kj::Exception> networkException;
     // If the connection has failed, this is the exception describing the failure.  All future
     // calls should throw this exception.
@@ -446,6 +461,68 @@ private:
 
   kj::TaskSet tasks;
 
+  // =====================================================================================
+
+  class ResolutionChain: public kj::Refcounted {
+    // A chain of pending promise resolutions which may affect messages that are still being
+    // processed.
+    //
+    // When a `Resolve` message comes in, we can't just handle it and then release the original
+    // promise import all at once, because it's possible that the application is still processing
+    // the `params` or `results` from a previous call, and that it will encounter an instance of
+    // the promise as it does.  We need to hold off on the release until we've actually gotten
+    // through all outstanding messages.
+    //
+    // To that end, we have the resolution chain.  Each time a `CapExtractorImpl` is created --
+    // representing a message to be consumed by the application -- it takes a reference to the
+    // current end of the chain.  When a `Resolve` message arrives, it is added to the end of the
+    // chain, and thus all `CapExtractorImpl`s that exist at that point now hold a reference to it,
+    // but new `CapExtractorImpl`s will not.  Once all references are dropped, the original promise
+    // can be released.
+    //
+    // The connection state actually holds one instance of ResolutionChain which doesn't yet have
+    // a promise attached to it, representing the end of the chain.  This is what allows a new
+    // resolution to be "added to the end" and have existing `CapExtractorImpl`s suddenly be
+    // holding a reference to it.
+
+  public:
+    kj::Own<ResolutionChain> add(ExportId importId,
+                                 kj::Own<const ClientHook>&& replacement) {
+      // Add the a new resolution to the chain.  Returns the new end-of-chain.
+
+      this->importId = importId;
+      this->replacement = kj::mv(replacement);
+
+      auto result = kj::refcounted<ResolutionChain>();
+      next = kj::addRef(*result);
+
+      filled = true;
+      return kj::mv(result);
+    }
+
+    kj::Maybe<kj::Own<const ClientHook>> find(ExportId importId) const {
+      // Look for the given import ID in the resolution chain.
+
+      const ResolutionChain* ptr = this;
+
+      while (ptr->filled) {
+        if (ptr->importId == importId) {
+          return ptr->replacement->addRef();
+        }
+        ptr = ptr->next;
+      }
+
+      return nullptr;
+    }
+
+  private:
+    kj::Own<const ResolutionChain> next;
+
+    bool filled = false;
+    ExportId importId;
+    kj::Own<const ClientHook> replacement;
+  };
+
   // =====================================================================================
   // ClientHook implementations
 
@@ -547,10 +624,11 @@ private:
         // Remove self from the import table, if the table is still pointing at us.  (It's possible
         // that another thread attempted to obtain this import just as the destructor started, in
         // which case that other thread will have constructed a new ImportClient and placed it in
-        // the import table.)
+        // the import table.  Therefore, we must actually verify that the import table points at
+        // this object.)
         auto lock = connectionState->tables.lockExclusive();
         KJ_IF_MAYBE(import, lock->imports.find(importId)) {
-          KJ_IF_MAYBE(i, import->client) {
+          KJ_IF_MAYBE(i, import->importClient) {
             if (i == this) {
               lock->imports.erase(importId);
             }
@@ -564,17 +642,9 @@ private:
       }
     }
 
-    kj::Maybe<kj::Own<ImportClient>> tryAddRemoteRef() {
-      // Add a new RemoteRef and return a new ref to this client representing it.  Returns null
-      // if this client is being deleted in another thread, in which case the caller should
-      // construct a new one.
-
-      KJ_IF_MAYBE(ref, kj::tryAddRef(*this)) {
-        ++remoteRefcount;
-        return kj::mv(*ref);
-      } else {
-        return nullptr;
-      }
+    void addRemoteRef() {
+      // Add a new RemoteRef and return a new ref to this client representing it.
+      ++remoteRefcount;
     }
 
     kj::Maybe<ExportId> writeDescriptor(
@@ -673,9 +743,11 @@ private:
   public:
     PromiseClient(const RpcConnectionState& connectionState,
                   kj::Own<const ClientHook> initial,
-                  kj::Promise<kj::Own<const ClientHook>> eventual)
+                  kj::Promise<kj::Own<const ClientHook>> eventual,
+                  kj::Maybe<ExportId> importId)
         : RpcClient(connectionState),
           inner(kj::mv(initial)),
+          importId(importId),
           fork(connectionState.eventLoop.fork(kj::mv(eventual))),
           resolveSelfPromise(connectionState.eventLoop.there(fork.addBranch(),
               [this](kj::Own<const ClientHook>&& resolution) {
@@ -692,6 +764,23 @@ private:
       resolveSelfPromise.eagerlyEvaluate(connectionState.eventLoop);
     }
 
