feat: construct RpcPromise from a Promise

.changeset/rpc-promise-from-promise.md

@@ -0,0 +1,5 @@
+---
+"capnweb": minor
+---
+
+`RpcPromise` can now be constructed from a `Promise`: pipelined calls queue in order until it settles, so you can publish a capability that doesn't exist yet.

README.md

@@ -267,6 +267,28 @@ let profile = await api.getUserProfile(user.id);
 
 Whenever an `RpcPromise` is passed in the parameters to an RPC, or returned as part of the result, the promise will be replaced with its resolution before delivery to the receiving application. So, you can use an `RpcPromise<T>` anywhere where a `T` is required!
 
+#### Constructing `RpcPromise` from a `Promise`
+
+You can construct an `RpcPromise<T>` directly from a regular `Promise<T>`, allowing you to perform promise pipelining on a regular local promise. Pipelined calls will wait until the inner promise resolves, then will be delivered, in-order, to the resolution. This is useful when you plan to obtain some stub in the future, but you want to allow code to start queuing calls on it immediately.
+
+Wrapping a `Promise<T>` in this way is semantically identical to creating a local-loopback RPC and then invoking it. That is:
+
+```ts
+// this...
+let rpcPromise = new RpcPromise(myPromise);
+
+// is semantically the same as this...
+let rpcFunc = new RpcStub(() => myPromise);
+let rpcPromise = rpcFunc();
+```
+
+In other words, this means:
+* The result of the promise must be serializable.
+* If the promise resolution contains `RpcTarget`s or `Function`s, the `RpcPromise`'s resolution will replace them with stubs.
+* Ownership of any stubs in the Promise result is transferred away. If you want to keep your own copies, you need to `dup()` them.
+* If the promise rejects, the rejection propagates to all pipelined calls.
+* etc.
+
 ### The magic `map()` method
 
 Every RPC promise has a special method `.map()` which can be used to remotely transform a value, without pulling it back locally. Here's an example:

__tests__/index.test.ts

@@ -4,7 +4,7 @@
 
 import { expect, it, describe, inject } from "vitest"
 import { deserialize, serialize, RpcSession, type RpcSessionOptions, RpcTransport,
-         type RpcTransportWithCustomEncoding, RpcTarget, RpcStub, newWebSocketRpcSession,
+         type RpcTransportWithCustomEncoding, RpcTarget, RpcStub, RpcPromise, newWebSocketRpcSession,
          newMessagePortRpcSession,
          newHttpBatchRpcSession} from "../src/index.js"
 import { swapByteOrder } from "../src/serialize.js"
@@ -549,6 +549,7 @@ class TestTransport implements RpcTransport {
   private waiter?: () => void;
   private aborter?: (err: any) => void;
   public log = false;
+  public sentLog: string[] = [];
   private fenced = false;
 
   send(message: string): void {
@@ -557,6 +558,7 @@ class TestTransport implements RpcTransport {
     message = message.replaceAll("$remove$", "");
 
     if (this.log) console.log(`${this.name}: ${message}`);
+    this.sentLog.push(message);
     this.partner!.queue.push(message);
     if (this.partner!.waiter && !this.partner!.fenced) {
       this.partner!.waiter();
@@ -2378,6 +2380,211 @@ describe("RpcImportHook argument disposal", () => {
 
 // =======================================================================================
 