+    ~PromiseClient() noexcept(false) {
+      KJ_IF_MAYBE(id, importId) {
+        // This object is representing an import promise.  That means the import table may still
+        // contain a pointer back to it.  Remove that pointer.  Note that we have to verify that
+        // the import still exists and the pointer still points back to this object because this
+        // object may actually outlive the import.
+        auto lock = connectionState->tables.lockExclusive();
+        KJ_IF_MAYBE(import, lock->imports.find(*id)) {
+          KJ_IF_MAYBE(c, import->appClient) {
+            if (c == this) {
+              import->appClient = nullptr;
+            }
+          }
+        }
+      }
+    }
+
     kj::Maybe<ExportId> writeDescriptor(
         rpc::CapDescriptor::Builder descriptor, Tables& tables) const override {
       auto cap = inner.lockExclusive()->get()->addRef();
@@ -716,6 +805,7 @@ private:
 
   private:
     kj::MutexGuarded<kj::Own<const ClientHook>> inner;
+    kj::Maybe<ExportId> importId;
     kj::ForkedPromise<kj::Own<const ClientHook>> fork;
 
     // Keep this last, because the continuation uses *this, so it should be destroyed first to
@@ -784,8 +874,10 @@ private:
     // Reads CapDescriptors from a received message.
 
   public:
-    CapExtractorImpl(const RpcConnectionState& connectionState)
-        : connectionState(connectionState) {}
+    CapExtractorImpl(const RpcConnectionState& connectionState,
+                     kj::Own<const ResolutionChain> resolutionChain)
+        : connectionState(connectionState),
+          resolutionChain(kj::mv(resolutionChain)) {}
 
     ~CapExtractorImpl() noexcept(false) {
       KJ_ASSERT(retainedCaps.getWithoutLock().size() == 0,
@@ -806,9 +898,27 @@ private:
       return (count * sizeof(ExportId) + (sizeof(ExportId) - 1)) / sizeof(word);
     }
 
-    Orphan<List<ExportId>> finalizeRetainedCaps(Orphanage orphanage) {
-      // Called on finalization, when the lock is no longer needed.
+    struct FinalizedRetainedCaps {
+      // List of capabilities extracted from this message which are to be retained past the
+      // message's release.
+
+      Orphan<List<ExportId>> exportList;
+      // List of export IDs, to be placed in the Return/Finish message.
+
+      kj::Vector<kj::Own<const ClientHook>> refs;
+      // List of ClientHooks which need to be kept live until the message is sent, to prevent
+      // their premature release.
+    };
+
+    FinalizedRetainedCaps finalizeRetainedCaps(Orphanage orphanage) {
+      // Build the list of export IDs found in this message which are to be retained past the
+      // message's release.
+      //
+      // `capsToKeepUntilSend` will be filled in with
+
+      // Called on finalization, when all extractions have ceased, so we can skip the lock.
       kj::Vector<ExportId> retainedCaps = kj::mv(this->retainedCaps.getWithoutLock());
+      kj::Vector<kj::Own<const ClientHook>> refs(retainedCaps.size());
 
       auto lock = connectionState.tables.lockExclusive();
 
@@ -816,11 +926,13 @@ private:
       for (ExportId importId: retainedCaps) {
         // Check if the import still exists under this ID.
         KJ_IF_MAYBE(import, lock->imports.find(importId)) {
-          KJ_IF_MAYBE(i, import->client) {
-            if (i->tryAddRemoteRef() != nullptr) {
-              // Import indeed still exists!  We are responsible for retaining it.
-              // TODO(now):  Do we need to hold on to the ref that tryAddRemoteRef() returned?
+          KJ_IF_MAYBE(ic, import->importClient) {
+            KJ_IF_MAYBE(ref, kj::tryAddRef(*ic)) {
+              // Import indeed still exists!  We'll return it in the retained caps, which means it
+              // now has a new remote ref.
+              ic->addRemoteRef();
               *actualRetained++ = importId;
+              refs.add(kj::mv(*ref));
             }
           }
         }
@@ -836,24 +948,65 @@ private:
         resultBuilder.set(count++, *iter);
       }
 
-      return kj::mv(result);
+      return FinalizedRetainedCaps { kj::mv(result), kj::mv(refs) };
+    }
+
+    static kj::Own<const ClientHook> extractCapAndAddRef(
+        const RpcConnectionState& connectionState, Tables& lockedTables,
+        rpc::CapDescriptor::Reader descriptor) {
+      // Interpret the given capability descriptor and, if it is an import, immediately give it
+      // a remote ref.  This is called when interpreting messages that have a CapabilityDescriptor
+      // but do not have a corresponding response message where a list of retained caps is given.
+      // In these cases, the cap is always assumed retained, and must be explicitly released.
+      // For example, the 'Resolve' message contains a capability which is presumed to be retained.
+
+      return extractCapImpl(connectionState, lockedTables, descriptor,
+                            *lockedTables.resolutionChainTail, nullptr);
     }
 
     // implements CapDescriptor ------------------------------------------------
 
     kj::Own<const ClientHook> extractCap(rpc::CapDescriptor::Reader descriptor) const override {
+      return extractCapImpl(connectionState, *connectionState.tables.lockExclusive(), descriptor,
+                            *resolutionChain, retainedCaps);
+    }
+
+  private:
+    const RpcConnectionState& connectionState;
+
+    kj::Own<const ResolutionChain> resolutionChain;
+    // Reference to the resolution chain, which prevents any promises that might be extracted from
+    // this message from being invalidated by `Resolve` messages before extraction is finished.
+    // Simply holding on to the chain keeps the import table entries valid.
+
+    kj::MutexGuarded<kj::Vector<ExportId>> retainedCaps;
+    // Imports which we are responsible for retaining, should they still exist at the time that
+    // this message is released.
+
+    static kj::Own<const ClientHook> extractCapImpl(
+        const RpcConnectionState& connectionState, Tables& tables,
+        rpc::CapDescriptor::Reader descriptor,
+        const ResolutionChain& resolutionChain,
+        kj::Maybe<const kj::MutexGuarded<kj::Vector<ExportId>>&> retainedCaps) {
       switch (descriptor.which()) {
         case rpc::CapDescriptor::SENDER_HOSTED:
         case rpc::CapDescriptor::SENDER_PROMISE: {
           ExportId importId = descriptor.getSenderHosted();
 
-          auto lock = connectionState.tables.lockExclusive();
+          // First check to see if this import ID is a promise that has resolved since when this
+          // message was received.  In this case, the original import ID will already have been
+          // dropped and could even have been reused for another capability.  Luckily, the
+          // resolution chain holds the capability we actually want.
+          KJ_IF_MAYBE(resolution, resolutionChain.find(importId)) {
+            return kj::mv(*resolution);
+          }
 
-          auto& import = lock->imports[importId];
-          KJ_IF_MAYBE(i, import.client) {
+          // No recent resolutions.  Check the import table then.
+          auto& import = tables.imports[importId];
+          KJ_IF_MAYBE(c, import.appClient) {
             // The import is already on the table, but it could be being deleted in another
             // thread.
-            KJ_IF_MAYBE(ref, kj::tryAddRef(*i)) {
+            KJ_IF_MAYBE(ref, kj::tryAddRef(*c)) {
               // We successfully grabbed a reference to the import without it being deleted in
               // another thread.  Since this import already exists, we don't have to take
               // responsibility for retaining it.  We can just return the existing object and
@@ -865,9 +1018,17 @@ private:
           // No import for this ID exists currently, so create one.
           kj::Own<ImportClient> importClient =
               kj::refcounted<ImportClient>(connectionState, importId);
-          import.client = *importClient;
+          import.importClient = *importClient;
+
+          KJ_IF_MAYBE(rc, retainedCaps) {
+            // We need to retain this import later if it still exists.
+            rc->lockExclusive()->add(importId);
+          } else {
+            // Automatically increment the refcount.
+            importClient->addRemoteRef();
+          }
 