+describe("constructing RpcPromise from a promise", () => {
+  it("pipelines through a pending promise without pulling the resolution", async () => {
+    await using harness = new TestHarness(new TestTarget());
+
+    let {promise, resolve} = Promise.withResolvers<RpcStub<TestTarget>>();
+    using stub = new RpcPromise<TestTarget>(promise);
+
+    using counter = stub.makeCounter(1);
+    let result = counter.increment(2);
+    resolve(harness.stub.dup());
+    expect(await result).toBe(3);
+
+    // Only the final result was pulled: neither the promise's resolution nor the intermediate
+    // counter was transmitted.
+    let sent = harness.clientTransport.sentLog;
+    expect(sent.some(msg => msg.startsWith('["push"'))).toBe(true);
+    expect(sent.filter(msg => msg.startsWith('["pull"'))).toHaveLength(1);
+  });
+
+  it("queues calls made before resolution and delivers them in order", async () => {
+    let calls: number[] = [];
+    class Recorder extends RpcTarget {
+      record(i: number) { calls.push(i); return i; }
+    }
+
+    let {promise, resolve} = Promise.withResolvers<Recorder>();
+    using stub = new RpcPromise<Recorder>(promise);
+
+    let results = [stub.record(1), stub.record(2), stub.record(3)];
+    expect(calls).toStrictEqual([]);
+
+    resolve(new Recorder());
+    expect(await Promise.all(results)).toStrictEqual([1, 2, 3]);
+    expect(calls).toStrictEqual([1, 2, 3]);
+  });
+
+  it("accepts a promise for a target, a remote stub, or a plain value", async () => {
+    await using harness = new TestHarness(new TestTarget());
+
+    using target = new RpcPromise<Counter>(Promise.resolve(new Counter(1)));
+    expect(await target.increment()).toBe(2);
+
+    using remote = new RpcPromise<TestTarget>(Promise.resolve(harness.stub.dup()));
+    expect(await remote.square(3)).toBe(9);
+
+    using value = new RpcPromise<{foo: number}>(Promise.resolve({foo: 123}));
+    expect(await value.foo).toBe(123);
+  });
+
+  it("awaiting the RpcPromise yields the resolution", async () => {
+    using plain = new RpcPromise<{foo: number}>(Promise.resolve({foo: 123}));
+    expect(await plain).toStrictEqual({foo: 123});
+
+    await using harness = new TestHarness(new TestTarget());
+    using remote = new RpcPromise<TestTarget>(Promise.resolve(harness.stub.dup()));
+    let resolved = await remote;
+    expect(await resolved.square(4)).toBe(16);
+  });
+
+  it("reports rejection to queued calls, await, and onRpcBroken", async () => {
+    let error = new Error("nope");
+    using stub = new RpcPromise<Counter>(Promise.reject(error));
+
+    let broken: any[] = [];
+    stub.onRpcBroken(err => { broken.push(err); });
+
+    await expect(() => stub.increment()).rejects.toThrow("nope");
+    await expect(Promise.resolve(stub)).rejects.toThrow("nope");
+    expect(broken).toStrictEqual([error]);
+  });
+
+  it("does not report an unhandled rejection for an unused promise", async () => {
+    new RpcPromise<Counter>(Promise.reject(new Error("ignored")));
+    await pumpMicrotasks();
+  });
+
+  it("does not report an unhandled rejection for a discarded queued call", async () => {
+    using stub = new RpcPromise<Counter>(Promise.reject(new Error("ignored")));
+    stub.increment();  // result intentionally discarded
+    await pumpMicrotasks();
+  });
+
+  it("does not report an unhandled rejection for a discarded map() result", async () => {
+    using stub = new RpcPromise<number>(Promise.reject(new Error("ignored")));
+    stub.map(i => i);  // result intentionally discarded
+    await pumpMicrotasks();
+  });
+
+  it("disposes the eventual target when disposed before resolution", async () => {
+    let disposed = false;
+    class Disposable extends RpcTarget {
+      [Symbol.dispose]() { disposed = true; }
+    }
+
+    let {promise, resolve} = Promise.withResolvers<Disposable>();
+    let stub = new RpcPromise<Disposable>(promise);
+    stub[Symbol.dispose]();
+
+    resolve(new Disposable());
+    await pumpMicrotasks();
+    expect(disposed).toBe(true);
+  });
+
+  it("delivers a call initiated before disposal", async () => {
+    let disposed = false;
+    class DisposableCounter extends Counter {
+      [Symbol.dispose]() { disposed = true; }
+    }
+
+    let stub = new RpcPromise<DisposableCounter>(Promise.resolve(new DisposableCounter(1)));
+    await pumpMicrotasks();
+
+    let result = stub.increment(2);
+    stub[Symbol.dispose]();
+
+    expect(disposed).toBe(false);
+    expect(await result).toBe(3);
+    expect(disposed).toBe(true);
+  });
+
+  it("keeps an adopted RpcPromise lazy", async () => {
+    await using harness = new TestHarness(new TestTarget());
+
+    using counter = new RpcPromise<Counter>(harness.stub.makeCounter(1));
+    expect(await counter.increment(2)).toBe(3);
+
+    let sent = harness.clientTransport.sentLog;
+    expect(sent.filter(msg => msg.startsWith('["pull"'))).toHaveLength(1);
+  });
+
+  it("consumes the source when adopting an existing RpcPromise", async () => {
+    await using harness = new TestHarness(new TestTarget());
+
+    let source = harness.stub.makeCounter(1);
+    using wrapper = new RpcPromise<Counter>(source);
+
+    // The source was neutered: using it now reports the standard disposed error, and disposing
+    // it is a harmless no-op that doesn't affect the wrapper.
+    await expect(source.increment(1)).rejects.toThrow(
+        "Attempted to use RPC stub after it has been disposed.");
+    source[Symbol.dispose]();
+
+    expect(await wrapper.increment(2)).toBe(3);
+  });
+
+  it("stays lazy when a deferred promise is resolved with dup()", async () => {
+    await using harness = new TestHarness(new TestTarget());
+
+    using counter = harness.stub.makeCounter(1);
+
+    // Resolving with the RpcPromise itself would let the native promise machinery assimilate it
+    // as a thenable, pulling the resolution. dup() returns a non-thenable stub, which the
+    // resolution adopts, keeping calls pipelined.
+    let {promise, resolve} = Promise.withResolvers<RpcStub<Counter>>();
+    using stub = new RpcPromise<Counter>(promise);
+
+    let result = stub.increment(2);
+    resolve(counter.dup());
+    expect(await result).toBe(3);
+
+    let sent = harness.clientTransport.sentLog;
+    expect(sent.filter(msg => msg.startsWith('["pull"'))).toHaveLength(1);
+  });
+
+  it("preserves brokenness of a bare stub it was constructed from", async () => {
+    await using harness = new TestHarness(new TestTarget());
+    using stub = new RpcPromise<TestTarget>(<any>harness.stub.dup());
+
+    let errors: any[] = [];
+    stub.onRpcBroken(error => { errors.push(error); });
+
+    harness.clientTransport.forceReceiveError(new Error("test disconnect"));
+    await pumpMicrotasks();
+    expect(errors).toStrictEqual([new Error("test disconnect")]);
+  });
+
+  it("keeps disposal idempotent when constructed from a bare stub", async () => {
+    let disposals = 0;
+    class Disposable extends RpcTarget {
+      [Symbol.dispose]() { ++disposals; }
+    }
+
+    let inner = new RpcStub(new Disposable());
+    let outer = new RpcPromise<Disposable>(<any>inner);
+    inner[Symbol.dispose]();
+    outer[Symbol.dispose]();
+
+    await pumpMicrotasks();
+    expect(disposals).toBe(1);
+  });
+
+  it("resolves when awaited after construction from a bare local stub", async () => {
+    // Regression test: the constructor previously adopted a bare stub's hook directly, producing
+    // a promise whose pipelined calls worked but whose await rejected, because non-promise hooks
+    // don't implement pull().
+    using stub = new RpcPromise<Counter>(<any>new RpcStub(new Counter(1)));
+
+    expect(await stub.increment(2)).toBe(3);
+    let resolved = await stub;
+    expect(await resolved.increment(3)).toBe(6);
+  });
+});
+
+// =======================================================================================
+
 describe("HTTP requests", () => {
   it("can perform a batch HTTP request", async () => {
     let cap = newHttpBatchRpcSession<TestTarget>(`http://${inject("testServerHost")}`);