-          kj::Own<ClientHook> result;
+          kj::Own<RpcClient> result;
           if (descriptor.which() == rpc::CapDescriptor::SENDER_PROMISE) {
             // TODO(now):  Check for pending `Resolve` messages replacing this import ID, and if
             //   one exists, use that client instead.
@@ -876,29 +1037,26 @@ private:
             import.promiseFulfiller = kj::mv(paf.fulfiller);
             paf.promise.attach(kj::addRef(*importClient));
             result = kj::refcounted<PromiseClient>(
-                connectionState, kj::mv(importClient), kj::mv(paf.promise));
+                connectionState, kj::mv(importClient), kj::mv(paf.promise), importId);
           } else {
             result = kj::mv(importClient);
           }
 
-          // Note that we need to retain this import later if it still exists.
-          retainedCaps.lockExclusive()->add(importId);
+          import.appClient = *result;
 
           return kj::mv(result);
         }
 
         case rpc::CapDescriptor::RECEIVER_HOSTED: {
-          auto lock = connectionState.tables.lockExclusive();  // TODO(perf): shared?
-          KJ_IF_MAYBE(exp, lock->exports.find(descriptor.getReceiverHosted())) {
+          KJ_IF_MAYBE(exp, tables.exports.find(descriptor.getReceiverHosted())) {
             return exp->clientHook->addRef();
           }
           return newBrokenCap("invalid 'receiverHosted' export ID");
         }
 
         case rpc::CapDescriptor::RECEIVER_ANSWER: {
-          auto lock = connectionState.tables.lockExclusive();
           auto promisedAnswer = descriptor.getReceiverAnswer();
-          KJ_IF_MAYBE(answer, lock->answers.find(promisedAnswer.getQuestionId())) {
+          KJ_IF_MAYBE(answer, tables.answers.find(promisedAnswer.getQuestionId())) {
             if (answer->active) {
               KJ_IF_MAYBE(pipeline, answer->pipeline) {
                 KJ_IF_MAYBE(ops, toPipelineOps(promisedAnswer.getTransform())) {
@@ -920,13 +1078,6 @@ private:
           return newBrokenCap("unknown CapDescriptor type");
       }
     }
-
-  private:
-    const RpcConnectionState& connectionState;
-
-    kj::MutexGuarded<kj::Vector<ExportId>> retainedCaps;
-    // Imports which we are responsible for retaining, should they still exist at the time that
-    // this message is released.
   };
 
   // -----------------------------------------------------------------
@@ -944,7 +1095,6 @@ private:
 
       if (lock->networkException == nullptr) {
         for (auto exportId: exports) {
-          KJ_DBG(&connectionState, exportId);
           auto& exp = KJ_ASSERT_NONNULL(lock->exports.find(exportId));
           if (--exp.refcount == 0) {
             clientsToRelease.add(kj::mv(exp.clientHook));
@@ -1042,21 +1192,24 @@ private:
 
   public:
     inline QuestionRef(const RpcConnectionState& connectionState, QuestionId id,
-                       kj::Own<kj::PromiseFulfiller<kj::Own<const RpcResponse>>> fulfiller)
+                       kj::Own<kj::PromiseFulfiller<kj::Own<const RpcResponse>>> fulfiller,
+                       kj::Own<const ResolutionChain> resolutionChain)
         : connectionState(kj::addRef(connectionState)), id(id), fulfiller(kj::mv(fulfiller)),
-          resultCaps(connectionState) {}
+          resultCaps(connectionState, kj::mv(resolutionChain)) {}
 
     ~QuestionRef() {
       // Send the "Finish" message.
-      auto message = connectionState->connection->newOutgoingMessage(
-          messageSizeHint<rpc::Finish>() + resultCaps.retainedListSizeHint(true));
-      auto builder = message->getBody().getAs<rpc::Message>().initFinish();
-      builder.setQuestionId(id);
-
-      builder.adoptRetainedCaps(resultCaps.finalizeRetainedCaps(
-          Orphanage::getForMessageContaining(builder)));
+      {
+        auto message = connectionState->connection->newOutgoingMessage(
+            messageSizeHint<rpc::Finish>() + resultCaps.retainedListSizeHint(true));
+        auto builder = message->getBody().getAs<rpc::Message>().initFinish();
+        builder.setQuestionId(id);
 
-      message->send();
+        auto retainedCaps = resultCaps.finalizeRetainedCaps(
+            Orphanage::getForMessageContaining(builder));
+        builder.adoptRetainedCaps(kj::mv(retainedCaps.exportList));
+        message->send();
+      }
 
       // Check if the question has returned and, if so, remove it from the table.
       // Remove question ID from the table.  Must do this *after* sending `Finish` to ensure that
@@ -1157,7 +1310,8 @@ private:
           question.paramCaps = kj::mv(injector);
 
           questionRef = kj::refcounted<QuestionRef>(
-              *connectionState, questionId, kj::mv(paf.fulfiller));
+              *connectionState, questionId, kj::mv(paf.fulfiller),
+              kj::addRef(*lock->resolutionChainTail));
           question.selfRef = *questionRef;
 
           message->send();
@@ -1170,7 +1324,7 @@ private:
       auto forkedPromise = connectionState->eventLoop.fork(kj::mv(promise));
 
       auto appPromise = forkedPromise.addBranch().thenInAnyThread(
-          [](kj::Own<const RpcResponse>&& response) {
+          [=](kj::Own<const RpcResponse>&& response) {
             auto reader = response->getResults();
             return Response<ObjectPointer>(reader, kj::mv(response));
           });
@@ -1244,7 +1398,7 @@ private:
               }));
 
         return kj::refcounted<PromiseClient>(
-            *connectionState, kj::mv(pipelineClient), kj::mv(resolutionPromise));
+            *connectionState, kj::mv(pipelineClient), kj::mv(resolutionPromise), nullptr);
       } else if (lock->is<Resolved>()) {
         return lock->get<Resolved>()->getResults().getPipelinedCap(ops);
       } else {
@@ -1346,11 +1500,12 @@ private:
   class RpcCallContext final: public CallContextHook, public kj::Refcounted {
   public:
     RpcCallContext(const RpcConnectionState& connectionState, QuestionId questionId,
-                   kj::Own<IncomingRpcMessage>&& request, const ObjectPointer::Reader& params)
+                   kj::Own<IncomingRpcMessage>&& request, const ObjectPointer::Reader& params,
+                   kj::Own<const ResolutionChain> resolutionChain)
         : connectionState(kj::addRef(connectionState)),
           questionId(questionId),
           request(kj::mv(request)),
-          requestCapExtractor(connectionState),
+          requestCapExtractor(connectionState, kj::mv(resolutionChain)),
           requestCapContext(requestCapExtractor),
           params(requestCapContext.imbue(params)),
           returnMessage(nullptr) {}
@@ -1360,8 +1515,9 @@ private:
         if (response == nullptr) getResults(1);  // force initialization of response
 
         returnMessage.setQuestionId(questionId);
-        returnMessage.adoptRetainedCaps(requestCapExtractor.finalizeRetainedCaps(
-            Orphanage::getForMessageContaining(returnMessage)));
+        auto retainedCaps = requestCapExtractor.finalizeRetainedCaps(
+            Orphanage::getForMessageContaining(returnMessage));
+        returnMessage.adoptRetainedCaps(kj::mv(retainedCaps.exportList));
 
         KJ_ASSERT_NONNULL(response)->send();
       }
@@ -1374,8 +1530,9 @@ private:
         auto builder = message->getBody().initAs<rpc::Message>().initReturn();
 
         builder.setQuestionId(questionId);
-        builder.adoptRetainedCaps(requestCapExtractor.finalizeRetainedCaps(
-            Orphanage::getForMessageContaining(builder)));
+        auto retainedCaps = requestCapExtractor.finalizeRetainedCaps(
+            Orphanage::getForMessageContaining(builder));
+        builder.adoptRetainedCaps(kj::mv(retainedCaps.exportList));
         fromException(exception, builder.initException());
 