__tests__/workerd.test.ts

@@ -5,7 +5,7 @@
 /// <reference types="@cloudflare/workers-types" />
 import { expect, it, describe } from "vitest";
 import { RpcStub as NativeRpcStub, RpcTarget as NativeRpcTarget, env, DurableObject } from "cloudflare:workers";
-import { newHttpBatchRpcSession, newWebSocketRpcSession, RpcStub, RpcTarget } from "../src/index-workers.js";
+import { newHttpBatchRpcSession, newWebSocketRpcSession, RpcStub, RpcPromise, RpcTarget } from "../src/index-workers.js";
 import { v, wrapServerTarget, type ServiceValidator } from "../packages/capnweb-validate/src/internal/core.js";
 import { Counter, TestTarget } from "./test-util.js";
 
@@ -126,6 +126,17 @@ describe("workerd compatibility", () => {
     }
   })
 
+  it("can wrap a native promise in a userspace promise", async () => {
+    // Wrapping in RpcPromise (rather than RpcStub) exercises the rpc-thenable adoption path,
+    // which pipelines calls on the native thenable without eagerly awaiting it.
+    let factory = new NativeRpcStub(new CounterFactory());
+    let stub = new RpcPromise(factory.getNative());
+    expect(await stub.increment()).toBe(1);
+    expect(await stub.increment()).toBe(2);
+
+    expect(await stub.value).toBe(2);
+  })
+
   it("can pipeline on a native stub returned from a userspace call", async () => {
     {
       let factory = new RpcStub(new CounterFactory());

__type-tests__/rpc-base-cases.test.ts

@@ -150,3 +150,29 @@ api.invoke((name: string, attempt: number) => {
 
 // @ts-expect-error headers argument must be Headers
 api.roundTripHeaders(new Map([["x-id", "1"]]))
+
+// An RpcPromise can be constructed from a promise for a plain value, an RpcTarget, or a stub,
+// keeping the resolution's type in each case.
+expectType<RpcPromise<number>>(new RpcPromise(Promise.resolve(42)))
+expectType<RpcPromise<PointTarget>>(new RpcPromise(Promise.resolve(new PointTarget())))
+expectType<RpcPromise<PointTarget>>(new RpcPromise<PointTarget>(Promise.reject(new Error("x"))))
+
+// The target type is inferred exactly from a promise for a stub -- no explicit type argument.
+const promisedFromStub = new RpcPromise(Promise.resolve(pointStub))
+type _PromisedFromStubInfersTarget = Expect<Equal<typeof promisedFromStub, RpcPromise<PointTarget>>>
+
+async function assertAwaitedConstructedPromiseShapes() {
+  const target = await new RpcPromise(Promise.resolve(new PointTarget()))
+  expectType<RpcStub<PointTarget>>(target)
+
+  const value = await new RpcPromise(Promise.resolve(42))
+  expectType<number>(value)
+}
+
+void assertAwaitedConstructedPromiseShapes
+
+// @ts-expect-error a non-thenable value cannot back an RpcPromise
+void new RpcPromise(42)
+
+// @ts-expect-error a bare stub cannot back an RpcPromise; pass a promise for the stub instead
+void new RpcPromise(pointStub)

src/core.ts

@@ -561,9 +561,63 @@ export class RpcStub extends RpcTarget {
 }
 
 export class RpcPromise extends RpcStub {
-  // TODO: Support passing target value or promise to constructor.
-  constructor(hook: StubHook, pathIfPromise: PropertyPath) {
-    super(hook, pathIfPromise);
+  // Internally, an `RpcPromise` is constructed from a `StubHook` plus a property path. The
+  // application may instead pass a promise for the eventual resolution; calls made before it
+  // settles are queued and delivered, in order, once it does.
+  constructor(hook: StubHook | PromiseLike<unknown>, pathIfPromise?: PropertyPath) {
+    if (hook instanceof StubHook) {
+      super(hook, pathIfPromise ?? []);
+    } else {
+      if (pathIfPromise !== undefined) {
+        throw new TypeError("RpcPromise constructor expected one argument, received two.");
+      }
+
+      let kind = typeForRpc(hook);
+      if (kind === "rpc-promise") {
+        // Adopt an existing `RpcPromise` directly, transferring ownership of its hook: the source
+        // promise is neutered, as if disposed, and must not be used afterwards. In particular,
+        // adoption keeps the promise lazy -- assimilating it as a thenable would instead force its
+        // resolution to be pulled -- and preserves hook-local behavior such as brokenness. This
+        // applies only to promises, not bare stubs: a non-promise hook may not implement pull(),
+        // so a bare stub takes the generic path below, which adopts the stub into the resolution
+        // payload.
+        let raw = unwrapStubAndPath(<RpcStub><unknown>hook);
+        if (raw.pathIfPromise!.length > 0) {
+          // Property promise: get() returns an independent hook, and properties have no
+          // disposer, so there is nothing to neuter.
+          super(raw.hook.get(raw.pathIfPromise!), []);
+        } else {
+          let adopted = raw.hook;
+          raw.hook = DISPOSED_HOOK;
+          super(adopted, []);
+        }
+      } else if (kind === "rpc-thenable") {
+        // Workerd-native RpcPromise/RpcProperty: wrap in a TargetStubHook, which pipelines calls
+        // directly on the thenable and awaits it only on pull().
+        super(TargetStubHook.create(<RpcTarget><unknown>hook, undefined), []);
+      } else {
+        // `Promise.resolve()` natively handles the hazards of assimilating an arbitrary thenable
+        // (`then` getters with side effects, self-resolution, cross-realm thenables), so the
+        // resolution callback below only ever sees settled, non-thenable values.
+        //
+        // The resolution is adopted with "return" semantics, taking ownership of any stubs
+        // within (including a stub as the root value). This is the same representation used for
+        // the resolution of a local async call: pull() delivers the value, pipelined calls
+        // forward through the payload without forcing a pull, and a single-stub payload forwards
+        // onBroken(), preserving brokenness.
+        //
+        // A rejection is adopted as an ErrorStubHook rather than left to reject the backing
+        // promise. This way the promise chains backing queued calls never reject -- the calls
+        // land on the ErrorStubHook, which disposes their arguments -- and the error only
+        // surfaces through pull(), so a discarded pipelined call can't produce an unhandled
+        // rejection event.
+        let promiseHook = new PromiseStubHook(Promise.resolve(hook).then(
+            value => new PayloadStubHook(RpcPayload.fromAppReturn(value)),
+            err => new ErrorStubHook(err)));
+        promiseHook.ignoreUnhandledRejections();
+        super(promiseHook, []);
+      }
+    }
   }
 
   then(onfulfilled?: ((value: unknown) => unknown) | undefined | null,