         message->send();
@@ -1388,8 +1545,9 @@ private:
         auto builder = message->getBody().initAs<rpc::Message>().initReturn();
 
         builder.setQuestionId(questionId);
-        builder.adoptRetainedCaps(requestCapExtractor.finalizeRetainedCaps(
-            Orphanage::getForMessageContaining(builder)));
+        auto retainedCaps = requestCapExtractor.finalizeRetainedCaps(
+            Orphanage::getForMessageContaining(builder));
+        builder.adoptRetainedCaps(kj::mv(retainedCaps.exportList));
         builder.setCanceled();
 
         message->send();
@@ -1626,7 +1784,7 @@ private:
         break;
 
       case rpc::Message::RESOLVE:
-        // TODO(now)
+        handleResolve(reader.getResolve());
         break;
 
       case rpc::Message::RELEASE:
@@ -1720,8 +1878,11 @@ private:
     }
 
     QuestionId questionId = call.getQuestionId();
+    // Note:  resolutionChainTail couldn't possibly be changing here because we only handle one
+    //   message at a time, so we can hold off locking the tables for a bit longer.
     auto context = kj::refcounted<RpcCallContext>(
-        *this, questionId, kj::mv(message), call.getParams());
+        *this, questionId, kj::mv(message), call.getParams(),
+        kj::addRef(*tables.getWithoutLock().resolutionChainTail));
     auto promiseAndPipeline = capability->call(
         call.getInterfaceId(), call.getMethodId(), context->addRef());
 
@@ -1753,6 +1914,12 @@ private:
             contextPtr->sendReturn();
           }, [contextPtr](kj::Exception&& exception) {
             contextPtr->sendErrorReturn(kj::mv(exception));
+          }).then([]() {
+            // Success.
+          }, [&](kj::Exception&& exception) {
+            // We never actually wait on `asyncOp` so we need to manually report exceptions.
+            // TODO(cleanup):  Perhaps there should be a better, more-automated approach to this?
+            taskFailed(kj::mv(exception));
           });
       answer.asyncOp.attach(kj::mv(context));
       answer.asyncOp.eagerlyEvaluate(eventLoop);
@@ -1851,6 +2018,51 @@ private:
   // ---------------------------------------------------------------------------
   // Level 1
 
+  void handleResolve(const rpc::Resolve::Reader& resolve) {
+    kj::Own<ResolutionChain> oldResolutionChainTail;  // must be freed outside of lock
+
+    auto lock = tables.lockExclusive();
+
+    kj::Own<const ClientHook> replacement;
+
+    // Extract the replacement capability.
+    switch (resolve.which()) {
+      case rpc::Resolve::CAP:
+        replacement = CapExtractorImpl::extractCapAndAddRef(*this, *lock, resolve.getCap());
+        break;
+
+      case rpc::Resolve::EXCEPTION:
+        replacement = newBrokenCap(toException(resolve.getException()));
+        break;
+
+      case rpc::Resolve::CANCELED:
+        // Right, this can't possibly affect anything, then.
+        //
+        // TODO(now):  Am I doing something wrong or is this not needed?
+        return;
+
+      default:
+        KJ_FAIL_REQUIRE("Unknown 'Resolve' type.") { return; }
+    }
+
+    // Extend the resolution chain.
+    auto oldTail = kj::mv(lock->resolutionChainTail);
+    lock->resolutionChainTail = oldTail->add(resolve.getPromiseId(), kj::mv(replacement));
+    lock.release();  // in case oldTail is destroyed
+
+    // If the import is on the table, fulfill it.
+    KJ_IF_MAYBE(import, lock->imports.find(resolve.getPromiseId())) {
+      KJ_IF_MAYBE(fulfiller, import->promiseFulfiller) {
+        // OK, this is in fact an unfulfilled promise!
+        fulfiller->get()->fulfill(kj::mv(replacement));
+      } else if (import->importClient != nullptr) {
+        // It appears this is a valid entry on the import table, but was not expected to be a
+        // promise.
+        KJ_FAIL_REQUIRE("Got 'Resolve' for a non-promise import.") { break; }
+      }
+    }
+  }
+
   // ---------------------------------------------------------------------------
   // Level 2